Before the next outage.
Battery Backup Time & Inverter Size Calculator
Pick your appliances, set the battery capacity and see exactly how long the power will last — or tell us how long you need and we'll size the battery bank and inverter for you.
Private · Local · No account
- 20 appliance presets
- 3 battery types
- Private, in your browser
Battery backup & inverter sizing
Live - local onlyYour battery estimate will appear here
Pick your appliances or enter total load, set the battery type and voltage, then press Calculate.
This is an estimate. Actual backup time depends on battery age, temperature, and load variation. Confirm final sizing with your installer and equipment manuals. Everything runs locally in your browser.
A clear answer, before the long explanation.
Set your load
Pick appliances from the catalog or enter a total wattage directly. Adjust quantities to match your actual setup.
Choose battery & voltage
Select the battery type (lead-acid, tubular or lithium), system voltage and inverter efficiency. Each type has a different usable capacity.
Read the result
See your estimated backup time or the recommended battery bank and inverter size, with the formula and a shareable summary.
What this estimate assumes
Backup time uses the formula: time = (Ah × V × DoD × efficiency) ÷ load. The depth of discharge (DoD) depends on battery type: lead-acid 50%, tubular 70%, lithium 90%. Only the usable portion of the battery is counted.
Inverter sizing rounds up to the nearest standard VA rating (650, 850, 1000, 1200, 1500, 2000, 2500, 3000, 3500, 5000, 6000, 8000, 10000) using a power factor of 0.8. Battery bank sizing uses standard 12 V units in 100–300 Ah sizes, configured in series for the system voltage and parallel strings for capacity.
Appliance wattages are typical averages — your actual consumption varies by model, usage pattern and load. For backup-time estimates, the result assumes a constant load, which is conservative for refrigerators and pumps that cycle on and off.
The formulas
Backup time (h) = (Ah × V × DoD × efficiency) ÷ load (W)
Required capacity (Ah) = (load × hours) ÷ (V × DoD × efficiency)
Inverter VA = load (W) ÷ 0.8 power factor
Worked example: a 150 Ah, 12 V tubular battery (70% DoD) powering 93 W of appliances at 85% inverter efficiency provides (150 × 12 × 0.70 × 0.85) ÷ 93 ≈ 11.4 hours of backup.