Solar Battery Calculator: Inverter, Panels and Off-Grid Days

Size your home battery bank in kWh, peak continuous inverter wattage, and solar panel requirements for complete grid-tie or off-grid backup.

How to size a solar battery bank, inverter and array

Battery capacity = daily consumption × days of autonomy ÷ (inverter efficiency × depth of discharge). For 24 kWh a day and 1.5 days of autonomy with LiFePO4, the tool suggests 43 kWh, a 5,625 W continuous inverter and seventeen 400 W panels.

The four calculations

  • Battery (kWh) = daily use × days of autonomy ÷ (inverter efficiency × DoD). Depth of discharge is set at 90% for LiFePO4 and 50% for AGM lead-acid.
  • Battery (Ah) = kWh × 1,000 ÷ bank voltage (12, 24 or 48 V).
  • Inverter: continuous rating = simultaneous peak × 1.25; surge rating = peak × start-up multiplier (1.8 by default, for fridge or air-conditioner motors).
  • Solar array (kWp) = daily use ÷ peak sun hours ÷ 0.80 (losses), rounded up to whole 400 W panels.

Example with the default values

A home uses 24 kWh a day and wants to last 1.5 days without sun: 36 usable kWh. With a 93% inverter and LiFePO4, the gross battery is 36 ÷ (0.93 × 0.90) ≈ 43.0 kWh, or 896 Ah at 48 V. With AGM it would take 77.4 kWh. For a 4,500 W peak, the inverter must handle 5,625 W continuous and 8,100 W surge. With 4.5 peak sun hours, the array is 24 ÷ 4.5 ÷ 0.8 ≈ 6.67 kWp, i.e. seventeen 400 W panels.

Assumptions and limits

  • The array covers one average day of consumption; it adds no surplus to recharge the battery after cloudy days and ignores lower winter output. Use the sun hours of your worst month if you need year-round autonomy.
  • Battery charging efficiency, temperature and the charge controller are not modelled.
  • Results are orders of magnitude for off-grid or backup systems; have the design checked by an installer.

To see whether a battery pays off on a grid-connected home, use the solar battery ROI simulator; for an office UPS, use the UPS runtime calculator.

Frequently Asked Questions

What battery size do I need for 24 kWh a day?

For 1.5 days of autonomy with LiFePO4 and a 93% inverter, the tool gives about 43 kWh. For a single day, about 28.7 kWh is enough.

Why must a lead-acid battery be almost twice as large?

AGM batteries are only discharged to 50% to preserve their life, versus 90% for LiFePO4. For 36 usable kWh you need 77.4 kWh of AGM versus 43 kWh of LiFePO4.

How do I choose the inverter rating?

Continuous output should exceed your simultaneous peak load by 25%, and the surge rating must absorb motor start-ups. For a 4,500 W peak, the tool suggests 5,625 W continuous and 8,100 W surge.

Should my battery bank be 12, 24 or 48 V?

For the same energy, a higher voltage reduces current: 43 kWh is 896 Ah at 48 V but 3,584 Ah at 12 V. Beyond a few kWh, 48 V keeps cable sizes and losses down.

Is this tool really free?

Yes, all tools on Kristools are 100% free and accessible without registration.

Is my data protected?

Yes. All processing is performed directly and securely inside your browser. No personal data is sent to our servers.