Who runs it
Blackout Calc is built and maintained by DEV2REV, a small software company in Porto, Portugal. It is an independent site, not owned by an energy supplier or a retailer.
Why it exists
When the power goes out, the first question is what you can keep running, and with what. Most sizing charts add up running watts and stop there, then the generator stalls the first time the fridge or sump pump starts. This site sizes for both running and starting watts, shows the battery energy a power station needs, and shows the working behind every number.
How the numbers are calculated
The calculator uses the standard sizing method for generators and inverters, plus a simple energy total for batteries:
“Hours used” is hours a day × days of backup. For outages under 24 hours, each item counts its hours a day, up to the length of the outage, to err on the safe side. The 0.85 allows for inverter and battery losses, and the 0.8 for solar panel losses. Generators get 25% headroom and inverters 20%, so neither has to run flat out. Only one motor is assumed to start at a time.
Sun hours are the hours of full sun in a day. The calculator starts at 4, a cautious US average. Change it to match where you live and the season.
Preset running and starting watts are labelled example figures for common appliances. They come from generator makers' published wattage charts (Champion, Honda and the Westinghouse owner's manual chart) and, for the CPAP, ResMed's AirSense 11 user guide. The RV presets are rough totals for a rig running its air conditioner. Starting watts here are the total draw at start-up, where some charts list only the extra on top of running watts. Motor loads vary a lot by model and age, so the rating label on your appliance always wins.
The default electricity prices are recent averages: $0.18 per kWh from the US EIA (August 2026), €0.28 from Eurostat (EU households, first half of 2026) and £0.26 from the Ofgem price cap (October to December 2026). Enter your own rate for a closer figure.
The other tools
Power station runtime divides the battery's watt-hours, × 0.85 for inverter and battery losses, by the total running watts. It checks the total against the inverter's continuous rating, and the moment the largest motor starts against the surge rating.
Solar charging time divides the energy to add by the panel watts × 0.8, capped at the station's maximum solar input. The result is in hours of full sun, then in days at your sun hours.
Generator cord size finds the thinnest copper wire that keeps the voltage drop, 2 × length × resistance × amps, within 5% or 3%, and the load within 80% of the cord's amp rating. Resistances are for stranded copper at 75 °C from the US National Electrical Code (NEC Chapter 9, Table 8). Cord ratings are common cord-set ratings: 16 AWG 13 A, 14 AWG 15 A, 12 AWG 20 A, 10 AWG 30 A, 8 AWG 40 A, 6 AWG 50 A. The rating marked on the cord you buy always wins.
What the estimate leaves out
- Several motors starting at the same moment, for example after power returns
- 240 V loads such as many well pumps, which need a generator or station with 240 V output
- Battery ageing, cold weather and the reserve some stations keep back
- Generator fuel use, which depends on the model and how hard it is working
For an exact figure, measure the appliance with a plug-in energy meter and enter the reading.
How the site is paid for
The site is free and shows ads from Google AdSense. Some product links are affiliate links, which are always labelled. Neither advertisers nor retailers have any say over the calculator or its results. Details are in the privacy policy and affiliate disclosure.
Found a mistake?
If a preset looks wrong or a result doesn't add up, email hello@blackoutcalc.com or use the contact page. Corrections are listed with the date they were made. Last reviewed November 2026.