Observed arrival · 2026-09-11
Usable kWh: Runtime Math for Backup Power
An independent resource for sizing home battery backup by appliance, outage plan, and usable watt-hours rather than the number printed on the box.
Field notes
The site separates advertised capacity, outlet-level usable energy, and appliance demand, then applies a stated efficiency and duty-cycle formula to estimate runtime. One example turns a 3,600 Wh battery into 3,060 Wh at the outlet and estimates 58 hours for a refrigerator averaging 52.5 W. Its comparison method also checks startup surge and names certifications such as UL 2743, UL 9540, and UL 1973 rather than treating all portable batteries as equivalent.
Observed signals
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Editorial observations of this landing page, not a rating.
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