How to decide on a system voltage?

[RandomJoe]

I have been dreaming and making calculations for systems of varying sizes, trying to decide just how large I want to go right now.  I'm not interested at this point in going whole-house or grid-tied, as this is primarily an experimenter/playtoy thing for me to have some fun with.

My dilemma right now is what battery bank voltage to go with.  My primary daily use for the system is going to be my ham shack, where ALL of my gear is 12V.  Radios, scanners, even amplifiers.  For that matter, a lot of my networking gear is 12V as well.  I also have a fair collection of 12V lighting in the form of RV fluorescent lights and some modified LED drop lights that I can use for "emergency lighting" in case the grid goes down.

The problem, though, is that I would also like to have sufficient capacity in the batteries to run "essentials" in an outage (primarily the refrigerator).  This could primarily be an issue with battery-bank sizing, as I don't necessarily need to be able to run everything strictly off solar, nor does it need to run all day.  I do have a generator, which could run the appliances directly, but could also be used to provide bulk charging to the battery bank.  I just don't want to have to run the generator at night or when I'm gone, but would like the food in the fridge to keep!

So at what point (wattage or otherwise) does it simply become infeasible to stay with 12V?  I certainly don't need to stay there, as I could just run the rigs with the power supplies I already have, that just seems a bit wasteful - and might push my inverter requirements up higher than I'd like.  On the other hand, I notice most if not all the higher-wattage components (panels and inverters in particular) don't come in 12V anyway.

I have had some people mention pulling off just a portion of the battery bank for 12V items, but I'm worried about unbalancing the battery charges that way.  I have a fairly sizable draw, especially when I'm actively using the rigs and not just listening, and as I mentioned before this would be the primary use for the system.

Some usage numbers for the primary loads, in case they help:

Ham bench would be around 50AH (at 12V) per day.  That is, nominally.  Certainly in a pinch I could greatly reduce that.  And if I was having a really active weekend it could go significantly higher.  (Another reason an oversize battery bank wouldn't necessarily bother me!)

Refrigerator pulls around 110W while running.  The Kill-A-Watt shows an average of 1.6kWh/day usage.  But this would be an outage-only load, and I could be recharging the batteries with the generator part of the day.  Primarily I just need enough AH in the batteries to run this.

[RandomJoe]

Just for fun, let's see if I'm doing my math right...

I wanted to know how much battery I'd have to have just for the refrigerator:

1600WH / 12V = 133AH x 4 = 533AH to keep in the top 25% of battery charge, assuming full recharge each day.

Then thought, how much solar panel?  My area has 4.98 hrs insolation in winter:

133AH / 4.98H = 26.7A x 15V = 400W  (I don't remember just where the 15V came from, probably someone else's examples?)

So probably a bit more solar panel than I want to buy just for an emergency backup.  But it does tell me I'd have to have a charger I could plug into the generator that could push 30A.  And the generator would have to run 5 hours to get the batteries charged.

Is that correct?

My original plans for the ham bench were:

50. AH x 5 = 250AH battery for one day, so:
    
250. AH x 3 = 750AH to stay in top 20% for 3 days no sun.
     
     

That seemed a bit excessive, I don't usually go that long without sun, so I dropped it to 50% over 3 days no sun:

50AH x 2 x 3 days = 300AH.

Looking at what is required for the fridge, it appears I might not be too far off with - say - a bank of six T-105s (660AH).  That would provide an excellent reserve for my regular use, and keep the fridge going in a pinch.

On the solar side, I figured (for the ham bench):

50AH / 4.98 = 10.04A x 15V = 150W

So preferably something around 175-200W of panels.

So far so good?!? :-)

[electronbaby]

How old is your fridge? Are there newer / more efficient units?

The cheapest part (and most misunderstood) of an RE system is conservation. If you can figure out a way to reduce your power requirements, you will not have to spend an arm and a leg on your PV array.  However, everyones needs and habits are different and there is no "one size fits all" solution. Only you can figure out your power usage. (it sounds like you are well on your way).

It looks like your heading in the right direction (although I didnt go over your math too detailed).

One thing to remember is that with batteries, you have to start out with an inherent 80% - 85% efficiency. (roughly 20% averaged loss from chemical storage)

Solar PV you have to start out with a 75% efficiency from nameplate rating on modules. This is not including your hours of insolation (lat.) or your declination. (a 130 watt module will only put out 97.5 watts max under "most" conditions).

