Modfied Sine wave more viable as complete solution?

[joe4324]

Piggy-backing on my AC vs DC thread,   I've decided to push for a pure AC system with a much smaller 12v DC backup system for which I already have the batteries for.

So for the AC main house-system, I need to figure out the inverter(s).  Pure sine typically seems to be 3-5X more expensive than mod sine.   It has obvious advantages I'm well aware of.  But that price.........

Generally speaking, the entire home-stead was nearly entirely built on a 1750 xantrex mod-sine inverter and generator.  The 1750 inverter ran every tool but the compound miter and table saw.  Plus lighting/water pumping/small air compressor etc etc.  

A off-grid system I used to live on, had pure sine in the office with the electronics and mod-sine for the rest of the house.   This seemed to be a good mix of matching cost vs needs.

My gut reaction is that going the 'brute force' pure sine method most commonly advocated is over-kill and somewhat not in the spirit of what we are trying to do.  Not to mention mega expensive.  

A outback 3.6kw pure sine inverter is typically around $2,000.  It seems to be one of if not the most popular units for grid-tie/off grid homes.  For that same price  I could have  say 600watts of pure sine and 8,000+ watts of modified sine.  Most of the devices I seem to use don't mind the mod-sine.  Maybe over a long time they would develop problems?

I mentioned in the other post that our backup cooking (that will be used fairly often I suspect) will be induction cook-tops.  These draw some serious juice,  up to 1300watts at full load,  maybe with 2 going at once sometimes.  I doubt I would be very often using them to maximum draw but I need to design for that.   I've called the companies and they tell me the induction cooktops will run just fine on mod-sine.

Overall I know the homestead will be extremely power-efficient, but I need to design for the worst case draws.

What are you opinions of mod vs pure sign giving what you know about my setup?   Money is as always the most limited resource and I want to spend what we have were it most needs to go.

[Volvo farmer]

I disagree that a 3.6KW sine wave inverter is throwing "brute force" at a problem. The fact is that motors run hotter on MSW. Sure, maybe no problem for a circular saw which has a 5% duty cycle. Try running a refrigerator or water pump on one though, and my instinct tells me that you might shorten the life of of it because of the ugly waveform and extra heat.

One thing you might not have considered is that every inverter has an idle current that it uses, even with zero loads. The more inverters you use, the more inefficient things become, especially on the low end of the spectrum.

Having lived with a 3.6KW pure sinewave inverter for a few years, my feeling is that it is worth twice what I paid for it. I can plug in anything I want, from toasters, to chest freezers, to LCD televisions, to mig welders, and it just works.

Everyone has different goals and objectives with their off-grid system. A pure sinewave inverter is like a Martin guitar and a msw inverter is like a Sears guitar. In the end they both make music, but if you are using it every day for a number of years, I think you would come to appreciate the finer qualities of the better inverter over time.

The bitterness of poor quality remains long after low pricing is forgotten!

-Leon M. Cautillo

[westbranch]

Hi, new here, but have been on another forum for a good few years, I vote for a hybrid system, mod and TSW.  Just have to identify which circuits are TSW and MSW.

EJ

[joe4324]

I guess its all a bit relative,  your 3.6kw outback is probably all things the considered the best residential inverter on the market.  Or at least the most popular from my experience,  I'm using one of them to power my netbook right now.  I won't for one minute say its over-kill because I wish I could put one in the homestead (and we just might!)  But I've also lived on much less, and sometimes almost nothing at all.  When I was living in Hawai'i I used one of those crappy die-hard 400 mod sine inverters to power my little cabin,  I charged a 60lb deep cycle battery by carrying it up a hill and putting it into my truck when I'd go to town.   Crude... but it worked for better part of a year and cost almost nothing.  Sure its Hawai'i and almost everything is easier there.

