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How do I figure out how much solar energy for emergency power?

whitenack | Posted in General Questions on

Hi all,

We have a new house that is all electric. For efficiency, we did not include any fireplaces. This leaves us vulnerable in the event of a power outage during a bad winter. We live in Central KY, so the winters here are usually not too bad, but we do have times where the temps get below freezing for extended periods of time.

We built the home with lots of insulation, so we are able to heat in comfortably with mini splits. We also have a HPWH, so our hot water demands are hopefully low.

I would like to start researching a small solar array that would allow me to keep the essentials going. Things like heat, hot water, the fridge, stove, and maybe a few outlets around the house for a few lights, charging mobile devices, etc.

How do I go about figuring out how many KWh I need? Most websites say to look at your electric bill to see your usage,but that just shows total usage, not how much could be cut out (temporarily) in the event of an emergency.

My utility company gives me daily usage stats, so I guess I could check and see how much power I use when we are out of town for a few days. That would not tell me how much the stove is using, but would give me the fridge and heat numbers.

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  1. GBA Editor
    Martin Holladay | | #1

    Solar panels, batteries, a charge controller, and an inverter, sized for backup at a house that uses fossil fuels for space heating and domestic hot water, could cost between $10,000 and $20,000. If the house uses electricity to provide space heating and hot water, it would be hard to even imagine the size of a battery big enough to get the house through a few cloudy days. Such a system would cost a fortune and would be a maintenance headache.

    It's far cheaper to spend $1,200 to $2,000 on a gasoline-powered generator.

  2. NormanWB | | #2

    If you are connected to the grid, you may not be allowed to use the panels during a power outage, as it creates a safety issue for the people working to restore power. If you have access to natural gas, a stand-by generator can provide enough to run your house for $6,000 or s installed. Much cheaper than PV, if you do not intend to use PV to offset your power bill..

  3. Expert Member
    Dana Dorsett | | #3

    Norman: There are inverters for both standalone PV and PV + battery solutions that do the auto-switching to island the system from the grid when the grid goes down, avoiding the safety hazard.

    A high-R house in central KY won't have plumbing freeze- up issues even during a Polar Vortex event.

    A small woodstove may be useful for grid-down comfort, without punching huge holes in your efficient building envelope.

    Last I heard KY was still in the first-world. Power outages that go beyond a day or two are extremely rare. The capital budget for equipment to cover trailing-edge 0.001% risks should be sized accordingly.

  4. whitenack | | #4

    Thanks for the replies

    Haha, yes, Dana. Although barely, KY is still considered first-world (I shouldn't joke, there are places in the world much worse off than we are). However, I am not so much worried about the typical storm related power outages that may put you down for a few hours or even a day or two. I was more worried about some natural disaster that wipes out the local power plant, and it will be weeks before power is restored. It's not a huge fear of mine...I'm not obsessed with zombie-apocalypse scenarios, but I have a young family and this is the first house I've lived in that hasn't had some option for backup heat.

    Martin, I hadn't thought about a backup generator. Duh! Yes, that would be an easier and cheaper installation. What about the long-term returns of a solar system vs. a generator, though? I plan on living at this location for another 40-50 years, God willing. A PV system would allow me to offset my energy bills, which a generator would not.

    I guess I've got some more studying to do.

  5. AlanB4 | | #5

    Heat is the biggest consumer, if all you need besides heat is fridge and a few LED lights then panels to grid tie (switchable) and a battery will probably do nicely. Hopefully a Tesla Powerwall will drop in price in the next few years, buy one, integrate it properly and your golden.
    In fact if its freezing you can do without your freezer, just move everything to your trunk. Use up your fridge items quickly. A few 10W led bulbs will need very little juice. Plan some canned food storage for meals, get a BBQ and a propane tank for outdoor use (they do still work in winter).

