Home Battery Backup Calculator
Check which appliances you want to keep running, enter your battery size, and see how many hours of power you get - with a side-by-side comparison of popular home batteries. All math runs in your browser. Nothing uploaded.
Batteries needed to reach target ?
Brand comparison at this load
Estimated installed cost shown. Actual prices vary by installer, region, and incentives. Hours shown per single battery unit.
Home Battery Sizing and Backup Duration
Calculating Backup Hours
Backup hours depend on two things: how much your appliances consume per day (measured in watt-hours) and your battery's usable capacity. If your fridge, heat pump, and internet use 5,000 Wh/day and you have a 13.5 kWh battery with 90% DoD (12.15 kWh usable), you get about 58 hours (2.4 days) of backup. More appliances = fewer hours.
Limitations of Single-Battery Estimates
The calculator assumes continuous draw at the average daily rate. Real outages vary: a 3-day winter outage might spike heating load, while a summer outage might reduce AC load. Also, battery output power is limited - a 13.5 kWh battery might only discharge at 5 kW continuous, so high-power appliances (electric stove, dryer) may not work even if you have enough energy.
FAQ
What is Depth of Discharge and why does it matter?
Depth of Discharge (DoD) is the percentage of your battery's total capacity that you can safely use before recharging. A Powerwall 3 allows 100% DoD (you can use all 13.5 kWh), while most other home batteries allow 90-96% to protect battery longevity. Lower DoD = fewer usable hours per battery. Always check your battery's spec sheet.
Should I size my battery for essential loads only or whole-home?
Essential loads (fridge, internet, heating pump) need far less capacity than running your entire house. Most people install enough storage for 1-3 days of essential load, because whole-house backup would require 3-5+ batteries. Start with essential and add more if you have longer outages in your area.
How accurate is the brand comparison cost?
The estimated cost is a rough guide - it assumes average installer pricing and does not include labor, permits, electrical work, or regional incentives. Tesla offers IRA credits, some states have rebate programs, and costs vary widely by region. Get quotes from local installers for actual pricing.