But,  its $2,000... for a single piece of hardware.  Maybe that's just the cost of doing it right for most people but its a huge expense I really have to scrutinize.  The solar system where I am living now is massive, 20kwh worth of GNB batteries,  two of those 3.6kw outbacks, two MX60's on 3kw of panels in the heart of the sun-belt.  I think the total installed cost was upwards of $50,000!!!   Its because my uncle wanted to live a completely 'normal' grid tied life.  Its impressive but I'd call this pure brute force.

I think our total budget is going to be $5,000 give or take,  We do have the convenience of not needing to invest in solar or wind RE upfront because of the bio-fuel generator, so this is essentially just for batteries/charger/inverter and interconnects.  I gotta stretch this as far as I possibly can, and if $2700 needs to go into charge controller and inverter than thats just the way it will be, but then the batteries..........

attrition......

[Warren]

Hello,

We used a modified sine inverter for about ten years in our off grid house. The inverter ran a 19 cf fridge, 15 cf freezer, clothes washer, lights and electronics. We really had no motor problems during that time. The inverter did cause a noise problem with our radio. The surge protectors for our computer had to be replaced yearly and the ballasts in our light ran fairly warm. Some electronic equipment did not work with the modified sine inverter such as the more expensive bread machines, a UPS and an early Apple computer.

About five years ago we purchased a pure sine inverter. There was a noticeable difference. The fridge did seem to start easier,  running electronics was not a problem and the inverter did not create noise on the radio. In the long run the pure sine inverter allows you more options.

Hope this helps

[wpowokal]

What you are really asking is others to justify a cheep system for you, to still plan an induction stove shows you have not really grasped the situation. You tell us on pure sine wave "It has obvious advantages I'm well aware of.  But that price.." I should rest my case there.

I may have missed it but how many amp hour of batteries and what re source are you planning/ or have.

You probably have done a load analysis, you know time of day use V incoming power, expected load V battery storage, battery charge efficiency.

For my money pure sine wave is the only way to go, accepting that I have a bigger budget than yourself, but you will eventually buy one, efficiency is also a factor you may wish to consider, not only the inverter efficiency but the battery charge efficiency.

I don't mean to be negative but I can only go by what you have posted.

allan

[joe4324]

I'm not asking anyone to justify anything.  In less than four months this 'paper' system I am barely even beginning to outline will be installed and working whatever it turns out to be.  Unless of course the price is too high and then nothing gets installed and we don't use the place and keep using our rentals in town which would suck.  

I don't care how much better anything is if its unaffordable at this time we either have to wait or simply do without.  And two thousand dollars for (1) component of many is expensive in my world.  So price is a HUGE disadvantage of pure-sine.  Big enough that I'm raising the question and getting surprising responses as if cost is the least important factor.  Our budget will never be big enough and because of that something will suffer.  Judging by the few responses I have so far if I posted a similar thread in the vein of cheaper/fewer batteries vs cost and performance I'd get chewed out for not planning on putting in a 40kwh bank.  There's gotta be a balance somewhere.  I thought this was 'otherpower'  we build windmills from car parts and turbines from cage fans.  Do we just throw money at it now?  

A major motivation for building this homestead is to prove you can do an awful lot of stuff without a lot of money if you just think about what your doing and use a little elbow grease.  I get sick of all this it only costs $70,000 to go 'green' crap.  I drive a car thats 30 years old, cost me a thousand dollars,  another thousand dollars of thoughtful modifications and its given me 50,000 miles of almost 'free' drive on waste oil.  Thats what two grand means to me, A car that goes anywhere roads exist for a little time, rubber and brake pads.  

If you read my original post again and preferably my other post about AC vs DC, you'll see a lot of information about my project and also evolution in my thoughts.  I really enjoy challenging what 'feels' right and I was almost dead-set on running a largely DC house/homestead.  Instead after 35 posts I've almost completely changed my mind.  I still think pure DC systems are wonderful, but maybe for my own 'personal' cabin,  not so much the common space anymore.

Also,  the manufacturer of the induction cooktops we are going to go with says they will work fine and still be warrantied under MOD sine.   The cooktops are going to be the single biggest draws. So why not really consider mod-sine?   If it turned out it was 'good' enough for us, we could 'cheep' out and put $1500 more into batteries.