One thing you will see if you look close enough (and I know you want to start this out as a hobby) but you will find it is much more efficient and easier on the wallet to go with a grid tie system if you live in an area that has reliable electricity delivery. You will need to purchase minimal equipment, and you can use all your existing appliances with knowing you will get the quickest return on your investment.

If you need to design battery storage in from the beginning, make sure you design the system so it is scalable and made to be grid tied some day. That way you can have best of both worlds, and most importantly, it serves your needs well (keeping in mind they might change in the future).

Good luck and welcome aboard. :-)

[Jeff]

Nice to see someone getting started on RE!

The ONE thing that is the hold-up for most people is: GET STARTED!

You have a good-to-great choice of batteries, the heart of any/most RE systems. I think I'm not over-stepping my experience in saying: get started on your battery bank. You can always use them for your 12v applications you mentioned. Of course, it's always best to buy your battery bank all at once, but some cannot do that. If you are ready to get started on any system though, it will never hurt to buy just the batteries all at once if you can. Even if you don't have any other way to convert to 110v, or a way to charge them as yet.

The first step is get started! You'll find many ways to start using the 12v power it seems. Even if you need to keep a few seperate for some 24v applications, you won't have that option if you don't get the batteries, will you?

If it takes even a year before you put those batteries into a more complex system, you can always keep them in tip-top shape with a small "wall-wart". A small 1-2amp charger that is.

Looking forward to seeing how you progress, and WELCOME!

[DamonHD]

"You gotta be in it to win it!"

Yes, get started, gently, with almost anything, and you'll learn a lot.

I have a very small off-grid system, originally 20Wp PV and now several times that size and with a little wind on the side too.  I slightly wish that I'd bought the 100Ah battery that I was offered rather than 'cunningly' economising with a 40Ah, but never mind.

But it gave me the confidence to commission and install a grid-tie PV system 10x even the new, inflated off-grid system's size, and I intend to triple the size of my off-grid system this year given my positive experience.

I'm not addicted.  I can stop any time I want to.  Really.  B^>

Rgds

Damon

[RandomJoe]

Ah yes, after I posted I was doing a little more figuring and realized I need to throw in the inefficiency numbers.  I'll have to revise my figures a bit there.

The fridge is a pretty good unit, I think.  It's only 5 years old, and I could probably make it even more efficient if I'd get around to filling it up!  I'm single, and there's not much in there...  I do drink a lot of water, so have thought about buying a bunch of water jugs to take up the space and help hold the cold.  (Then I just have to remember to keep them filled!)

I've done quite a few of the usual conservation measures.  CFLs everywhere, the house was already well insulated, programmable tstat to set back the A/C during the day (usually never runs when I'm at work), things like that.  

I'm not terribly keen on grid-tie for a couple of reasons, first is that my utility company doesn't give that great of a deal on it - the very best I could do is reduce my electricity charge portion of the bill to zero (would still have the usual fees), I can't actually make money on them!  Considering my monthly bills average in the $70-75 range, we aren't talking much savings.  There are also no installation subsidies, according to findsolar.com the only thing I could get is the federal rebate.

Also, a major motivation here is backup power - while extended outages are still relatively rare, they are becoming more frequent.  Worse, the two extended outages that have affected me were both mid-winter during ice storms (also getting worse in recent years in my area) which is a miserable time to have no power.  If I put in an RE system, I darn sure want to be able to use it in an outage!  I understand it's possible to have batteries on a grid-tie, but it does seem to be the exception rather than the rule.

Is the derating of the solar panels really that bad?!?  I know I've been seriously unimpressed with the "45 watt" panels I got from Harbor Freight, but just figured that was more due to the usual HF quality than anything else.  (Best I get is usually 25W.)  I remember reading that there is some derating due to the difference between "test conditions" and "real", and there's also the loss due to voltage mismatch - for which I intend to use an MPPT controller.

[RandomJoe]

Yes, it does get addictive... ;-)

As I mentioned in another post, the primary reason I even wandered into this particular field was due to power outages.  Last winter I sat in the dark and cold for two days because I didn't have an independed means of powering things.  Never again!

I've already started somewhat small.  I bought one of the Harbor Freight 45W kits, and currently have the panels in a temporary frame sitting on the roof, keeping a 100AH AGM battery charged on the ham bench.  I have to be careful how much I use, of course, as the HF panels will only put back around 9AH per day, but the past couple of weeks I've been running my radios and an RV fluorescent light a few hours in the evenings.

