diy solar

diy solar

Hello, from Southern California (not LA)

Dnele928

Dnele928

New Member
Joined
Jul 30, 2023
Messages
4
Location
La Jolla, Ca
Now that I’m here, I’m looking for some tips on an off grid solar+battery system for my cabin in the Sierra foothills. Power cut offs are frequent enough to just give up and go off grid. I have a number of the components identified, but I have some questions to resolve. Initially involving realistic battery power availability.

Number one, help me with the arithmetic, if I have four 12 volt, 100 Ah batteries, in series for 48volts, do I have: 12x100, 1200Ah x 4 = 4.8 kwhr, or 48x400 = 9.2kwhr?

Next, since the power band for a 48 volt system has an effective range down to 40-42 volts, what kwhr can I expect to draw when operating on only batteries?

Cheers!
 
But depending on the battery type, the multiplier could be 12.6, or 12.8
Giving you 50.4V or 5040Wh
Or 12.8V to 51.2V or 5120Wh
 
In parallel the capacity doubles and voltage stays the same. In series the voltage doubles and capacity stays the same.
 
ampsarus said:
In parallel the capacity doubles and voltage stays the same. In series the voltage doubles and capacity stays the same.
Click to expand...
Well... capacity is the same no matter how they are arranged...
But Ah the calculation number increases in parallel.
In series, the Ah stays the same, but voltage increases.
 
Power availability is probably frequent enough to just keep grid connection.
It is a fraction the price of running a gas generator and less hassle.
Also, electric heat may be convenient alternative to getting propane or oil tank refilled.

Various inverter and AIO options to switch between PV & grid, or blend the two.
 
Thank you all, I understand now. I was thinking AGM batteries would be ok, but upon investigation, I see the useable voltage range is 12.8 to 11.8 vdc. One volt isn’t going to cut it. Now, looking at LiFePO4 batteries, I can get nominal 48vdc with a volt range of 58-40vdc range, that has 2560Wh. With four of these, I’ll have over 10kWhr.

My power use analysis shows I’ll use 3.5-4.0 kW each night, looks safe. I’ll need to add a 5000W step up transformer to run my Mini-split AC’s. They use about 800 watts, with a 50% duty cycle, for 2-3 hours in summertime evenings. My plan is to put about 2/3rds of the solar panels facing west, getting a lot of sun from noon to sunset, the other 1/3rd will go on my carport roof facing east to get some morning sun. It’s a 4000W panel/controller/inverter system. It has an input for my generator if I need to top up on low sun days. This gets me off grid.

My motivation is based on the low reliability of the grid in my area (Sierra foothills). Got shut down three times for 6-12 hours each time, three times last week. It just keeps getting worse.
 
Lead chemistries can only be discharge to 50% without risking sulphating the plates or grids. Derate the kW/hr. capacity by 50% and use that for sizing. The other issue with lead is voltage droop under load heavy load. Voltage droop increases as state of charge decreases. Running the mini-split from them will be the biggest challenge; you might find that under load the voltage droop only provides functional power down to about 80% state of charge, for example. The health of the cells is the main factor in voltage they can supply under a given load. If that becomes an issue you can scale out more AGM in parallel, or consider Lithium Iron Phosphate where you can discharge to 5% and they have much less voltage drop over their entire state of charge, and they have superior capacity to deliver amperage without voltage droop at any given state of charge. If you go that route though the AGM have to go elsewhere - can't mix them in the same system.
 
You must log in or register to reply here.
Back
Top