Don't get me wrong, I want it all.  Huge batteries,  unlimited pure sine passively cooled, hell I want a push-button generator start inside the kitchen.   But what I get is a different story which is why its so good to post here and get as much input as I can.  I did feel like your post was negative but I appreciate it just the same.  

I think we need at LEAST 250Ah @48 worth of storage,  We will be keeping weird hours forever so there is no sweet spot of use/charging for us.  Heavy draws like dishwashing/clothes washing will of course planned but generally speaking computers/lights/cooking could happen at anytime.

Our primary source of power generation is a bio-fuel generator for now.  Until we can save up for RE.  The inverter question is raised here because I am working on envisioning the backbone of our whole RE system sans pv or wind.

I have my heart set on a outback FX80 charge controller, I think its big enough to handle our inputs for as far as I can see,  and is very evening matched in AC input power as our generators output (about 3.0kwh) and will be a great platform for a 48v pv array.

I'd type more but its getting to late!

[wpowokal]

Joe please don't take the responses the wrong way most including myself are answering your post from personal experience.

The biggest most outstanding point of your posts is the  induction cook tops, yes I'm sure they will work on MSW but...... when living off renewable energy we do not use such devices, propane fired cook tops are the go or perhaps a wood stove.  

A large battery bank is not necessary the smart answer there are charging losses to consider, perhaps a little reading here will help.

 http://photovoltaics.sandia.gov/docs/BattIntro.htm

The bio fuel generator will allow you to input power at peak times, usually evenings, this will reduce the size battery bank you require.

Choosing an AC system is basically smart, I have taken that road, and yes Modified sine wave will work for most applications as you already have stated you fully understand, I and others are sharing our experience which is what I understand you asked for.

This will be my last post on this as usual I regret wading in but am too stupid to not so best of luck.

allan

[jacobs]

We have a fairly large system of 2400 watts of PV's, 51 kw batteries and a 15 year old 3kw modified sine wave inverter that cost $3000 new.  We've been using modified sine wave inverters for the last 28 years and have never had any problems with starting or running any motors including our submersible well pump, refrigerator, and freezer.  Yes, there are a few electronics that don't like msw but I've found them to be few and far in between.  Maybe the newer cheep msw inverters aren't as good as the old ones were....Don't know.  I'm very pleased with what we have and see no reason to upgrade.  Msw inverters are less complex which means less to go wrong.  KISS for me.

[BigBreaker]

The induction top has its own switching transformer (25khz is a typical frequency) so I doubt it cares how it is driven.  You may even want to see if it can run on DC since it will be on a dedicated circuit anyhow.  MSW shouldn't be a problem though and doesn't need special arc quenching switches.

I'd run gas lines while you are installing the induction tops.  You may find a propane, wood gas or other cooking gas to be more useful in the future.  To the induction haters on the board...  it's not that much worse than a microwave for efficiency, IE 90%.  Gas is about 40% for reference.

[SteveCH]

Our home has been PV off-grid since '85. We use a Trace 2512 [old now, means 2500 w. 12 v.] modified sine wave for everything that isn't 12 v. It is on all the time. Search draw is .33 watt. They claim efficiency at 85-96%, though I have no idea how accurate that is.

We have five computers, all Apple, big lcd TV, stereo stuff, and so on [Kitchen Aid mixer, etc.], Grunfos well pump, vacuum cleaner. The usual stuff. Lighting and fans and greenhouse swamp cooler are all 12 v. as are the fridge and freezer. The only thing I run a generator for is the table saw and an air compressor.

I have mulled over getting a sinewave to replace the Trace. They are costly. This Trace cost us about $1000, though that was around 1990 I think. I cannot justify the expenditure however. We have had no issues I can tell from the mod. sine thing. If there are any, we can't find them in our home.

I can't imagine an induction stove, so.....

So, at least in our case, the mod. sine inverter is just fine. We have lost no electronics, no problems.

