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diy solar

the right number of batteries in case of power failure

PPz

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Sep 4, 2020
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I live in Quebec, where the cost of electricity is low (10 cents/kwh = 7.7 US cents/kwh). I consume around 17400 kWh/year for my entire house. My consumption splits into 3 parts:

- around 5000 kwh/year for my heating and air conditioning (geothermal unit), average temperature required (winter): 22C (71 F)
- 4000 kwh/year for my electric car,
- 8000 kwh/year for the rest, including water heating (around 1600 kwh/year) and everything else. My fridge consumes 365 kwh/year.

In winter, my geothermal consumption is around 800 kwh/month (25 kwh/day).

I can do without heating throughout the season, except in winter. My fridge has to run all the time. I can wash elsewhere.
I can lower my heating temperature to 15C (60F) in case of the grid failure. Estimated consumption at this temperature: 18 kwh/day

To sum up, I estimate that my minimum consumption in winter should be 20 kwh/day.
The geothermal unit operates on 220V single-phase

Installing solar panels doesn't seem a good choice to me (cost of electricity, depreciation of equipment, aesthetic constraints on the part of the city, etc.).


So I'd like your opinion on whether I should build a powerwall to last me 3 days or more (estimated maximum duration of power cuts), i.e. 60+ kwh actually usable.
If necessary, I could add a gasoline generator (noisy), if the outage time gets longer.


What do you recommend, as a first approach? One or more commercial LiFePO4 packs (with BMS, etc.), a home-made battery kit?
And the inverter must generate 220V, I remind you.

Thanks for your comments.

2024 - december - geo kwh.jpg
 
Lets make this simple. You really want to know how large a battery bank you would need to last you three days if you have no grid, no solar and no generator.

For the sake of illustration, lets say your 24 hour load consumption is 30 kW. That's 30/24 = 1.25 kW/hour. This is a small load. A single 5.1 kW server battery would then last you: 5.1/1.25= 4.08 hours if discharged to 0% from 100%. But that is not practical. So lets recalculate with 100% to 20%, or an 80 percent discharge. 5.1x.8= 4.08 kW of power. 4.08/1.25= 3.26 or 3 hours and 15 minutes. 24 hours /3.26= 7.36.

So the answer is 8 batteries would supply your needs if starting at 100 percent charged for 1 day. But you want a 3 day capacity, so your answer is 24 batteries would last you a little more than 3 days if starting at fully charged and drawing about 1.25 kW an hour load. I estimate in USA dollars this would cost you about $35,000 depending on the battery chosen. Plus you will need an inverter, but not a large one because your load is not large. BUT, YOU WILL NEED A LARGE CHARGER TO RECHARGE THE BANK. For example, if you charge each battery at 50 amps rate, this would take about 2 hours to recharge the battery. But the power would be 1,200 amps at about 56 volts. Cut that to 600 amps and you will need about 4 hours to recharge. That's still charging at 33,600 watts. On a 240 volt AC circuit, that's 140 amps. Cut it down to 100 amps and you will need about 7 hours to charge the bank.

If recharging with a generator, get a big one, like 100 amps 240 output so roughly a 30,000 watt generator.

Hope this makes some sense?
 
Feel you need to re-evaluate back up requirement, as illustrated above 3 days backup with given needs is going to be costly.
Are power cuts a regular and costly experience for you? Do power cuts equate to more cost than cost of a back up system?
 
If you're looking at battery backup, you're looking at the worst case scenario. The times you're using the most power. That's wiinter, at 27.2kwh/day. That means for 3 days you want at minimum of 90kwh of battery.

But in reality, 150kwh for 5 days. The reason is this. Say you lose power for 3 days, it comes back, now you have to charge the batteries up again. It's winter, so you're already going to be pulling 30kwh from the grid per day, how much can you really recharge your batteries while you're consuming power from the grid?

Solar panels can help, used ones are cheap, just wire them up. I don't know what a city looks like in canada, but I'm sure there's a way. It'll ease the burden in a big way, both while hte power is out, and while recharging the batteries.
 
A large generator makes more sense cost wise run on nat gas or propane
I’m with @rodrick on this one. If you’re just looking at covering the occasional three day power outage, it’s going to be a much better answer.

Personally, I would prefer a hybrid approach, a single EG4 18Kpv or 12Kpv and a couple of power pro wall mount batteries will give you plenty of power for a day or two (28.6 KWHR), and then an inexpensive generator and a ChargeVerter would charge your batteries back up in something like six hours, and then you are good for another day or two. Easy solution, but not inexpensive. I am guessing $11K usd plus a generator, which can be $500-$1K depending on whether you want cheap or quiet, though a single $4000 Honda inverter generator will probably run your entire house.
 
Thank you all (@wpns @rodrick @fatjay @1234enough @glandpuck) for your enlightened advice!

In the light of your answers, I think I'll go for a generator (petrol/propane) to make up for the rare power cuts, and keep a bank of LiFePO4 (A123) - 4 kW batteries that I own and that I'll keep exclusively to power my electronic devices for as long as possible (+ 1 UPS & extended battery). With a 10-15 kW generator, I'll be comfortable (without too much maintenance) in the event of power cuts.
 

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