Battery runtime calculator
Choose a battery or power station, tick the devices you want running in an outage, and read the estimated hours. It runs in your browser; nothing is sent anywhere.
Data as of 8 Nov 2026, source: battery and power station figures from manufacturer pages, see Batteries and Power stations; U.S. DOE Energy Saver, Estimating Appliance and Home Electronic Energy Use (archived copy, 31 Dec 2023)
Tesla Powerwall 3: 13.5 kWh per unit (Datasheet states nominal battery energy 13.5 kWh AC, beginning of life, 25 C; no separate usable figure), continuous 11.5 kW, peak not stated in kW. Assumption: capacity and power add up linearly with the number of units. Check the maker's limits for stacking. Charge at start, reserve and conversion loss are assumptions of ours, not datasheet values: edit them.
| Use | Device | Running watts | Share of time running, % | Start surge watts | Basis |
|---|---|---|---|---|---|
| Refrigerator | duty 33%: DOE rule, plugged-in time divided by three; watts and start surge are our typical values | ||||
| Router and modem | typical value, edit it | ||||
| CPAP machine | typical value, edit it; only runs at night: set the duty to the share of the outage you use it | ||||
| LED lights (6 bulbs) | typical value, edit it | ||||
| Laptop and phone chargers | typical value, edit it | ||||
| Well pump (about 1 hp) | typical value, edit it; motor start surge is usually several times the running watts, check the nameplate | ||||
| Microwave oven | typical value, edit it; 8% is about 2 hours of a 24 hour day | ||||
| Furnace blower | typical value, edit it | ||||
| Window or mini-split air conditioner | typical value, edit it; compressor start surge is a typical multiple of 3 |
Within the stated power limits. Full-watts total 225 W against 11,500 W continuous.
How the estimate is made
Hours = capacity per unit x units x (charge at start minus reserve) x (1 minus conversion loss) divided by the average load in watts. Average load is each device's running watts times its share of time running, added up.
Capacity, continuous kW and peak kW come from the datasheets and product pages in the tables. The percentages for charge at start (100), reserve (0) and conversion loss (5) are our assumptions; no datasheet gave them, so change them. The Enphase continuous figure is in kVA and is treated as kW here. Powerwall 3 is entered at its highest rating, 11.5 kW; an install set to 5.8, 7.6 or 10 kW has a lower limit.
The DOE page gives the method (watts x hours / 1000), the refrigerator rule (plugged-in time divided by three) and states that its calculator wattages are samples only. It prints no wattage table, so every wattage below is a round sample number chosen by us. Enter your own from the nameplate or a plug-in meter.
The refrigerator share of 33% follows the DOE rule that a refrigerator runs at full watts for about a third of the time it is plugged in. The calculator does not model solar recharging during the outage, battery temperature, or the way a hybrid inverter limits power per circuit. See Methodology.
The capacity and power figures behind each option are in the battery table and the power station table, each with its source link in the product notes. Dated sources are listed on sources. For a battery choice that also depends on price, read the home battery cost guide and the battery incentives. Sizing and wiring a backup system is work for a licensed electrician following NEC and local rules; this page only estimates hours (more on about).