Powering Fridges from Inverters

[OuttaSight]

Fridge-freezers... one of the worst kinds of load you can tackle with a small solar system like mine.

They don't consume that much power on average when running (mine draws about 125W) and the fact that they mostly run at less than 50% duty cycle even makes them quite attractive loads to run 24x7 (an average load of just 62W means mine consumes about 1.5kWh per day).  I could easily power it with the ~1kW of solar panels I have... except for one thing...  They have unbelievable inrush surges when starting (like 10x the operating power).

My main solar system has a 24V bank of batteries and a 1kW pure sine inverter that feeds my work laptop, the house lights, some hi-fi and AV kit and a computer room upstairs with another PC and some comms kit.  That works just fine but on a sunny day like today, the battery gets full by lunchtime and then I end up wasting a lot of solar power in the afternoon that can't be used and can't be stored.

Most of the time the main system inverter sits around about 200W output constant load during the day.  I once tried running the fridge-freezer on it with nothing else plugged in and it just about worked.  It overloaded for about half a second (more than 1150W on the power display) and it beeped.  But after that it sat at at the bottom of the scale.  I gave up on it because the inrush current meant that I couldn't risk running anything else at the same time.

Today, it was especially sunny so I decided to see if I could run the fridge-freezer and the other loads at the same time.

I had a few spare bits kicking around: A really old 100Ah battery (that just refuses to die even though I often neglect it in the garage for months), a spare 1kW pure sine inverter (left over from when my main system was 12V) and a 30A variable lab PSU that I use for charging batteries, starting small cars, blowing stuff up

Here's what I did:

The fridge-freezer only needs 125W when running, so the main solar inverter can deliver this with the other constant loads it has to supply.  The fridge-freezer only needs the 1-2kW surge for a fraction of a second and the 1kW inverter can deliver a 2kW surge.  So I used the spare inverter and battery as a buffer.  The 12V battery is only there to act as a big capacitor. In fact, I might try one of those 1 Farad car hi-fi capacitors instead of the battery (but I happened to have a battery kicking around).

Once the fridge-freezer is running, the 30A PSU runs the load from the house solar mains (drawing about 9-11A @ 13.8V, so it can recharge the 12V battery at the same time).  I'm using an old deep cycle battery but a car battery would do fine as it only has to deliver a "starting" current before going back to bed... floating to 13.8V after a couple of minutes recovering from the "start".

Obviously, the wife isn't too happy about a big boat battery next to the fridge in the kitchen but I think I can hide it all under the fridge. We had to raise the fridge up on decorative concrete blocks because there was a water pipe at the base of the wall that meant the fridge wouldn't fit in the space next to the units if it was on the ground...  So, there's a convenient bit of storage space under the fridge.  If I can get away with using the 30A supply (as the load is only nominally 9A) and a 1F capacitor for the surge, then I won't even need a battery.

Or, I might keep some sealed batteries under there as then I can maybe run the fridge overnight. I can run the fridge on the main bank via the charger until we go to bed and then I usually transfer the PC & comms stuff to grid power (easy as it's on a UPS) and I turn off the main inverter.  The fridge can then run for about 8 hours off it's dedicated battery (it should consume no more than about 46Ah), until the morning when the main inverter is restarted and we have solar power again.

A bit mad, but it might work!  The dedicated 1kW inverter can supply the inrush surge and power the fridge (plus I get another 100-ish Ah of battery capacity in the system for it) and the solar system sees a higher average power utilisation during the day.  And I didn't have to buy a new 2kW pure sine inverter to replace my main 1kW one

[DamonHD]

Very interesting...

My interest is in keeping our freezer from thawing out if we have an extended power-cut.  It's not happened for a long long time since domestic electricity in London is pretty damn reliable, but I'm still interested "in theory".

My fridge/freezer's consumption is a little over 1kWh/day, so in theory ~160Ah of 12V battery would provide a day's power on top of the ~18h that the freezer in particular is rated to keep the food safe for if you don't open the door (IIRC).

But the size of the surge is a real eye-opener, and means that you/I have to waste extra energy in what is, most of the time, a massively-oversized inverter.

