How to Size Your Battery Bank
Plain math for DIY LiFePO4 banks—loads, hours, days of autonomy, usable capacity, and worked examples before you shop. All items listed include affiliate links. Use of links or discount codes will pay a small commission to Mike’s Solar Garage LLC at completely no cost to you.
The plain math
1) List loads in watts. 2) Multiply by hours used per day = Wh/day. 3) Divide by battery voltage (often 48V) for Ah/day. 4) Multiply by days of autonomy. 5) Divide by usable depth of discharge (LiFePO4 DIY often plans ~80–90% usable). Add inverter inefficiency (~10%).
Example: 1500W critical load × 4 hours = 6000 Wh/day. At 48V that is 125 Ah/day. Two days autonomy ≈ 250 Ah before losses. With ~10% inverter loss, plan about 280 Ah usable — close to a 280–314Ah 48V class pack or stacked 100Ah racks.
Next
Compare form factors on WallMount vs server rack, then shop reviews on batteries.
Units that keep you honest
Work in watt-hours (Wh) or kilowatt-hours (kWh). Amp-hours alone hide voltage: a 100Ah 12V pack is not the same energy as a 100Ah 48V pack. Convert Ah × V = Wh when you compare 12V, 24V, and 48V options. Most garage and whole-home DIY banks on this site live at 48V because inverter efficiency and cable size get easier.
Usable capacity and headroom
LiFePO4 can use a high fraction of nameplate capacity, but planning 100% depth of discharge leaves no margin for aging, cold, or a longer outage. Add about 20–30% headroom above daily need × days of autonomy. Then confirm the inverter continuous draw will not exceed the bank’s continuous discharge rating when big loads run.
Examples in plain English
Cabin weekend: low daily Wh, two nights autonomy, modest array. Whole-home critical panel: higher Wh/day, one to three days autonomy depending on storms. Garage tools: shorter runtime but high surge—battery C-rate and soft starts matter as much as kWh. After the math, pick a form factor on WallMount vs server rack or a budget path on EG4 vs budget 48V.