  6. Expert Member
    Dana Dorsett | | #6

    Unless you're on a grid-isolated island I've never heard of in KY, power plants aren't the weak link in the grid-power chain. The local powerplant is backed up by all other powerplants on the grid. Massive grid failure (Puerto Rico style) is the only scenario that keeps you out of power for weeks. Both the distribution grid and transmission grids in central KY have quite a bit of redundancy (thank you TVA, PJM!)

    Grid planners in KY are also further along than most states with smart-grid planning & development, making the grid more reliable in general, but also "self healing" for re-routing power when major links go down. It's definitely a first-world power grid environment, nowhere near as vulnerable as NJ/NY was during Superstorm Sandy.

    The investment return on investment of a gas generator is net negative, but a grid-attached PV may have a real return, and PV + battery could too. At the moment KY is great place for commercial electricity ratepayers to have a battery for managing demand charges, but since demand charges aren't (yet) a standard feature of residential rate structures there's really not much incentive to go there, other than your emergency backup scenario (but a fossil burner generator is an order of magnitude cheaper.) If going solar it might be worth investigating the upcharge for a "battery-ready" inverter, in case demand charges become the new paradigm for PV owner electricity rates (as was very recently approved by regulators in the parts of MA served by the large utility Eversource.)

    The cost of lithium ion batteries is still in freefall on a double-digit learning curve. At some point in the next decade it will be "worth it", even if the primary use is as backup rather than demand-charge mitigation.

  7. STEPHEN SHEEHY | | #7

    I was researching some of these issues after our power in Maine was out for almost a week in late October, early November.
    1. Your minisplits needs 240 volts, as does your well pump. Most gasoline generators I found that had 240 volt output were much more than just $2000.
    2. I got two quotes for battery backup for my solar power. A Tesla Powerwall 2 was almost $14,000 installed. An LG battery was almost $12,000 installed. For that, you get about 12-14 kwh of battery storage.
    3. You need a generator, whether you have battery backup or not, for those periods when the sun doesn't shine for days.
    4. Both systems quoted to me could not be charged by the generator. That meant that in an outage, I'd need to go back and forth between battery and generator.
    5. I keep hearing about some inverter that lets you use solar power without a battery when the power is out, but I haven't actually found such a device. So on a sunny day, during an outage, you get no power.

    After my research, I bought a 7,000 watt Honda generator and hired an electrician to install an exterior receptacle that accommodates a 10 guage wire that can handle the 30 amp generator output. The whole thing cost about $6,000, with the electrical work and hundred foot cord. I tried to shop around but most dealers didn't have any Hondas available. It's a nice, quiet machine with inverter technology that lets me run my minisplits.

  8. GBA Editor
    Martin Holladay | | #8

    Here's what I do: I use a 5,000 watt Honda generator that cost a little over $2,000. It has both 240-volt and 120-volt output, but I just use the 120-volt output.

    Using a generator to charge a battery bank is fairly simple. Most off-grid inverters include a battery charger.

    Off-grid homeowners start up the generator when it's been cloudy for a few days. You plug your house into the generator. When the inverter senses the generator power, it automatically charges the battery at a rate of about 90 amps (tapering down to 40 or 50 amps when the batteries are close to full). While the generator is running, there is also plenty of extra power to run the washing machine or other appliances. I don't use electricity for space heating or domestic hot water, but it's easy to power our deep-well pump off the batteries.

    If you are grid-connected, you don't need PV or a battery system for occasional power interruptions. Buy a generator if you want. Run an extension cord from your generator to your refrigerator, your TV, and a few lights. Stay warm with a wood stove.

  9. STEPHEN SHEEHY | | #9

    Martin: I was repeatedly told that neither the LG nor Tesla batteries could be charged with a generator. The Tesla quote came from ReVision Energy, the largest solar installer in Maine. Is it that I'm not off grid?

  10. GBA Editor
    Martin Holladay | | #10

    My house has old-fashioned lead-acid batteries -- the kind used for golf carts or forklifts. Simple old technology, and very rugged. Easy to charge with almost anything, including automotive battery chargers in a pinch.