And the fact that you have two more phantom loads with the waste inherent in your charger, and keeping the battery on float.

I'm not knocking your scheme at all: thanks for writing it up!

Rgds

Damon

[OuttaSight]

Hi Damon,

Ha haha.. Yeah it's a bit of a Frankenstein system but I was just so annoyed today at having all that lovely sunshine go to waste.  If I can make use of more of it (even at a poor efficiency rate from triple converting it!!!) then it's better than just throwing it all away when the main bank is full.

Fridges are doubly hard to run from inverters because not only do you have to over size the things massively but they also have to be pure sine inverters as well.  Compressors don't like mod-sine.  If that weren't a problem, I'd have just bought a 5kW 24V inverter as my main one.  But I went pure sine so that my TV and hi-fi wouldn't make a buzzing noise (they did on my first inverter - a cheap 600W mod-sine one).

[Ungrounded Lightning Rod]

Have you considered getting a spare 24V inverter just for the refrigerator?

You could rewire the refrigerator so the thermostat disables the inverter (and its associated losses) when the compressor and any fridge fans are not supposed to be running, rather than controlling the motors directly.

The timer and light can run from your always-on regular inverter.

Other things to look out for on refrigerators is a defrost heater and an anti-condensation heater on the outside case.  You'll probably want to run the defrost timer on your regular inverter so it will cycle properly.  The defrost heater may be big enough that you'll want to keep it off your regular inverter.  In that case you'd probably need a relay to control it if you want to power it from your aux inverter, ANOTHER aux inverter, and/or to replace the element with one that will work from 24V directly.

Unfortunately a self-defrosting fridge with a defrost heater actually needs the heater.  Otherwise (as I discovered with a fridge where the early models had too short a defrost cycle) ice builds up in the cooling coils and eventually blocks airflow, making the fridge stop cooling.

[dnix71]

I had a small upright freezer that used about 120 watts running but pulled 800+ to start. Even after adding a big start cap (there was no cap, the model came with and without), the startup inrush was more than my sine wave inverter could handle.

The 1 Farad car caps only cost about $18 now. http://www.sonicelectronix.com/item_18671_Raptor+C1F.html

If that works, it's cheaper in the long run than a car battery.

It was cheaper for me to buy an Engel 12v fridge than to buy a huge sine inverter and battery just to run a freezer. The Engel doesn't have that much space, just 43 quarts, (there is an expansion mod that's on my to-do list). But it only pulls 36 watts running, and will freeze if you turn the thermostat down just a little.

http://www.12voltgifts.com/engel-12-volt-fridge-freezer/medium-large-engel-12-volt-ac-dc-fridge-free
zer-43-quart-mt45f-u1.htm

The expansion mod adds a small box over the top with a basket in it. Turn down the thermostat a little and the bottom freezes while the top just stays cold.

[kurt]

most not all modern refrigerator designs the defrost timer only advances when the compressor is running they defrost every 8 hours of compressor run time some do 12 hours but most do 8 some of the more expensive modern refrigerators have electronic control boards that control the defrost cycle with a frost censor and whatnot i would avoid one of those if you intend to modify it to run on on 2 inverters.  

[FishbonzWV]

A few years back, someone posted a similar thread.

If I remember correctly, they used a long extension cord to mitigate the inrush current. I have never tried this but it might be something to experiment with.

Bonz

[dnix71]

A homemade choke coil (ferrite plus thin wire wrapped around it) would do the same thing.

Wouldn't choking the start current prevent the compressor from spinning up?

The freezer I had had a thermal relay on the start circuit. After about 1 second the heat from the inrush would trip out the start windings. If the compressor didn't keep going on the run windings, it would short-cycle.

[Ungrounded Lightning Rod]

A homemade choke coil (ferrite plus thin wire wrapped around it) would do the same thing.

Ferrite plus thin wire, especially many turns of it, in series with a high current load, is a fire waiting to happen.

