Home battery backup planner: size an outage energy budget
Estimate battery energy for your household's measured loads, compare outage duration and losses, and separate portable backup from whole-home installation.
Start with the appliances you need and the hours you need them. This planner estimates stored energy for a portable backup scenario. It does not establish that an inverter can start a motor, that a transfer will be uninterrupted, or that a home circuit can be connected safely.
Your scenario
Enable JavaScript and enter documented operating watts to calculate.
Inputs stay in this page. Nothing is sent to a server or saved automatically. The download is a local arithmetic worksheet, not a product compatibility approval.
Quick answer: choose the loads before the battery
The fictional starting list averages 55W: a 100W fridge running 40% of the time, a 10W router, and 20W of lights running 25% of the time. Over 24 hours that is 1,320Wh delivered to loads. At 85% delivery efficiency, no extra standby draw and no retained reserve, nominal storage need is about 1,553Wh. The same assumptions for 12 hours need about 776Wh. These are not measured household loads.
Keep each energy boundary clear
The load list uses appliance-side watts. Required nominal Wh equals outage hours × (average appliance watts ÷ delivery efficiency + additional battery-side standby watts), divided by the fraction of charge you choose to use. If your efficiency measurement already includes standby, leave additional standby at zero. The household tool reuses the RV planner's load arithmetic; it assumes no charging during this outage rather than forecasting solar production.
Portable station, dedicated backup or fuel generator?
| Approach | Useful question | What the Wh total misses |
|---|---|---|
| Portable battery station | Can selected plug-in loads run for the needed hours? | Continuous/surge output, ports, battery condition and transfer behavior |
| Dedicated backup system | Does a specific pump or device need an approved automatic backup? | Controls, installation, maintenance and model compatibility |
| Fuel generator | Is extended outdoor generation practical? | CO exposure, fuel, running/startup capacity and connection method |
| Installed home battery | Which circuits and installation are required? | Professional design, transfer equipment, local requirements and complete system cost |
A portable battery does not authorize feeding a home wall socket or improvising house wiring. Have a qualified installer assess permanent circuit connections. For a fuel generator, CDC says to operate outside, at least 20 feet from doors, windows and vents. A battery station's absence of combustion exhaust does not certify its ventilation, charging or electrical installation.
Work through the difficult appliances separately
- CPAP backup: documented draw, sessions and an approved power path
- Fridge runtime: measured cycling and additional loads
Enough modeled energy is a planning input, not a promise that a medical device stays powered or a basement stays dry. Record manufacturer limits, test the intended arrangement under appropriate supervision, and revisit the budget when the real load, battery health or weather differs.
Sources and scope
- CDC: use a generator safely
- ResMed: planning CPAP battery backup
- Zoeller: Aquanot Fit 508 dedicated backup pump system
Documentation reviewed October 24, 2026. Manufacturer ratings are source claims, not our equipment tests. Numeric examples are calculations under stated assumptions. No device, pump, battery, transfer switch or installation is certified by this tool.