What the calculator does—and does not do
The U.S. Energy Information Administration distinguishes power, measured in watts, from energy use over time, measured in watt-hours or kilowatt-hours. This tool applies that arithmetic to values you enter. It does not contain an appliance library, manufacturer performance claim, outage forecast, battery efficiency default, price, runtime promise, or product ranking.
The capacity result starts with raw load energy. The default planning allowance is 0%. If current equipment documentation or a qualified professional supports an allowance for your system, enter it explicitly; the result shows that assumption.
How surge is handled
The continuous output total assumes every listed load is running at once. Startup output adds only the largest difference between one load's startup and running watts. That is a visible one-start-at-a-time assumption. If multiple loads can start together, this output is too low; model that case separately and use current equipment guidance.
Limitations and safety boundary
This is planning math, not electrical, medical, fire-safety, or emergency advice. Do not use it as the sole basis for life-safety or medical-device backup. Follow current manufacturer instructions and utility, fire, building, and electrical requirements. Use a qualified electrician for hardwired circuits or transfer equipment.
FEMA's official outage guidance covers preparations and hazards that a sizing formula cannot evaluate. Review the FEMA power outage guidance and make an emergency plan appropriate to your household.
Formula reference
- Load Wh = running W × duty cycle ÷ 100 × backup hours.
- Raw Wh = the full-precision sum of every load's energy.
- Planning Wh = raw Wh × (1 + your allowance percentage ÷ 100), rounded up once.
- Continuous W = the sum of running W.
- Startup W = continuous W + the largest single startup increment.
Source terminology: the U.S. EIA electricity measurement explainer. Source and page checked 2026-07-27.
Frequently asked questions
What is the difference between watts and watt-hours?
Watts describe an instantaneous rate of power. Watt-hours describe energy used over time. This calculator keeps those outputs separate so a battery's capacity and output ratings are not confused.
Where should I get a device's running and surge watts?
Use the device label, manufacturer documentation, or a suitable meter. Motors and compressors may have startup requirements that are not represented by a simple running-watt reading.
Does the result tell me which battery to buy?
No. The result is generic load math, not a product recommendation or electrical design. Confirm capacity, continuous output, surge behavior, charging limits, compatibility, installation, and safety with current equipment documentation and a qualified professional.
Need utility-rate math instead? See the reviewed calculator directory and the source ledger.