Quick answer: A 13.5 kWh battery at 80% depth of discharge powering 500W of essential loads (fridge, lights, router) runs roughly 21 hours before requiring recharge.
Backup Hours Calculator
Estimate outage runtime for a specific battery model and selected appliance set, including running-load and startup-load checks.
How we calculate backup runtime
Runtime is calculated by dividing the battery's usable capacity (adjusted for depth of discharge) by the total connected load in watts.
Startup loads are checked separately against the battery's peak output rating to flag appliances that may not start reliably.
Frequently Asked Questions
- How long will a battery last during a power outage?
- Runtime depends on battery capacity, depth of discharge, and your load. A 13.5 kWh battery at 80% DoD powering 500W of essential loads (fridge, lights, router) will last about 21 hours.
- What appliances can I run on a home battery?
- Essential loads (fridge, lights, router, phones) draw 300–500W and run comfortably on most home batteries. Central AC (3,500W), electric dryers (5,000W), and EV chargers (7,200W) drain batteries quickly and may exceed the battery's continuous output.
- What is battery continuous output vs. peak power?
- Continuous output is how many watts a battery can supply indefinitely. Peak power is a higher burst for a few seconds (for motor startups). The Powerwall 3 delivers 11.5 kW continuous and 30 kW peak.
Reviewed April 2026
Methodology and source note
PowerSizing calculators use shared formulas, documented assumptions, and current planning inputs that are summarized on the methodology page. Use these tools for first-pass planning, comparison, and sanity checks, then confirm local code, pricing, utility tariff, and installer specifics before you buy equipment.