    If you can charge lithium-ion batteries with the grid, I can't think of any reason why you can't charge them with a generator. It happens in a Prius every day. When the batteries in your Prius need charging, it's the internal combustion engine that charges the lithium-ion batteries in your Prius.

  11. GBA Editor
    Martin Holladay | | #11

    I did some online research. It looks like lithium-ion batteries may need an automotive alternator rather than a generator for charging -- or a set of old-fashioned lead acid batteries as a buffer for the generator. The explanation gets a little complicated. Here is a link:

    Backwoods Solar Electric in Sandpoint, Idaho used to sell a homemade rig consisting of an automotive alternator hooked up to a lawnmower engine -- a good inexpensive way to charge batteries. That type of rig might work for lithium-ion batteries, too.

  12. calum_wilde | | #12


    The lithum battery in my phone, my laptop, and my tools are all easily charged from 120v/60Hz/single phase/AC. The power wall and other similar alternatives are also charqged from typical wall power. If ripple is required on top of the DC, then it's being created in the charger. But I have my doubts.

    So much of that article seems suspect to me. Modern alternators in automotive applications are regulated by varying the field strength within the alternator, not by an external power supply zener diode type of setup. This has been true since at least the early 80s and the technology hasn't changed much from the alternator side. (You can use a Dodge Omni regulator on a modern car if the ECUs field control circuit fails...)

    Modern alternators should be out putting 14.4 volts at idle unless they're very loaded down. That's the first thing I check if diagnosing a charging system fault.

    Any decent power supply for a charger should have (I would hope but I haven't torn one down) a filtering capacitor after the DC is generated, to remove the ripple.


    If the power wall is being charged by mains power it shouldn't matter if that 240vac coming from the grid or your generator. The only way it could matter is if the hookup method purposefully prevents it. But the battery charger/inverter SHOULDN'T care. If I were you, I'd ask about that setup on Reddit dot com/r/electricians. They're generally a great bunch of professionals with collectively a emence amount of knowledge. I wouldn't be surprised if you walked away with code references, product names, and a full explanation of how it would need to be done.

  13. brp_nh | | #13

    Pika Energy out of Maine has a variety of products including an inverter that can handle (I believe) grid tie and "islanding" when the grid is down:

    We have an all electric house in New Hampshire. Our outages have been rare/short, but we did buy a small inverter generator ($500, 1800 running watts) just for emergencies like a space heater in winter or fridge in summer. There's a good chance we'll never use it.

  14. GBA Editor
    Martin Holladay | | #14

    Thanks for the link. Interesting equipment -- the future is getting closer.

    Nevertheless, as far as I know, lithium-ion batteries still cost more than lead-acid batteries.

    And if all you need is 5,000 watts or 7,000 watts of backup power for emergencies, my advice is the same: buy a generator.

  15. STEPHEN SHEEHY | | #15

    Brian: I looked into Pika and was informed, by Pika, that its system doesn't allow pv power to be used when the grid is down, except through a battery. And again, you can't charge the battery with a generator.

  16. GBA Editor
    Martin Holladay | | #16

    Hunh? "Except through a battery"? If you have a PV system that you can use "through a battery," then you have an effective, fully functioning off-grid PV power system. (I guess what I'm saying is you always need a battery, because PV power fluctuates all the time, and isn't used directly. Clouds drift past the sun all the time.)

    Obviously, though, the inability to charge the batteries with a generator is a big downside -- and a big plus for lead-acid batteries. That said, it seems that the Pika system will work with lead-acid batteries (according to the video on the company's web page).

  17. brp_nh | | #17

    Stephen, thanks for the clarification, it's tough to decipher the exact capabilities from their website (and why I put "I believe"). It certainly makes sense to speak directly to Pika/Revision, as you did, and ask simple questions like "can this inverter supply power when the grid is down, without batteries?"

  18. Stockwell | | #18 i was quoted a system using this inverter, a generator and Li-ion batteries. So, it can be done. See these guys

  19. GBA Editor
    Martin Holladay | | #19

    How much was the quote?

    And what size system was specified for the quoted price?

