Scale & Stem
cm · L · °C
Length
Volume
Temp
Weight
Solar

Battery bank sizing

How much battery capacity (Ah at your system voltage) you need to ride through cloudy days without the lights going out.

Battery bank sizing
Higher voltage = thinner wires, smaller losses, smaller Ah. Default to 48V unless you have a reason not to.
No-sun days the bank must carry. Most off-gridders pick 2-3. More = much more cost.
Cold reduces usable capacity. LiFePO4 won't charge below freezing without internal heating.
Required capacity -- Ah at voltage
Total energy (nameplate) --
Usable energy --
Notes --
Capacity is at nameplate. Real-world capacity is lower due to depth of discharge limits, temperature, and battery age. The calc already accounts for DoD and temperature. Round the Ah number up to whatever battery sizes you can actually buy.

What you're sizing for

Off-grid battery banks have two jobs:

  1. Carry the system through nighttime (every day)
  2. Carry the system through bad weather (clouds, storms, snow)

The first is automatic, it's just one day's load. The second is "days of autonomy", how many no-sun days the bank can absorb before you have to start a generator or sit in the dark. Most off-gridders pick 2-3 days. Higher = much more battery cost.

The math

required_Ah = (daily_load_Wh × autonomy_days) / (system_voltage × depth_of_discharge × temperature_derate)

System voltage matters a lot. At 12V, a 2400 Wh/day system needs 600+ Ah of batteries which is a wall of lead-acid. At 48V the same system needs 150 Ah, far cheaper and easier to wire. Higher voltage = thinner wires, smaller losses, more battery options. Most modern off-grid builds run 48V unless they're tiny.

Depth of discharge is how deep you can pull the battery before it gets damaged. Lead-acid: 50%. AGM: 50-60%. LiFePO4: 80-90%. Going deeper kills cycle life. The calc divides by DoD because you size for usable capacity, not nameplate.

Temperature derate is the gut-punch nobody warns you about. Lead-acid loses ~25% capacity at 32°F (0°C). LiFePO4 won't charge below 32°F at all without internal heating. If your batteries live in an unheated garage or shed, derate hard or insulate them.

Lead-acid is dying

LiFePO4 (lithium iron phosphate) has gotten cheap enough that the old "lead is cheaper for off-grid" argument doesn't hold anymore. LiFePO4 lasts 3-5x as many cycles, handles 80-90% DoD instead of 50%, weighs a third as much, and the price per usable kWh is now competitive with new lead-acid. Recommend LiFePO4 unless you've already got lead.

What this calc doesn't cover

  • Charge controller sizing (depends on panel array and battery voltage)
  • BMS / battery management for lithium (always required)
  • Series/parallel wiring layout for your specific battery model
  • Grounding, fusing, and safety (consult an electrician for permanent installs)

Further reading