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Solar Battery Size Calculator

Size a battery bank from daily energy, autonomy days, depth of discharge, inverter losses, and peak load.

Inputs

Load and battery assumptions

Energy the battery must supply per day. Use essentials-only kWh for partial backup, or 24-hour kWh for whole-home.

Hours/days the bank must carry the load with no sun and no grid. 1 day is the usual outage target.

Share of nameplate you plan to cycle. Lithium 0.9, lead-acid 0.5. Staying inside the warranty value protects cycle life.

DC-to-AC and recharge losses. 0.9 is a typical lithium plus inverter figure; older equipment runs lower.

Usable kWh of one battery unit as rated (a Powerwall-class module is 13.5 kWh usable).

Continuous AC output of one module. Used to confirm the bank can carry the largest load, not just the energy.

Biggest load you must run at once (well pump, furnace blower, fridge, A/C). Surge is a separate inverter question.

Result

Nameplate bank size
—
Headroom above requirement
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Usable energy installed
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Modules to install
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Continuous output installed
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Modules for energy alone
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Modules for power alone
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Usable energy required
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What to check

  • Daily energy: use 12 months of metered kWh for whole-home, or a measured essentials list for partial backup.
  • Autonomy target: the longest outage you must ride through with no sun (most US homes size 1-2 days).
  • Depth of discharge and cycle life from the manufacturer datasheet, not a generic figure.
  • Module ratings: confirm whether the published kWh is usable or nameplate, and the continuous kW per unit.
  • Inverter round-trip efficiency: 0.85-0.95 typical; older or undersized inverters run lower.
  • Utility interconnection limits on backup circuits and export, plus AHJ permit requirements.

Related calculators

Planning estimate only. It sizes energy and continuous power; it does not select equipment, verify surge capability, or determine code compliance. Installer quotes, manufacturer datasheets, the locally adopted National Electrical Code (Article 706 covers energy storage systems), and utility interconnection rules govern.

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