Home Emergency Inverter Power Supply

I'm considering building a home emergency supply to mostly keep the gas furnace (1/2 HP motor) going plus some lights on
extension cords, during winter power outages. No solar (this is in Michigan - :-)) but I think the expertise I need is here.
I know how to make an auto transfer from line to inverter power.

What would those experienced in this suggest in the way of a charger (from the power line), batteries,
and sine-wave inverter? What fusing would be needed, and how should the charger be cut out when a power
failure occurs? I haven't measured the furnace power yet but it should be typical for one using a 1/2 HP motor.


  • System2
    System2 Posts: 6,290 admin
    Re: Home Emergency Inverter Power Supply

    Well, lets look at this. Your typical 1/2 HP AC induction motor will draw as much as 9 amps while running. MUCH more while starting, possibly as much as 40 amps, unless it's capacitor start, or capacitor run. Now if you're taking this power out of a 12 volt battery system, the draw will be 10 times that amps, plus extra for inefficiencies in the inverter and wiring etc. AND, if you're using a "modified sine wave" inverter, add another 20% on the motor draw.
    So, you can bet on a minimum of a 100 amp draw from your batteries.
    Think of it this way, how long will the starter motor on your car continue to crank over an engine that won't start, before the battery goes dead? Your furnace would possibly run twice that long on a normal car battery before it went dead and your house stays cold.
    What you're looking at are relatively huge amounts of power for a battery backup system, which will require a huge financial investment and a lot of maintenance. It's my opinion, that you should be looking at a generator for that emergency use.
    Good luck
  • System2
    System2 Posts: 6,290 admin
    Re: Home Emergency Inverter Power Supply

    Thanks, Wayne, for the advice - which is pretty valid, certainly for long term use. However I'm willing to partly "camp out" (We've done it
    for four days after an ice storm with just a fireplace to feed). There are some mitigating circumstances. The furnace motor is capacitor
    start. And I'd want to use deep-cycle batteries (and a "pure" sine-wave inverter). And we'll turn down the thermostat (and wear more sweaters).

    The reasons I am considering this instead of a generator are: for automatic power transfer; infrequent use (which is bad for gasoline
    generators); a generator would have to be outside (buried in a couple of feet of snow...); and I'd have to go out in that to refill the
    dang tank.

    So I'm comparing this or an autostart, autotransfer, natural gas motor-generator......which is also lots of pennies. So if you, or anyone, can suggest
    appropriate components for me to price, I would sure appreciate it. I'll measure the actual furnace current draw soon.

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Home Emergency Inverter Power Supply

    consider not using the electric backup for resistive space heating and it works better. you could get some propane heaters for some heat backup. if you must use the battery power(large battery bank) for heat, it should be something like an electric blanket at its worst so as to keep the power consumption from being rediculously high. i might add that several electric blankets add up to a sizable draw too so the e blankets are only on a larger system and only are just throwing ideas out to you. consumption of power will vary from blanket to blanket so read the specs if it lists them. the following site lists some typical values and lists an electric blanket at 200w.
    this is basically a backup system for your house and something like the outback inverters would fill your requirements nicely as it would charge your battery bank from household utility ac power and use the battery power through the inverter for the home ac power when the power goes out. of course you'll need other things like the wiring, fuses/breakers, switches, etc. and if it's a selfinstall it would probably need inspected by local inspectors or at least you should have an electrician inspect it for geographic areas that haven't any local inspector.
    the battery bank would be large due to your requirement of lasting up to 4 days even for just the lights and you don't want to deplete the batteries below 50% so that doubles the size of the battery bank right there. it is doable though.
  • System2
    System2 Posts: 6,290 admin
    Re: Home Emergency Inverter Power Supply

    I would use this battery backup mostly to run the blower motor for
    a gas furnace, Niel. Usually in this climate when ice storms bring
    down the wires the temperatures don't go much below freezing at
    first, so the heat demand isn't the highest. Hot water is also gas
    heated and requires no electricity. With battery power I would just
    not be able to cook, having an electric stove -- but then we usually
    eat out anyway in these somewhat stark circumstances.

    Can this system be made to self-wrap? I noticed it didn't wrap
    my long lines in my last post.


    P.S. Happy Christmas, All.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
    Re: Home Emergency Inverter Power Supply


    Your posts look fine for here. What browser are you using--sometimes there is a wordwrap option that can be turned on or off.

    Regarding your system, look at this guy--basically a UPS with external battery type function (includes a nice battery charger function too):

    http://store.solar-electric.com/xaswpl25in.html 2,500 watt pure sine, with generator control and other functions...

    It costs $1,850 (110 vac, for 220 vac, you would need two).

    Go to www.xantrex.com for the specs. and installation manuals.

    The next question, is how much energy (power * time) do you need... Your furnace may take 700+ watts to run, but if it runs only 5 minutes per hour, instead of needing 0.7 kWhrs per hour to run, you would only need 1/12 of that or 5/60 * .7 kW = 0.058 kWhr per hour (basically, a 60 watt light bulb running continuously).

    For batteries, they are typically rated in amp*hours, so a 100 amp*hour (at 20 hour rate) automotive sized 12 vdc battery would give (very approximately) 1.2 kWhr of energy over twenty hours, or 1.2/20 = 0.060 kWatts or ~60 watt light bulb for 20 hours. Now, a automotive battery is not a deep cycle storage battery, so you will want to use a real storage battery and discharge it to only about 50% of capacity (to give long life). So, from a "car sized" storage battery, your 60 watt light can run about 10 hours.... Look here for several options for storage batteries:


    So, if you want 10 days of energy backup (no power, no generator), you will need to run a "60 watt load" for

    10days * 24 hours * 0.060 kW * 1/50% discharge * 1/80% (inverter/system inefficiencies) = 36 kWatt*hours of battery storage.

