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I'm considering partial electrical backup incase of power failure.

Considering a solar solution.

My power company tells me my usage per month is 60.0 kWh

So I assume me daily usage is on average about 60/30 = 2 kWh. Does that make sense?

So I need a battery that can store 2+ kW and a charging source that supply the same daily.

Gotta a feeling I'm way off.

Given I use 60 kWh per month - what kind of capacity do I need?

Thanks.

*** CORRECTION ***

I misread the bill.

My average DAILY usage ranges from 10-50 kWh - i guess the major factor is AC. During the winter months it's 10 kWh avg daily.

Regardless, let's say I plan for 20 kWh max daily (not using AC) - so what kind of setup do I need.

Also, I'm doing an emergency backup only (furnace + fridge + some lights) - so let's say I need 10 kWh daily. I will find the actual numbers using a watt meter - but let's use that as a starting point.

Apologies for the misread - but the above number seems to be a fair starting point.

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  • That is very low usage unless you have mostly gas for water heater, cooking and heat and no air conditioning.
    – manassehkatz-Moving 2 Codidact
    Commented Sep 20, 2023 at 23:52
  • I checked the bill - that's mainly AC during the summer. For colder months - it's much lower and I made sure I was looking at electrical section. Yes - cooking, heating and water heater is gas. But given the above sample, I'd like some direction. Most probably the battery will be Li PH4 12V. In fact, since this is a partial backup during power failure, I'd think daily consumption would be less than 1 kWh. I'm assuming my math above and understanding above is correct.
    – dashman
    Commented Sep 21, 2023 at 0:00
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    Don't plan for a pack that matches your consumption. Aim for a pack that runs forty percent greater or higher. One should not charge to one hundred percent, nor discharge beyond twenty percent remaining.
    – fred_dot_u
    Commented Sep 21, 2023 at 0:07
  • 3
    You need to consider peak instantaneous draw in addition to daily average. It's probably not a nice constant 83W draw 24/7 but rather short peaks of at least hundreds of watts, possibly even a thousand or a couple thousand. 2kWh of usable battery capacity could power your house for an average day, but the inverter in front of it will need an instantaneous watt rating high enough to run the peak critical loads that you want to run at the same time.
    – nobody
    Commented Sep 21, 2023 at 0:25
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    It's also possible that, if you go away to work on weekdays and are at home on weekends, your daily power varies significantly from that monthly average.
    – nobody
    Commented Sep 21, 2023 at 0:26

3 Answers 3

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Gotta a feeling I'm way off.

You're way off. It might be a problem with units - kW and kWH are quite different things and people confuse them constantly, at least until they get energy basics under their belt. Batteries are sized in kWH.

The EPA reports that the average house uses 30 kWH per day or 1.2 kWH per hour. This is 5 amps at 240V.

I can tell you from personal measurement that a basic refrigerator takes 1 kWH per day (averaging 42 watts - really - you can't go by how much heat comes off it, because it didn't make that heat, just moved it.) People tell me their fancy deluxe fridges with vertical full-height fridge and freezer section and ice in the door take 2-3 kWH per day.

My power company tells me my usage per month is 60.0 kWh. So I assume me daily usage is on average about 60/30 = 2 kWh. Does that make sense?

No chance of that. Something has gone haywire with your data collection.

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  • Agreed, 60kWh per month is implausibly low usage for just a fridge and air conditioning, never mind everything else that’s normally in a house or apartment.
    – nobody
    Commented Sep 21, 2023 at 20:46
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Battery capacity

You need the maximum daily figures, not average daily figures. Assuming that you have a smart meter, your power company should be able to provide you with those figures.

If they cant then either double it to 4. Or buy/rent a measuring device.

Invertor size

This needs to handle instaneous loads. Many gadgets have a higher draw at startup, such as ACs, motors, fridges.

Either check these on the labels of your gadgets, or double the daily usage to 4 for your invertor.

The last thing you want is for the invertor to shut down or the voltage to drop. Voltage drop will dim the lights and sustained voltage drop may actually harm some appliances.

Depending on the design, its frequency may drop, which would be worse.

Summary

Select a system where you can add another battery, that way you can try it with a figure of 2 per day before buying the 2nd battery

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Home Appliance Energy Used Air conditioner 900 to 4,100 watts an hour Heater -Gas Lighting 0.61 kilowatt-hours per month Television 58.6 kilowatt-hours Desktop computer 60 to 250 watts Laptop 15 to 60 watts DVR 8 to 15 watts Video game console 90 watts Refrigerator 300 to 800 watts
Freezer 30 to 100 watts Oven- Gas Microwave 0.12 kilowatt-hours per five minutes Coffeemaker 0.4 to 0.26 kilowatt-hours Dishwasher 0.5 kilowatt-hours per load Ceiling fan 0.9 kilowatt hours per day

That ads to way over 2kWh per day, unless you do not run the fridge, or Dishwasher or washing machine, and forget about HVAC

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