[Airstream]

SteveCH has a valid comment - though please note the TRACE 2512 was & is rock solid equipment (I have two) AND the $1000~ paid in 1990 has the buying power of $1750 in our 2010 Dollars so to purchase a $2200 outback is an exception deal.

Don't spend the money twice, go with Trace/Xantrex or Outback the first time.

[joe4324]

Sorry if I was a little short, I do appreciate the responses!

About the induction cooktops,  I already have them and will be using them.  Even if they are harder to run than I think they will be we will figure it out.  We will use wood/solar and now a bio-diesel pump stove I'm working on.  The Induction cooktops will be a back up to these, but still used often I'm sure.

Propane is not an option for us.  We have the mis-fortune of being too idealistic and for other reasons mostly political we won't use fossil fuels for day to day inputs.  Our cooking fuel has to be renewable too. Some of the original earthships in new mexico used resistance electric cooking a primary means to be self sufficient.  This was when 200 watts of PV cost a fortune!   Most of them now use propane I am sure but wow.

[joe4324]

Exactly,  and that is kind of the flip side of this whole discussion.   I'm not at all against rallying the money for the outback inverter.  I'm just raising the question of "is it really worth it" in my case.  And I still don't know.   Running a separate pure sine circut wouldn't be 'that' much more costly, but should be weight against putting that money up towards the big inverter instead also.

[joe4324]

The wiring costs would be about the same,  the only real cost would be extra box fuses, conduit and disconnects.  Maybe as low as a couple hundred bucks.  (minus inverters) I do have to weight that against just getting the bigger pure sine though.

[Bruce S]

Joe;

 I want to weigh in hear about the induction cook tops.

First I have to say my wife happens to be a professional chef, so staying up with the latest is a must for her.

We have a very highly rated NG stove, not a Viking of anything of that level, but at the moment the most expensive item in our kitchen :-(, and that includes the under-counter fridge.

We also have the smaller induction cook top (2), got them at 1/3rd normal going price.

Nice units, does exactly what they're supposed to.

I've run this 1 little puppy off a good sized MSW attached to a couple old 1000CCA batteries during a chili cook-off; there were a lot of on-lookers and questions.

Since you are looking for ways to keep costs down, I would back-off getting propane or fuel powered units.

Ours even though small show a much better efficiency that even the natural gas stove:( that I paid dearly for.

These are real world uses not labeled or sales pitches.

NOW: having said that, NG stoves and propane stoves can be re-purposed for methane, if you go the route of building a storage system for it.

Hope this little post helps with the decision to keep those cook tops:)

Once the price comes down, we'll probably sell the NG stove, put in induction cook-top and new double ovens .. but when time IS money these things are a must.

Keep the posts coming they are very interesting.

Cheers;

Bruce S

BTW: WE took 1st place with Salsa:)

[Airstream]

Rock solid used "stepped pseudo-sine" SW4024, 4048 or 5548's are out there, much better return on your investment than 90% of the China inverters getting badged with recognizable name brands these days.

If you are counting on cheap mass market inverters to get a five or seven MSW kilowatt cheap powerhouse source I'm fearful you will be disappointed - UL/CSA/CE designations aside think longevity, just one weak or low Quality Assurance binned component and you're out of luck and behind on the dollar game.

Either of the $1300 outback inverters in the classified right now would be a heavy duty power source and capable of stacking, etc.. and give you only ONE inverter to suffer line/idle losses from...

[scottsAI]

Joe4324,

OK, you wanted another way. Here is cheap!!!!

Consider using a used UPS cost $50! 1kw. Hard to buy a good MSW for that!

APC SmartUPS 1kw and larger are sine wave. Efficiency is 88%+ loaded. Please check from memory. Lightly loaded bad, 1kw uses 24w doing nothing, 3kw 47w.

I have used (1kw, 1.5kw has 24v battery), (2.2kw, 3kw, (have 5kw not used) has 48v battery). SETI computer farm.

