Battery sizing
Battery Bank Calculator: How Much Solar Battery Storage Do You Need?
Battery capacity is the difference between a solar setup that feels reliable and one that only works on perfect sunny days.
Estimate battery sizeThe simple battery formula
If you use 5 kWh per day and want 2 days of backup, you need 10 kWh of usable energy. If your battery chemistry allows 80% usable discharge, the rated bank would be about 12.5 kWh. If it only allows 50%, the rated bank would need to be about 20 kWh.
Why usable capacity matters
Battery labels can be misleading if you compare only amp-hours or rated capacity. What matters is usable energy after accounting for battery chemistry, depth of discharge, temperature, inverter losses, and how much reserve you want to keep.
LiFePO₄ batteries are common in DIY solar because they usually support deeper discharge, longer cycle life, and lighter weight than lead-acid batteries. Lead-acid and AGM can still work, but they often require a larger bank for the same usable energy.
kWh vs amp-hours
For system planning, kWh is easier because it works across voltage. Amp-hours only become meaningful when voltage is included.
A 100Ah 12V battery is about 1.2 kWh. A 100Ah 48V battery is about 4.8 kWh. Same amp-hours, very different energy.
How many days of autonomy?
- 0.5–1 day: portable power and non-critical weekend use
- 1–2 days: basic home backup, RVs, and modest cabins
- 2–3 days: more comfortable off-grid cabin planning
- 3+ days: higher reliability, but cost rises quickly
More battery is not always the best answer. Sometimes reducing loads or adding more solar recharge gives better value.
Battery buying checklist
- Usable kWh, not just amp-hours
- Battery chemistry and cycle life
- Maximum charge/discharge current
- Built-in BMS protections
- Compatibility with inverter and charge controller
- Warranty, support, and expansion options