Choking would shift the phase of the run current and further reduce the power factor (not a good idea, especially with an inverter, which has to interrupt the reactive current twice every cycle).  But it wouldn't appreciably limit the current unless the inductance was so high that the motor would have additional difficulty starting.

[TheCasualTraveler]

Where would the capacitor be? I'm guessing across the input to the inverter?

[hydrosun]

I'm surprised no-one commented on your statement that compressors don't like working on a modified sine wave inverter.  I ran a small apartment size refrigerator on an old Trace 800 watt inverter for a few years. I did add more insulation to cut down on the run time. On another system we ran a midsize refrigerator on a small 12 volt 700 watt inverter as a test. It would start it and run most of the time. Occasionally it wouldn't quite get started and the over temp relay on the fridge would shut it down. We then got a bigger inverter. All the trace DR inverters were designed to be able to back up homes when the power went down, that included the refrigerator.  So I think if the inverter is large enough you should be able to run your refrigerator on a modified sine wave inverter. The heavier transformer based inverters will surge higher than the small high frequency cheap inverters.  So a Xantrex DR 1500 would be able to run your refrigerator and all your other loads. If you are concerned with noise you could have yours small inverter to just power your sensitive electronics.

Chris

[Ungrounded Lightning Rod]

Replacing the 12V car battery.

It filters the output of the charger, provides an amp-second per volt of droop for covering the compressor startup surge, and doesn't spit acid fumes into the kitchen, need watering, or fail after a few years.

[DamonHD]

What is the actual service life of one of these things under heavy load and lots of cycling?  They seem to be curious beasts internally; half man, half sausage ... I mean, half electrolytic cap, half battery, half rubber mattress...  Well, something like that.  Anyway, it seems to me that there's plenty of scope for mechanical strain and something like metal fatigue to set in.

Rgds

Damon

[OuttaSight]

Wow... What a response!  Thanks everyone.  Fridges are obviously an area where the mad inventors get together and have a kitchen party.

It's not really a permanent install - I was just trying it out as an experiment (having already discovered that the start-up surge is right on the limit of what my 1kW inverters can deliver) and I'm not planning on disassembling my fridge-freezer.

Mine is a an electronic one (well at least the thermostat is a logic controlled one with a LED bar graph for the setting) so that probably rules out most of the ideas for hacking the thermostat / defrost timers, etc.  It's a self-defrosting one and it does have a door heater as well, plus fans to active cool the radiator and circulate the cool air in-between the freezer and chiller compartments (it's an upright 50:50 fridge freezer). So maybe not your AAA rated fridge... although it is only three years old.

Here you can see my creation which I've dubbed "Fridgenstein" . Sitting on the counter are some of my other solar powered kitchen appliances. I have a 400W toaster oven, a 900W kettle and (out of shot) a 400W rice cooker and (in a cupboard somewhere) a 200W automatic egg boiler (actually an egg steamer but it makes "boiled" eggs).  The cooker, washing machine and fridge remain out of bounds for now  Although, I do run the cooker hood lights and fan from solar power.

One poster mentioned that if the inverter can't produce the required start-up surge, the compressor can fail to start and I did have this happen once (while doing the dishes in the evening).  The charger was sitting at 15A and the fridge was doing nothing.  The compressor had stalled on start-up, drawing 180W while not moving (hope nothing was damaged). I cycled the power and the fridge started up as normal after a couple of minutes.

It almost made it though the night on its dedicated battery (quite a feat for the 12 year old & often neglected battery).  After sun-down, it ran on the house bank via the charger and then at about midnight I turned off the house inverter (with the bank depleted to 60% DoD).

At about 4am, I woke up and had a look at it... still working. The compressor was running.  About 6am I woke again and this time the battery had died with the inverter locked out on a low voltage alarm.  I fired up the house inverter and the charger sat pegged at 30A for about 30 minutes (with the compressor running) but I had to stop it as the house bank was now drained down to 80% DoD.  The fridge carried on with its own battery and I let the sun start to charge the house bank.

It's not nearly so blue-sky sunny today so that will probably be it for the experiment.  I'll let it run until noon and then I can say that I've powered a fridge for 24 hours on solar power but I also know that even a dedicated 2kW surge rated inverter isn't enough to guarantee that the fridge will operate normally.

