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diy solar

Please check my math

Notpowersmart

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Jul 26, 2020
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Just starting with simple math to determine "ballpark" number and type of batteries. I am assuming no recharge and not including surge, inverter efficiencies, max discharge, etc. (I know these are important, but I will add later)

Usage
From my electric company (I rounded)
monthly consumption = 2,000 KWH
average daily consumption = 2,000 / 30 days = 66.7 KWH per day = 66,700 Watts hours per day

Battery storage
Voltage = 48
Amp Hour rating = 150
KWH = 48 * 150 = 7.2
Watt hours = 7.2 *1000 = 7,200

I would need 66,700 / 7,200 = 9.2 batteries

Thanks!
 
The math seems about right.

Some things to consider:
  • During the day, when the sun is shining you will be using power as you produce it. This is power that does get stored, consequently, the math slightly over estimates the storage need.
  • Your battery will typically be running at greater than 48V. (53 or 54V is probably a more typical) Consequently, the math slightly over estimates the storage need.
  • The storage and conversion efficiency is not 100%. Consequently this will mean the math slightly under estimates the storage need.
So, lets assume the under and over estimation in the above items cancel each other out. There are still other things to consider:
  • Average usage over 30 days is good a reasonable first approximation, but you really need to use a daily peak if you plan on going off grid.
  • If you are planning on going off grid, you need to have a few times the daily need of storage in order to handle a string of cloudy days.
BTW: How are you going to hook up the .2 battery? ?
 
One more thing: Have you gone through the house to make it more energy efficient? If you can do things like LED lighting to reduce the usage, it will typically be much cheaper than the extra solar production and storage.
 
Thanks for responding! We have gone through the house, re-insulated, installed new windows, changed out bulbs, etc. It's a large, old house. We are going off grid, we are looking at "living" through a long power outage for a couple of days...

Thanks again!
 
Just starting with simple math to determine "ballpark" number and type of batteries. I am assuming no recharge and not including surge, inverter efficiencies, max discharge, etc. (I know these are important, but I will add later)

Usage
From my electric company (I rounded)
monthly consumption = 2,000 KWH
average daily consumption = 2,000 / 30 days = 66.7 KWH per day = 66,700 Watts hours per day

Battery storage
Voltage = 48
Amp Hour rating = 150
KWH = 48 * 150 = 7.2
Watt hours = 7.2 *1000 = 7,200

I would need 66,700 / 7,200 = 9.2 batteries

Thanks!

You could go with bigger cells
280 amp hours * 51.2 nominal volts = 14336 watt hours
14336 watt hours * .8 depth of discharge = 11468.8 watt hours
11468.8 watt hours * .85 inverter efficiency = 9748.48 watt hours
66,700 watt hours / 9748.48 watt hours = ~7 batteries
Lets make it 8
8 batteries * 16 cells = 128 cells
Plus you will need 8 BMS's
280 amp hour prismatic lifepo4 cells are currently the forum favourites
 
Thanks for responding! We have gone through the house, re-insulated, installed new windows, changed out bulbs, etc. It's a large, old house. We are going off grid, we are looking at "living" through a long power outage for a couple of days...

Thanks again!

You'll need 9.2*2*2 = 37 of FLA/AGM for 2 days of off grid power and keep going for 2 days at your average power consumption to keep them from dropping no more than 50% SoC.

For LFP, it's 9.2*2/0.8 = 23 batteries.

Pretty daunting. Assuming you'll have about 15kW of solar?
 
Nice! I can't imagine the cost.
So you can charge and discharge LYP batteries down to nearly -50 F ? Literally, cool.
 
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