Battery Backup Calculator
When the power goes out, how long will your battery keep the essentials running? Pick what you need to keep on and a battery size for an instant estimate.
1. Your battery
Lithium batteries safely use ~90% of their capacity; lead-acid only ~50%. This sets your usable energy.
2. What do you want to keep running?
Add the appliances you need during a brownout to see how long your battery lasts.
Estimated backup runtime
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Add appliances to estimate
- Total load
- 0 W
- Usable energy
- 0 kWh
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Usable energy
A battery's usable energy is its size × how deeply it can safely discharge — about 90% for lithium, 50% for lead-acid.
Your load
Add up the running wattage of everything switched on at once. A fridge, a few lights, a fan and WiFi is a light load; add aircon and it climbs fast.
Runtime
Runtime = usable energy ÷ load. A 5 kWh lithium battery (4.5 kWh usable) running a 300 W load lasts roughly 15 hours.
This is an estimate. Real runtime is a little lower after inverter losses, and appliances that cycle (fridges) or surge on startup (non-inverter aircon) behave differently. For an exact figure, ask us for a proper assessment. Also try the appliance cost calculator to see what everything costs to run.