That means my only real options are:

A SunnyBoy 1100LV grid tie inverter (these work at 21-60V so I can recycle my 600Wp 35V array).

A minimum 2kW house inverter (as this affords a 4kW surge and means I won't blow up the inverter if I'm running the 900W kettle at the instant that the fridge fires up).  The house battery bank would have to be upgraded as well... 1kW is already a discharge rate of C/5 on my 220Ah 24V bank.

I'm not sure who to believe on the mod-sine debate.  Some manufacturers say that it's ok to run fridges on mod-sine, some say not.  Some manufacturers even say you shouldn't run fluorescent lights on their mod-sine products... Err... Hello... I don't have ANY tungsten lights in my house.  All I know is that the mod-sine inverter I had made all my appliances buzz (even the lights) and almost caused a plug-in kWh meter to catch fire so I'll have nothing more to do with them :/

Then there's the solar input...  I've been building up the system for about a year and I can now (on average) extract about 1-2kWh per day in the summer.  I could utilise more power than that on long summer days but the fridge-freezer consumes a whole 1.5kWh per day by itself, leaving (on average) nothing for the rest of the house!

Looks like you need a hell of a lot of solar to run a standard fridge-freezer 24x7 (don't even think about the 365).

It's easy to rig up the spare kit I have for the odd really sunny day if I want to break a record on utilisation but, given our generally inclement weather here, I'll be sticking to the grid for my refrigeration loads for now.

The SunnyBoy looks like a better bet for my next big upgrade.  It will plug straight into my existing solar array and it just plugs into an AC wall socket to feed the grid (no hardwire installation required).  That can offset the fridge (and cooker and instant heating shower and immersion water heater) without having to actually deliver the power for those loads directly. Cookers and showers are immense loads but (like kettles) are very low duty cycle in a 24 hour period.

I don't know what arrangements there are with the UK utility companies for discounts / export tarifs.  They have been very slow here to organise anything and if (as happened with Sharkey on the other forum I hang out on) the meter I have counts up regardless of whether the power is flowing in or out of the house, I could end up paying for the units I deliver to the grid!  That won't do at all...

[electrak]

As to MSW inverters, it depends on how they function. Some are 120V 60+ 60- splits, some are 120V to neutral, some have 1 step from square, some have a lot more, most sine wave inverters if looked at close enough are modified wave, just smoothed more. YMMV

[DamonHD]

Hey!  Our kitchen sometimes looks a little like that when I'm trying out yet another mad scheme, though mine are all demand-side control rather than off-grid!  (And I suspect that the yellow sticky above the dishwasher plug saying "not between 4pm and 9pm" will do more for our carbon emissions than all the fancy electronics and National Grid real-time home automation yadda yadda!)

Where are you based?  In the UK AFAIK you can't just plug in a grid-tie given the G83/1-1 regs: you need to have a part-P competent electrician wire it in for you.  I wish that weren't the case (and I recently wrote to DECC suggesting why it may be an unnecessary inhibition to home 'nano'generation http://www.earth.org.uk/note-on-G83-lite.html to which I haven't yet had a full reply) but please tell me I'm wrong.

Rgds

Damon

[OuttaSight]

I tried a search on this and there was only one reference to so called "enhanced mod-sine inverters" and that was a theoretical discussion that by using a two voltage step (instead of the simple dead gap period square wave type) you could reduce the harmonics significantly to produce a much closer approximation of a sine wave but without the expense and complexity of a full high frequency PWM generator.

The system relies on generating four power line voltages (+crest, +some ideal mid-voltage, GND, -some ideal mid-voltage and -crest) and then rapidly switching between these rails in series to a 50/60Hz timebase.  This meant that the switching takes place at low frequency and provides a relatively low THD figure plus lower switching losses than high frequency PWM pure sine types.

I couldn't find any manufacturers that claimed they had enhanced mod-sine outputs.