But I've had to seriously modify my operating habits to keep within that meager 9AH, thus the desire to step up the system size.  Also, the 100AH battery won't run much in an outage.

My end-game would be a whole-house system (including A/C if I want to really dream, but that adds an insane amount to the power budget) but I need to make a firm decision I'm going to stay here long-term before I go that far.  There's a part of me that wants to move out of town and get 10-20 acres so I have room to do some bigger things, but I also dislike long commutes so...!  I also won't do loans so it'll take a few years to save up to the whole-house price tag.

[Jeff]

Another good way to "up" your charging power is to invest in either making, or buying a solar panel tracking system. I made one for my 45watt HF panels, and now I get close to 2amps continuously from about 9am-6pm. A HUGE difference from the solid mounting which only gave me 1/2amp at best for most of those 9 hours during the summer!

[frackers]

I made my decision based on the price and availability of inverters.

If you are not going for grid connect or full home system then, like me, you'll probably be looking at a modified-sine inverter (they are WAY cheaper). The number of batteries determines your total system power storage independently of the system voltage but the higher the voltage, the less losses you'll get from cables and connectors. Go to too high a voltage and the inverter price starts going up!!

I traded these off and went for 24volts - trucks use 24 volt systems so are common and a 3kw Chinese inverter can be had for $500. I'm using T105 style deep cycle batteries, 3 strings of 4 eventually, that will give me a total storage capacity of 16kwh but since I'll only use the top 30% of that then I'll be able to run a 1kw load for the best part of 5 hours.

[PaulJ]

   Given the amount of 12V gear you already have, and the fact that you really only need backup power when the grid goes, I'd say stick with the 12V.

   A couple of warnings:

   1)  "12V" can be up to 15V when equalizing flooded batteries- make sure your 12V gear can cope with/is protected from this.

   2)  The fridge complicates things a bit, a full size domestic fridge is probably close to the limit (but not out of the question) for a 12V system.

    My new efficient fridge also uses 110W running, but its startup surge is over 2kW for half a second or so. You'll need an inverter with that sort of surge rating as a minimum to start just about any fridge reliably.

 Also, a sinewave inverter will run a fridge more efficiently than a cheaper MSW inverter.

   Paul.

[la7qz]

Hi

Ham gear is generally designed to operate on 13.8V and many transcievers will start FM-ing at anything below 12V. To protect your ham gear when battery voltage is high (equalizing) AND give it a good operating voltage when the batteries are low, you might want to look at the MFJ-4416 battery booster.

I installed one on my boat to make sure I have communications in an emergency even if my battery voltage is dropping (from running the bilge pumps or other reasons), but I find that it is also an excellent solution for keeping my Kenwood TS-570D at top performance at times when the batteries are not fully charged. After all, the only time they are near 13.8V or above is when the batteries are well charged and have an active charging source (solar, wind or diesel engine).

The only problem with the MFJ-4416 is that at max 25A, the capacity is a little too low when you want to add other gear in addition to your transceiver. The internal 30A fuse blew once when I was testing the transmission at 100W with a low battery. I assume the drain from the battery was more than 30A when the low battery voltage needed to be boosted to 13.8V. In future, I will reduce my power if I need to transmit when the batteries are low. Another point is that I make my living as a marine electrician. One of my specialities is marine SSB installations. Most marine SSB radios are 150W, so the capacity of the MFJ is too low for these installations.

The manual is here:

http://www.mfjenterprises.com/man/pdf/MFJ-4416.pdf

PS.I do not like the idea of the RF sensing circuit in the MFJ and don't use it. It would also be a bit difficult for me to implement since the Kenwood has two antenna connectors and I use both for two different antennas. (a longwire with SG-230 antenna coupler and a 14MHz vertical dipole) The unit is installed between the circuit breakers for the ham gear and the radio, so I simply switch it off at the breaker when I'm not using the radio.

PPS. Usual disclaimer. I do not sell MFJ equipment, I just use it and occasionally install for my customers.

PPPS. You'll often find me on the Maritime Mobile net on 14297.5 at 1900z on Saturdays and Sundays and also occationally on the MMSN at 14300 a bit later in the evening.