All above support battery start and turn off beeping. In the manual.

For a long time I though the UPS did not support being overloaded. Discovered it was the battery not UPS, For a short time (longer than motor start) put 2kw load on a 1kw SmartUPS. Needs a good battery. They just keep working. Not a failure yet.

UPS can charge the battery, may take a while!

Control: on only when needed like for a Fridge, efficiency is just fine.

Due to the low cost, place an UPS on each motor or load needing sine.

My 3kw cost $125 including shipping, have 4.

Other things use MSW.  Quiescent current is a concern. My 70w to 150w use 0.11a no load. The 1kw Trip-lite uses 0.89a no load. I leave on the 150w for lighting at night. Units with a FAN use more Quiescent current. Modified a 350w with bigger heat sink. Broke something. Removing the fan cut half the quiescent current. Noticed several transformers in the larger units, each was around 250w. If only they switched them in/out as needed could reduce quiescent current down to 0.1a!

Battery: Start with Golf cart. Here $70 each, 6v @ 225ahr. My 8 supply 10.4kwhr for $470 (cheaper then) Golf cart are designed to be abused for 3-5 yr. With good care twice that? Other batteries just do not give the same value. So what if you have to replace in 10 years? Buy another set. Less investment in case you screw them up.

Saving so much on the battery, reward yourself with a watering system, catch them on sale. Now have a near equivalent to SLA. (Last I looked was about 30% the battery cost). Good watering and charging are the key to long battery life. Connect to each battery cell an independent vent tube, each goes outside. Now do not need to clean battery nor use a fan while charging!

Is this what you were looking for?

Have fun,

Scott.

[bob g]

my experience with APC ups inverters is the same, nearly indestructable

very good sine wave, and will surge to nearly twice rating for 3 seconds if the

battery bank can deliver the amps.

if overloaded they just click off on safety requiring a reset, easy enough

very inexpensive pure sine wave in my opinion that can be put to use with a bit

of thought.

also

check local telecom support companies, and follow ebay or craiglist for

exeltech mx series inverters, most now are 48volts and are hands down the best

inverters on the market, bar none

they have a 20 year plus mean time to failure, and the new ones are expecting twice that. exeltech is really good to work with and will repair any inverter that has an

out of warrantee failure for a flat fee of something like 100 dollars

they are modular and expandable as well, for 120/240 and threephase up to crazy kwatt capacities.

i bought several kwatts worth last year for an average of 125 dollars per kwatt capacity.

they are not the highest efficiency on the market, at only 88-89%, but they have the cleanest sine wave on the market at an advertised limit of no more than 2% with typ harmonic distortion under 1%

nothing wrong with outback, lots of folks have them, and are happy with them, however

they are not all that and a bag of chips!

there are other alternatives that can be used if you look around a bit.

then there is this

you can use a large modified sine wave inverter to start a heavy load and once started switch over to a puresine inverter of smaller capacity.

this is not difficult either in a great number if cases

bob g

[joe4324]

Scott!!!!

Now we are talking!   I'm sure I could fumble through this from here but I at least want to test this out.  You are causing my mind to 'buzz' about these being good in for small cabins/buildings needing a little bit of power for CHEEEEEEP.

Can you walk me through how exactly you would use the UPS inverters?  

[joe4324]

So I just found this:

http://cgi.ebay.com/APC-Smart-UPS-3000-SU3000RMNET-3000RM-5U-30A-Back-Up_W0QQitemZ390139459032QQcmdZ
ViewItemQQptZPCA_UPS?hash=item5ad61fb5d8

Are you saying that for less than $300 shipped I can get a used, but high quality 2.25kw pure sine inverter?  Do i tear it out of the 5U case?   Do I wire-it up as if the main battery bank was the UPS battery (if same voltage)  so the UPS functions as if it has a HUGE battery and I can feed the house with this?

Or do I get the much smaller ones, and plug them into the house outlets that are all MSW.  And I use these small ups-inverters to power only the loads that don't like the MSW?