The dead gap period square wave "mod-sine" manufacturers never quote THD figures for sine distortion harmonics because they are so horrendous as to be embarrassing and at the price point they sell them at in camping and car accessory shops, nobody who sells or buys them knows what harmonic distortion does to their equipment.

For the more engineering oriented web sites that make and sell both types of inverter, they usually explained that some appliances (such as compressors or pumps or some types of fluorescent lamps, any electronic speed control power tools and so on) may or may not run but might run hot or not properly or make a buzzing noise or emit lots of RFI noise on their cheaper mod-sine range.

My kWh meter that almost caught fire did so because it used a very simple power supply for its meter electronics that just had a R-C tank across the L-N input.  It managed to eek a few mA of AC current and tap it, rectify it to get about 5-10VDC to work the meter.  Worked fine on a sine wave at 50Hz.  Trouble was that on a typical mod-sine signal, the R-C tank soaked up the harmonic frequencies (remember it was deliberately tuned to a high frequency so as to limit the power it tapped at 50Hz sine). This caused a much larger HF current than expected to flow through the tank, cooking the resistor until it made a burning smell.  I measured 180C on the resistor when I took the unit apart and ran it on the mod-sine inverter and 40C when running on proper mains sine.

Other equipment may work but also run much hotter than intended.

If you know of any specific enhanced mod-sine makes and models of 2-3kW inverter, I'd be interested as they should be fine for all but hi-fi audio and more efficient than true sine PWM designs, and cheaper as well.

[OuttaSight]

Yeah, I'm in Sussex.

The SunnyBoy 1100LV comes with a regular 13A UK mains plug as its method of grid connection and is G83 certified.  I also looked at the Mastervolt Soladin700 which connects by a regular AC plug as well but it has a higher string voltage requirement that means I can't use my 35V array.  That's G83 compliant too.

link to Sunnyboy PDF specs

They both incorporate anti-islanding (which is one of the main requirements of G83) so that they disconnect from the grid if there is no grid supply or the grid supply goes out of voltage / frequency range of the inverter.

Whether you earn any credit for generation / export depends on whether your meter runs "backwards" if you make a surplus.  I've got a LCD digital meter so it probably runs "forwards" even when exporting power.  That's why I'm only playing with off-grid kit at the moment.

[Ungrounded Lightning Rod]

Not sure.

But since they're used for avoiding power supply voltage sag on boom-car audio systems they have large currents at hundreds to thousands of cycles per second continuously for hours at a time.  So I'd expect they'd be loafing (but being held at a good polarization-maintenance voltage) if you use them to occasionally start a fridge.

They'd make a big bang if they failed shorted.  B-(

[Norm]

  It seems that we must have a lack of experts

in refrigeration? or what gives? Why can't anyone

seem to redo a refrigerator, put in a seperate

compressor and DC motor? Seems like you would be

money ahead to just hire someone to install a

refrigeration system to tie into your refrigerator

or freezer?

[wdyasq]

Norm,

The compressor that would make this all possible is one by Panasonic (EKI series IIRC) that is a variable speed inverter run one. Being inverter run, it can also have 'soft start' and vary power to meet load requirements.

I would seem simple enough to just tack a special inverter front end on a properly built refrigerator and instant RE unit.

Unfortunately, cheap electricity and consumers who don't give a flip keep this stuff from commercial/widespread application. The United States market for such a unit is probably under 1000 units per year... How many of each door style will need building and what happens when SWMBO wants it in a color out of stock ....

Ron

[TheCasualTraveler]

     Been thinking about this for awhile and your right, powering fridges off inverters is interesting stuff. I have a few questions I have to ask.

First, why do you go to all the trouble of turning 12 volts to whatever house voltage is in the UK, then back to 12 Volts thru the battery charger and then back to house current again? Why not just run a wire from the main battery bank to the fridge inverter? The only reason I can think of is line loss if it is a very long run.

Now this thing with capacitors fascinates me also. Would putting a large cap across the input terminals of an inverter help other loads that need more power at start up (florescent lights, fans)? I suppose a condition where the inverter is sized properly but the battery bank is on the low side for supplying a large continuous current.