  20. Stockwell | | #20

    They were giving me rough numbers over the phone but we did not finalize numbers since I did not have exact loads calculated for my house yet. You can click around their site and get a good idea of a 19.5kWh Lithium Iron battery set is $14,900, with two of the Schneider XW+ 6848 @ $4550 ea....prices add up quickly.

  21. GBA Editor
    Martin Holladay | | #21

    Thanks. So that $14,900 battery stores the same amount of electricity that you can buy from the grid for between $2 and $4.

    Batteries are expensive. Grid electricity is cheap.

  22. _Stephen_ | | #22

    I've just been going through this whole procedure myself. We'll have an 11 kW solar array, and no means of hearing the home during an extended power outage, but adding a battery array just doesn't make sense. A small battery setup that will run the house for 6 hours is $20k, and a 7 kW natural gas generator is $2k.

  23. AlanB4 | | #23

    How much electricity do you need that $20K would only last 6 hours. $80K for a day of power is insane.

  24. GBA Editor
    Martin Holladay | | #24

    Your question is easy to answer -- or at least, easy to analyze so we can get a handle on possible answers.

    The average U.S. household uses 29.9 kWh/day of electricity.

    Kevin told us that the cost of a lithium-ion battery is $764 per kWh.

    So Stephen's proposed $20,000 battery would store about 26 kWh. That should last almost a full day, not 6 hours. So Stephen was either given a high bid for his battery installation, or his house uses more electricity per day than average.

  25. AlanB4 | | #25

    @ Martin
    Thats still crazy. A Tesla Powerwall costs $6200 (5500 + 700 supporting hardware) equaling 13.5kWh or $459/kWh. This does not include other charges of which there may be but lets assume he can live with 27kWh a day (emergency and all) then its $12400 a day plus other charges or $3100 for 6 hours...

    That said thats a lot of electricity, my average in winter is about 5kWh/day last month with the polar vortex and regularly being under 0F so i should get 3 days from 1 Powerwall (i could cut some lights, computer and washer/dryer use in an emergency). Mind you this does not heat my house since i have natural gas but my calculated furnace load at 0F (99% design temp) is between 2.4 and 3kWh/day assuming natural gas service continues.

    Now if i were going to buy batteries at such a cost i might as well use them more then once or twice a year, i should buy solar power to fill them and go off grid as much as possible which would be quick payback given my province's high electrify rates, off peak about 15ckWh and 25c/kWh on peak after transmission and other fees.

    Tesla batteries will get cheaper in the future, their production cost is claimed to be well under $150kWh and competitors will break into this market forcing cost reductions and worsening their margins.

  26. GBA Editor
    Martin Holladay | | #26

    I don't know if it's crazy or not. It's math.

    Summarizing: Depending on your source and brand, a lithium ion battery for backup power will cost you anywhere from $459 to $764 per kilowatt-hour. (My guess is that the price spread is due to two main factors: whether the inverter is included in the price, and whether installation is included.)

    Needless to say, GBA readers who want a good price for their lithium-ion batteries should shop around and get several bids.

    The cost per kWh for lead-acid batteries, the last time I checked, was about half the cost of lithium-ion batteries.

    Even if you specify inexpensive lead-acid batteries, it will cost you many thousands of dollars to buy enough batteries to power the average American house for a day.

    For temporary power in an emergency, a gasoline-powered generator is about an order of magnitude cheaper than a battery plus PV system. As long as you have enough fuel, your generator will power your house for 20 cloudy days in a row, whereas (assuming a long stretch of cloudy weather) your battery + PV system will give up after one day.

  27. AlanB4 | | #27

    I agree, deep cycle lead acid is cheaper today and depending on usage should last until lithium ion drops precipitously.
    In an emergency if people want to live as if there was no emergency then it will cost a huge amount, but most do not need 30kWh a day to survive, essential usage is far less.
    We have whole house natural gas electricity systems around here, not sure if they are common elsewhere, they cost $5-10k installed and apparently work great as long as the gas service is still available, its permanent wired and kicks in automatically if grid power fails.