I did not know these were pure sine...  is it just because they make so many of them?

[joe4324]

I miss having chili cook-offs,  I used to work in a office and we had a few every year.   I lost to a chili that had chocolate in it.  Chocolate?!!!

Thanks for the thumbs up to the induction,  it seems like amazingly appropriate tech for what we are trying to do.  To hear someone say I should get a few of these (which we plan to do) and 'save' money doing it over propane is refreshing.  4 remote cooktops would cost less than 1/2 of a cheap propane stove.  Not to mention gas-lines etc etc, I'll put that money into solar-panels...

[ghurd]

Bruce,

That veggie stuff you make for the MB diesel...

It has properties similar to diesel? And diesel has properties very similar to kerosene?

Basically, will the stuff go up a king sized wick?  Will it work in a Dietz lantern?

If yes, then Joe, you could have a look at these;

http://www.lehmans.com/store/catalog?Args=

Click- stove, cook stove, gas and kero, Perfection kero.

Mrs. Bruce needs one of those.  

G-

[bob golding]

you cant put flowerpots on an induction hob to take the chill off in the morning though.-;)) how do you work out that gas is 40% efficient? is that the amount of heat going into the heating of the pot? is the remaining 60% "waste" heat?

bob

[BigBreaker]

Apparently induction is really popular in Asia where they have a real efficiency kick and perhaps less access to NG service.  I saw the efficiency statistic for induction vs gas on wikipedia but it was referring a paper of some sort.

Open flame burners are not particularly efficient so 40% seemed reasonable.  Of course if you need the wasted heat anyhow, that might as well be 100% efficient.  I guess my real comment is that induction stove tops are in the same league as microwaves for efficiency and convenience and not necessarily wasteful.  Like a microwave, though you are going to want an alternative - likely a gas or wood fueled alternative.

If I were building a homestead, I would want a 1000 gallon propane tank for cooking and backup heating/electricity.  You just can't count on having the khrs for induction tops all the time.  In the winter when solar output low, and the "waste" heat of a combustion based cooking source is valuable, you are not going to want an induction top.

[Bruce S]

G-

on your questions of the Bio-D, yes!! BUT have to say I'm not sure how Joe is using his.

IF straight WVO; that won't wick until the cows start to sing Kumbiya.

MY current mix uses 7grams red-devil lye, 100mL Menthanol per 1L WVO, let sit over 2 hours pour off slowly strain again through old T-shirt.

It wicks nicely, but does smoke til the little, I keep the wick trimmed and short.

For the old white gas you-pump-it Coleman stoves, those mainly run off the vapors and will smoke a bit until the jets and surrounding area get hot then no smoke!! and lots of heat:).

NO!! Mrs. Bruce does not need another "item" still paying on first <.

[Bruce S]

JOE;

Chili; YES chocolate !! but bitter chocolate!! and go one better instant expresso coffee as well.

BUT I digress...

G- does have a very valid point on using the WVO you have on hand. Before going too much further, I'd kinda need to know where you stand and how you're using the WVO.

I answered G- BUT I use Bio-D.

SOoo if you're using straight WVO then may not be a happening thing, but can be a nice shift.

Pump up Coleman stoves can be found fairly cheaply at resale shop and most are in good shape. They can do an wonderful job for cooking w/alternative fuels.

I do this so I keep my Bio-D costs to a minimum and out of the house.

YMMV.

YES! keep the cook tops and spend the rest towards PVs and such.

I tend to run my inverters at about 80 max continuous load, but I do not normally run then 24/7 so...

Bruce S

[scottsAI]

Yes!

As with anything, be sure to check what your getting meets needs.

Download the manual, read carefully before you buy.

Not all APC UPS are SINE. Check the model.

I know the Smart UPS line over 1kw is SINE. Pro-line is NOT. (MSW)

How can it be so cheap? New they were anything but cheap!

Newer models more features, people getting rid of the old, room for the new.

Efficiency is a bit lower than a good sine inverter.