And finally, I've always wanted to try to run my fridge off a wall wort. It goes like this... The batteries, charged by solar have enough power to make the fridge run with power to spare on sunny days, but by the end of the night the fridge has drained the batteries down too far. So, adding a wall wort with a full wave bridge to the batteries would help it recover just a little between run times, just enough till the sun comes up again. Sure the fridge would still be grid tied but running on a fraction of normal grid power.

Hope this all didn't sound too goofy.

.

[Phil Timmons]

Any one look at doing a by-pass valve (loops around like a feed-back loop) around the compressor for start-up?  Lets the compressor come up to speed without head pressure and then closes slowly, sending the compressed freon down range to the condenser, just like normal operation.

We do that on High Pressure Cooling Pump water systems in Power Generation Land.  It lets a 1000 + HP pump and motor come up to speed with reduced head pressure, and then starts raising the pressure, as the valve closes.  We keep the in-rush to within 125% of the full load amps.

If it works for a refrigerator, it should work for an Air Conditioning unit, as well.  A/C typically have a high in-rush current on starting, as well -- for the same reason -- a hammered on hard-start with a high head pressure going to the condensing coil.

[freshair]

I've been running my fridge off a MSW inverter for over two years. My system is rather small and I found the fridge to be one of the best "loads" to put on it. What could I use it for during daytime hours??? Tried a few different things... 2 foot box fan running for 9hrs ran the batteries down so low, it took 4 days to bring them up to full charge.

So I tried the fridge. Only runs about 10 minutes every hour then has 50 to bring the batteries back up. I run it from about 6am to 6pm in the summer... winter till about 4. Then switch it over to "mains" at night. But there have been times when it was left on till 1-2am. Last weekend it ran for 60hours straight... Sun till Tu night. Never saw the batteries go below 12 volts. But it wasn't till Friday that the batteries came up to full charge.

Granted I have to watch the weather... this year its been pretty bad. Probably only used it about 4 months... to many rainy days. :-(

[bob g]

anybody just connected up a nice apc ups?

i would imagine an apc2200 or 2200xl would kick over just about any

refrigerator.

it goes like this, connect external batteries that have some amp capacity

rather than the puny little fellars that are internal to the units.

with a set of real batteries the ups will provide about 175% of full load capacity.

the 2200 series puts out 1600watts, 1600 x 1.75 = 2800watts surge, and they can support that for about 3 seconds before the overload kicks out.

around here old ups systems are relatively cheap, usually with dead batteries you don't want anyway, and the apc smartups series produces a nice pure sinewave of less

than 5% distortion.

as for efficiency, they are about 84% efficient, but for an intermittent load that might be acceptable.

bob g

[dnix71]

My parents used to live on a boat. The fridge always ran on an internal inverter no matter what it was plugged into. The inverter made 120 from 12v, but if you had a genset or were on shore power, the inverter would sense that and use the 120 directly.

When the inverter failed it was cheaper to replace the fridge than to replace the inverter.

It didn't cool very well, either. Milk wouldn't keep long and they had a separate ice maker.

[bob golding]

i have a belkin 1kva inverter, which is a re badged apc by the look of it. on a 120 watt load it draws around 180 watts. when it is  sitting with no load it draws around 50 watts at 36 volts. it will run most things but the 50 watts is a pain. i just use it as a back up for when the next cheap msw  inverter dies. i have around 5 dead ones at the moment. they seem to last around 8 months then i suspect the caps dry out. i did play with dehuminifiers for a while they have a  defrost valve operated from a solenoid. maybe one could use one of those as a valve assuming you can find someone to regas the fridge for you. maybe a car aircon system could be modified to use as fridge? interesting stuff. there is someone on here who modified a small chest freezer to use as fridge using an external thermostat. still got the surge problem though.

cheers

bob golding

[Jon Miller]

Hi Bob,

Had a go using a 1.5KVa UPS and it would kick a smaller fridge over and get it running but its almost as though it wont start it up quite right.  

The motor starts up, starts compressing, amps drop after 3 seconds and then the motor thermal device cuts in and the motor switches off.

