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Off-Grid Solar + Battery Sizer

Battery bank kWh and Ah plus the array and panel count for an off-grid site, with round-trip losses and derate applied.

Inputs

Load and autonomy

Metered or measured daily consumption at the load side. Off-grid systems need this measured, not estimated.

Days the bank must carry the load with no sun. 2-3 is typical for off-grid; winter storms drive the choice.

Share of nameplate you cycle. Lithium 80-90%, lead-acid 50%.

DC system voltage: 12, 24, or 48 V are common; 48 V keeps DC currents manageable.

Array and losses

Use the worst month (December in most of the US), not the annual average, or the array will be short every winter.

Soiling, wiring, and conversion losses outside the battery. 10-20% is typical.

Nearly all off-grid load energy passes through the battery, so this loss applies to the whole daily load.

Nameplate watts of one panel.

Result

Solar array required
2.87 kW
Battery bank amp-hours
520.8 Ah
Battery bank nameplate
25.0 kWh
Panels required
8
Energy the array must deliver daily
11.11 kWh
Usable energy the bank must hold
20.0 kWh

What to check

  • Daily energy measured over a real week, including phantom loads and any well pump or refrigeration.
  • Worst-month peak sun hours for your latitude and tilt, not the annual average.
  • Autonomy days that match your longest realistic cloudy stretch; a backup generator is usually cheaper than stretching the bank.
  • Battery round-trip efficiency, since almost all off-grid load energy passes through the battery.
  • Derate for soiling, wiring, temperature, and controller losses; 10-20% typical in a clean install.
  • Snow, shading, and panel tilt for the winter season when production is lowest.

Keep going

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Screening sizing for an ungridded site. It does not model shading, snow cover, temperature derate on panels, or charge-controller behavior, and it does not size a backup generator, DC protection, or grounding. Confirm with site-specific production modeling and the locally adopted National Electrical Code (Articles 690 and 706).

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