Standby they use more current than a good sine inverter, like the Outback.

My first search on eBay shows Smart UPS 3000 delivered for $208.45!

http://cgi.ebay.com/APC-3000-Smart-UPS-3U-Rack-Black-Cosmetic-No-Bats_W0QQitemZ290390993204QQcmdZVie
wItemQQptZLH_DefaultDomain_0?hash=item439ca6e934

Additional searching expect can do it for $150 or less. Above shipping is high.

Bought several 3k units with shipping $60 to $70. Total cost drives bidding.

Your questions:

Are you saying that for less than $300 shipped I can get a used, but high quality 2.25kw pure sine inverter?  Yes. Less than 4% THD.

Do i tear it out of the 5U case?   NO.

Do I wire-it up as if the main battery bank was the UPS battery (if same voltage)  so the UPS functions as if it has a HUGE battery and I can feed the house with this? Yes. Or use couple smaller for interment loads.

Front panel on UPS, starts up without AC. (don't connect UPS power cord to anything).

Plug in your load. Turn on. After first beep, you can turn beeping off.

Many remove the beeper inside UPS.

Do not do the MSW thing, nor plug in UPS except to grid power.

Have fun,

Scott.

[bob g]

i have a stack of apc model 700's that are also 24volt dc 120vac output and are

puresine as well, they can be had very cheaply.

beware some of the newer apc units will not restart if you cut out the beeper, by removal from the pc board,

my fix is to just inject the beeper with a squirt of rtv! just fill it up and it will be still on but barely audible.

otherwise i have not figured out how to turn the darn things off.

bob g

[ghurd]

Smash the beeper, without damaging the board or beeper leads,

and replace the beeper with a 1.2K or 1.8K resistor?

Know any common (meaning cheap on ebay) UPS models in the 1.5~2kw range that use 24V?

The assortment is mind boggling.

G-

[Ungrounded Lightning Rod]

Induction heaters may be 90% efficient electricity-to-heat, but resistance heaters are 100%.  But if you're comparing to burning fuel you also have to multiply by the carnot cycle efficiency of the generation system and the transmission efficiency from there to the stove.  Going from fuel to heating a pot could be 'way lower than 40% by the time that's factored in - especially if you have a local fuel-driven generation system that doesn't have a power plant's massive optimizations.

The big gain in induction heaters (versus resistive) is that less is lost transferring the heat from the electric element to the pot and gas-stove-like lack of thermal inertia delays between changing the control setting and changing the heat input to the pot.

[Bruce S]

Very Very true.

One of the bigger deciding factors for us was the fact that you take the pot off the top the heat is gone.

No coil branding on the hands no extra heat going up and pealing paint off the ceiling.

At the chili cook-off I had more than one idiot put their hand right on the burner area right after us taking the steaming hot Habanero/Onion pot from it.

One item I have not looked into, due in part to lending out Kill-A-Watt was what if any standby power is being used.

Thanks for pointing out the Carnot factor!! I'll need to do some cipherin this weekend to see where they play in.

Sense ours was powered via solar charged 1000CCA heavy batts going to a MSW inverter. I'll probably come out the looser on this but safety did win out.

Thermal delay on these are no where near the same as the resistance types, and sizzle sound of Onions about to burn when adjusting the levels tells me they're as near NG/propane gas tops as I can hear.

Cheers;

Bruce S

[Ungrounded Lightning Rod]

Induction may actually be a tad FASTER than gas.

 - Gas puts the heat on the outside surface of the pot, from which it has to conduct inward.  Thermal resistance causes a temperature gradient so there's a bunch of extra heat stored in the outer layer that still heats for a moment until the temperature averages out.

 - Induction through the thickness of the metal, not just the outer surface - and perhaps a small bit into the food itself if it conducts well.  (It's still more concentrated nearer the outer surface because the eddy currents resist mag field penetration.  But for a thin pot it may be close enough to even-throughout as not to matter.  And even for a thick pan it's always more penetrating than just surface heating.)