Powerwall Backup Duration Estimator
Estimate how long your Tesla Powerwall or comparable home battery bank can keep essential loads running. Provide the number of batteries, their usable capacity, and the average outage load to see backup hours and days, with optional adjustments for reserve settings and solar recharge.
Actual backup duration depends on inverter efficiency, temperature, state-of-charge when the outage begins, and whether solar production continues. Validate with your installer before relying on the estimate for critical loads.
Examples
- 2 batteries, 13.5 kWh each, 1.8 kW load, 10% reserve, 4 kWh solar ⇒ Backup duration: 15.72 hours (0.65 days) • Usable energy available: 28.30 kWh (including 24.30 kWh from batteries and 4.00 kWh solar recharge).
- 1 battery, 13.5 kWh, 3.5 kW load, default reserve, no solar ⇒ Backup duration: 3.47 hours (0.14 days) • Usable energy available: 12.15 kWh (including 12.15 kWh from batteries and 0.00 kWh solar recharge).
FAQ
What if I run multiple load profiles?
Use the highest sustained load you expect during the outage window or run separate scenarios for day and night demand to bracket performance.
How do I include generator assist?
Add the expected generator contribution to the solar recharge input so the total usable energy accounts for supplemental charging.
Does the result factor in inverter efficiency losses?
The tool assumes manufacturer-quoted usable capacity. Reduce the usable kWh per unit input if you want to reflect inverter or temperature derates.
Additional Information
- Tesla Powerwall 2 units provide 13.5 kWh of usable capacity, though firmware reserves a configurable percentage for grid services or emergency holdback.
- Average load should reflect only the circuits you plan to back up—often refrigerators, lighting, networking, and critical HVAC zones.
- Solar recharge is treated as a one-time energy boost; real-world performance depends on daylight availability and inverter capability during outages.