I have thought about using a spinning reserve, another induction motor switched on before  but not tried.

Also, using a ups it would be possible to make a circuit to switch on the mains before the fridge wanted to start allowing it to be connected to the grid for starting, then switch that grid connection off and let the ups run as an inverter, this way I know works.

The fridge debate is one worth the effort.

does anyone remember those savaplugs, whats in them?

Regards

 

[OuttaSight]

Simple...  I had to use my whacky multi-conversion thingy because the start surge from the fridge will blow the house inverter that is servicing a 200W constant load plus intermittently a 800W kettle.  The 1kW spare inverter I had could only just about start the fridge when nothing else was connected to it.

If I was serious about powering the fridge from solar, I'd just upgrade the house inverter to a 3kW one to provide the fridge startup current within the base load capability (not relying on the surge rating which will shorten the inverter life) and service the other loads at the same time.  It would also mean I could run the microwave, washing machine and a "normal" 3kW kettle from the solar system (not at the same time!).

But that means doubling the standby current from 0.7A to 1.5A on the house inverter and spending north of £1,000.  I did this test using spare kit laying about the workshop for nothing .

Post analysis puts the multi-conversion thingy at about 55% efficiency.  You lose a lot of power as heat on a transformer based linear regulated 30A lab supply...

As for modding the fridge... Not really interested.  Only hardened mad inventors will take this route.  I'm interested in powering existing common household appliances from solar.  Using an inverter is a sop to this anyway.  It would be much more efficient to re-wire the house and all it's appliances for 24V DC operation but who wants to redesign the PSU of every computer, VCR, radio, TV and so on just to avoid using 230VAC as the power source?  Fine on a caravan or boat but not practical for the home.

[ghurd]

I just got off a house boat on Lake Cumberland.

Standard home appliances.  Fridge, Mr. Coffee coffee pot, microwave, mixed lights, LCD TV, etc, on the inverter if the diesel genny was not running.

Pair of 8D batteries and an antique Trace U2512 inverter.

The microwave put a serious drop on the battery voltage.  LOL

No problems except the GFCI outlets buzzed enough I shut the one in our bathroom off at night.  And the front deck FL buzzed, but it buzzed on genny power too.

I was surprised.

G-

[OuttaSight]

Simple...  I had to use my whacky multi-conversion thingy because the start surge from the fridge will blow the house inverter that is servicing a 200W constant load plus intermittently a 800W kettle.  The 1kW spare inverter I had could only just about start the fridge when nothing else was connected to it.

If I was serious about powering the fridge from solar, I'd just upgrade the house inverter to a 3kW one to provide the fridge startup current within the base load capability (not relying on the surge rating which will shorten the inverter life) and service the other loads at the same time.  It would also mean I could run the microwave, washing machine and a "normal" 3kW kettle from the solar system (not at the same time!).

But that means doubling the standby current from 0.7A to 1.5A on the house inverter and spending north of £1,000.  I did this test using spare kit laying about the workshop for nothing .

Post analysis puts the multi-conversion thingy at about 55% efficiency.  You lose a lot of power as heat on a transformer based linear regulated 30A lab supply...

[Ungrounded Lightning Rod]

As for modding the fridge... Not really interested.  Only hardened mad inventors will take this route.

Then I must be a hardened mad inventor.  (Got some patents to prove it.  B-) )

But modding a fridge to put a dedicated inverter inside is pretty trivial.  They generally use standard spade connectors for all the wiring and include a diagram.  Contact ratings on everything important (mainly the thermostat) are available.

[zap]

I have a Greenplug which I assume is the same as a savaplug.  I've never looked inside it but I assume it's just a "soft start" circuit?

I can't use a "Kill A Watt" meter on the fridge because it pulls too many amps and the KAW trips.  Using the Greenplug, the KAW is able to start the fridge.

I have an APC Smart-UPS 1250 and I might have to try that someday with the Greenplug.  The fridge is old and was free, the Greenplug is old and was free, and the UPS was free... not much to lose... and the fire extinguisher is always close by(it is old and was free too).