Owen

[RandomJoe]

I found out about the fridge this morning!  I guess I'd been lucky with the other motor-driven things I've tried to run off generator or inverter.  Up til now I'd only had one thing (9000 BTU mini-split A/C) that I couldn't get to start on the generator.  Interestingly, the 9000 BTU portable A/C unit does start and run just fine on it.  (The first time I had it on generator, I thought just the blower was running, kept waiting for the compressor to kick in.  Then realized it was blowing cold air!)

But I tried running the fridge with a 300W (600W surge) Samlex pure-sine inverter, and no joy!  The inverter made some noise, I saw a 100W draw, but nothing from the fridge then a few seconds later the inverter tripped.  My Kill-A-Watt isn't quick enough to catch that surge, of course, so had never shown it.  

I knew motor inrush could be pretty substantial (I work with 460V 3-phase stuff for a living) but had always heard 3x running, or maybe as high as 6x running.  2kW, though, for a fridge...  Wow!

I hadn't thought about the equalizing voltage yet.  Up to now I've only used SLAs, so it hasn't been an issue.  I may just turn off / disconnect everything during equalization, or switch over to the PSU.  The MFJ conditioner is another good idea, been thinking about getting one of those anyway for portable use.

[Jeff]

I have a 1200watt/3200peak MSW inverter & it starts my fridge with no problems. I have found a couple decent kits that will reduce the startup problems for those that need it. They might even decrease the load on the batteries enough that it saves some power.

http://www.azpartsmaster.com/Products/Compressor-Hard-Start-Capacitor---SUPCO-RSC10_SPP10.aspx

http://www.filterace.com/detail.aspx?ID=1112

[Ungrounded Lightning Rod]

Rule of thumb:

 - 12V up to 1 kW.

 - 24V up to 2 kW.

 - 48V above 2 kW.

(where the power rating is the greater of the max charging power and the max load power).

This is because:

 - Wiring losses (and approximate price of copper wire thick enough to keep them manageable) go up with the square of the current.  Cabling and miscellaneous electrical devices and fittings are readily available for average currents up 100A, which corresponds to 1.2, 2.4, or 4.8 kW of raw DC power at the various nominal voltages.  But above 100A they get harder to find and more expensive.  Also:  Working with wire suitable for more than 100A can be like wrestling full-grown anacondas.  (I'm talking from personal experience here.  I upgraded my townhouse to 200A service.)

 - But once you go over 50V you get out of the "low voltage" part of the electrical code - and are approaching "touch it and maybe die" risks when working with or around the live equipment.  So you're ahead, both financially and safety-wise, to stop at 48V even if it means using heavier wire and more expensive higher-current parts.

[veewee77]

RandonJoe,

Given the data you supplies, that your HAM stack uses 50AH/day at 12V for about 600WH/Day.

The fridge uses 110 watts and I am guessing about a 50% duty-cycle (if it runs constantly, there is a problem somewhere).

So, running this through the calculator I worte using parameters supplies by several collaborative online and printed resources byt the guys who do this every day, here is what your systems would require as far as solar panels and batteries to run the things you want to run.

The output from my calculator is as follows:

__________________

Renewable Energy System Sizing Report

Produced for

Random Joe

Random Joe, the total Watt-Hours average used per day is 1920.

Based on data acquired on the preceeding pages, this system is sized for a maximum of 1 days with little or no sunsine, cloudy, or overcast.

The batteries are less efficient the colder they are so there is a battery efficiency factor of 1.11 figured into the capacity of the batteries which will change based on the temperature where the batteries will be stored in the wintertime.

The batteries chosen have an Amp-Hour rating of 360 and a Voltage rating of 6 and it takes 2 of the selected battery to add up to the DC System Voltage of 12 Volts.

Solar Panels convert sunlight to electricity and the chosen Solar Panels for this system have a Voltage Rating of 12 and Peak Optimum Amp Rating of 5.00.

It takes 1 of the selected Solar Panels to add up to the DC System Voltage of 12 Volts.

4.9 is the average Sun-Hours per day as entered on the preceeding page.

Given all these factors, the System being calculated based on this information is described as follows:

Solar Panels Required: 13

Batteries Required: 4

____________________

The panels referenced here are 12V and about 60 Watts. As you can see, running that fridge is an expensive endeavor when relying on solar power alone. You inverter would have to be sized appropriately and the losses in conversion are already included in the calculations, as are losses based on temperature, and charging losses.

If you'd like to fiddle with the calculator, it can be found at:

http://dsjscrd.gotdns.com/calc

Enjoy!