    A gas generator with lots of fuel should last a while (assuming you get a good quality one, exercise it occasionally and rotate and/or stabilize the fuel. That said if you have solar + battery you should be able to recharge on solar, i would suggest more then one day of reserve but i assume some light will reach most people in a few days. I recall you saying your off grid?

    That said redundancy is a good idea, having solar+battery and generator + fuel reserve gives even more resilience.

  28. GBA Editor
    Martin Holladay | | #28

    Your suggested solution, "solar+battery and generator + fuel reserve" will work for those who can afford it. I think it's the approach used by Richard Branson for keeping the lights on at his private retreat in the Caribbean, Necker Island. It's nice to live without worries, isn't it?

    Oh, right. There was a hurricane. Disaster planning is tricky.

  29. AlanB4 | | #29

    Of course, everything comes down to those who can afford it.
    Cheapest solution would be a cheap walmart generator and a few extension cords, the things are low quality but cheap. I'm sure someone has mentioned this already
    A name brand gasoline generator will cost more for sure. How much gasoline to be kept in reserve is worth determining before an emergency (though more cost).
    As for solar/battery its hard to say, how much capacity is needed, how is it integrated, dual use (solar bought for net metering but can be flipped for battery for example). In that case the cost may just be the batteries because the solar is slowly paying for itself. Also in places with peak rates the battery could be used to shave costs, also slowly paying for itself (hopefully).

    Perhaps the best strategy is to figure out our budget then how much juice we minimally need?

  30. calum_wilde | | #30

    Alan wrote "my average in winter is about 5kWh/day"

    I wish I could get anywhere near that in the summer. O_o

  31. kbtstone | | #31

    Both SMA and SolarEdge make grid-tie inverters that will operate when the grid is down and the sun is shining. The SMA "Secure Power Supply" provides 2000 watts and the SolarEdge "Self-Sustaining Power Outlet" provides 1500 watts.

  32. AlanB4 | | #32

    @Calum I can't do it in summer either, and i try to avoid air conditioning but this house is an energy efficiency disaster...
    The shoulder seasons are the lowest for me, i can do under 4kWh/day but summer is central air plus dehumidifier, winter is furnace (vent/blower motor and circuitry).

  33. wisjim | | #33

    We're working on replacing our 19 year old lead acid batteries in the next year or so with Lithium, probably recycled Chevy Volt. The hard part is a BMS (battery maintenance system) because Lithium batteries are much fussier as to maximum and minimum voltage and charging rate. Finding a good Volt battery in the junk yard can save a lot of money, but it isn't a simple plug and play solution.

  34. user-2310254 | | #34

    I have watched a couple of videos from a guy in California who builds DIY Powerwalls (about 4 kwH) using recycled 18650 lithium batteries. In 2015, his materials cost was about $300.

  35. walta100 | | #35

    Be very careful with homemade lithium battery packs and chargers this technology lives on the bleeding edge of failure small mistakes can lead to massive failures. In this video simply connecting the battery terminals caused the cell to explode if part if a larger assembly nearby cell would have been damaged and are also likely to explode.

    I was in the room when a lithium battery ½ the size of an AA battery exploded. I thought it was a gun shot. The smoke it produced is worse than any I have ever experienced.


  36. marianelson | | #36

    I was doing homework last month to go for complete solar. Grid tied setup for home and solar thermal system for heating up the pool water which is approx. 500 yards from my home. It comes out real pricey if you compare it with regular generator. You can buy them under $1000 from Walmart and running also comes out cheaper.

    But this definitely is not the right solution if you want to go green. While researching I found some companies who are giving an option to lease the complete setup. I am not sure how this turns out in long term but one benefit is you don’t have to pay big amount upfront but still you take the advantage of technology by paying monthly. Once the lease period end (which may vary between 10 to 20 years), you will get an option to keep it as per the prevailing market rate. You can speak to Solarcity or take advice from

    But the biggest disadvantage I see is you won’t get tax credit as they are not owned by you after all.

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