    The numbers above are very rough--but will get you pretty close to the basics of how much equipment (and cash) you will need). Remember, if your furnace runs for 30 minutes per hour, then you will need 6x the amount of battery storage for 10 days of emergency usage. Conservation of electricity is the name of the game here.

    My recommendation, you could run a honda eu2000i (1,600 watt quite generator) at 250 watts (0.25 kWh) for ~15 hours on 1.1 gallons of gasoline or 17.6 gallons of gas for 10 days and power your fridge and microwave too (honda is $800-900 bucks shipped to your front door). Siphon the gas from your gas tank and keep a couple five gallon cans with gas preservative (change once or twice a year).

    Or, if heating the house is #1 priority, install a couple of direct vent natural gas heaters (that don't need AC power) in a couple rooms of your home for emergency use (kitchen, bedroom)... Will keep you safe and warm, without any noise or bother.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • SolarJohn
    SolarJohn Solar Expert Posts: 202 ✭✭
    Re: Home Emergency Inverter Power Supply


    you might consider doing what I've done. I have a very small generator, and a corn-burning stove. During a recent power failure, the stove kept my home warm. The generator provided the needed 100-watts to run the stove, and power for lights and a chest freezer. I only used about 2 gallons of gas per day, and about $6.00 worth of corn. I know of others who used more than $25.00 worth of gas for bigger generators, or even more for propane per day to keep their homes warm. My system not only did the job, it did so economically. You can find the details here: http://solarjohn.blogspot.com

    I'm in the process of upgrading my solar photovoltaic system so that I won't need the generator at all. If you don't think that solar power will work for you in Michigan, I suggest that you get a small generator (for fuel economy), and a good true sine wave inverter (not one of the cheap modified sine wave inverters). I understand why you don't want to use a generator, having just been through an experience myself. However, you have to get the power from somewhere. You might consider using the small generator to charge batteries during the day, saving you the trouble of refueling the generator at night.

    Good Luck!
  • BB.
    BB. Super Moderators, Administrators Posts: 33,477 admin
    Re: Home Emergency Inverter Power Supply

    Also, for the small Honda, Yamaha's and other generators, places like Wise Sales sells for $110 a 5 gallon gas can plus tubing (seems terribly over-priced--but their prices for the Honda generators shipped to your door are usually among the best) that siphons to the gas caps on the small generators... A 5 gallon can (plus 1.1 in the tank) of a eu2000i generating 250 watts (1/4 load) will run for around 80 hours non-stop (over three days).


    You may want to check the oil more often--but it is an option.

    Also, you will probably want a manual transfer switch so that you can place some of your home's circuits directly on the generator (and/or inverter) so that you don't have to hookup extension cords everywhere:

    http://www.nextag.com/home-generator-transfer-switch/search-html (don't know anything about nextag.com, just posting for reference)

    I did a six circuit one for my home (fridge, freezer, sump pumps, garage door, bedroom lights and outlets for radio/tv).

    The automatic transfer switches are nice (with a automatic generator)--but will cost you an arm and an leg: Around $5,000 for a Cummins/Onan 9-10 kW standby generator (3,600 rpm, not for off-grid living) and 200 amp auto-transfer switch. It is a nice system--but I really cannot justify it for 2 hour outages once every few years (been 50+ years since the last 5 day outage in the area where I was born and raised on the coast near SF).

    If you look at the fuel consumption of a 9 kW generator on natural gas, last I checked it was somewhere around $1-$2 per hour to run (even at low loads that most people would have if not running AC and/or electric heat/hot water). If you have an emergency, I am sure that it is worth almost anything to have the power--but, again, a large genset is not cheap to run and you need lots of fuel to keep it running (nat-gas is not a problem--unless you are in earthquake country--like me).

    One friend did install a small RV type genset in his crawl space for a nice installation--smaller system matched his power needs much better. Don't know if this was legal/to code however.

    And if you have to bug-out, your are not going to be able to take a large genset with you (the Honda eu2000i is less than 50lbs).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Brock
    Brock Solar Expert Posts: 639 ✭✭✭✭
    Re: Home Emergency Inverter Power Supply

    Ranecurl you are starting where we did. We are over in Green Bay, either across the lake or below you if your in the UP ;)

    Anyway I started with just my furnace and fridge on an inverter that had no solar, just a 1500w inverter and four 120 amp cheap deep cycle batteries. It worked out pretty well for it's intended use, my system has slowly grown since then.

    But it is very doable. What I would recommend is a 1500w+ sized sine wave inverter, I found out the hard way some furnace controllers don't like mod sine wave. Then wire the output of the inverter to the furnace and put the input of the inverter on a regular Edison plug so you can plug it in to a genset to charge the batteries if you ever need to.

    A friend of mine who system we just setup has a prosine 1800w inverter and only two 105 amp deep cycle marine batteries. He ran a 10-3 line out t the front of his garage and put the batteries and inverter there. We left an old set of jumper cables right there. He can run about 8 hours on batteries for his furnace and fridge, and then idle a car to keep the batteries charged off and run loads as long as he can idle the car.
    3kw solar PV, 4 LiFePO4 100a, xw 6048, Honda eu2000i, iota DLS-54-13, Tesla 3, Leaf, Volt, 4 ton horizontal geothermal, grid tied - Green Bay, WI
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Home Emergency Inverter Power Supply
    ranecurl wrote:
    Hot water is also gas
    heated and requires no electricity.

    You could use a small circulation pump on your hot water, to divert it thru a radiator for some heat. I assume you don't want the cold water pipes to freeze up? Force air heating an entire house, is a big blower power load.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,