How We Calculate

Every number on this site comes from published specifications and a small set of formulas, shown in full below. We don't lab-test power stations. We calculate from manufacturer data so you can compare models on equal terms, and so you can check our math.

1. Runtime

For each device we use its average power: running watts × the share of time it's actually on. Fridges, freezers, pumps, heaters and air conditioners cycle on and off, so their average is much lower than their running watts. For fridges and freezers, the average comes from ENERGY STAR's typical daily kWh.

The 85% figure is the share of stored energy that reaches the AC outlets after the inverter converts it to 120 V. Devices that charge from USB or the 12 V port skip most of that loss, so we use 90% for them. The inverter's idle draw is set to 0 W by default because it varies by model; add it under Settings for small loads.

When you set hours per day for each device, we add up the energy per day and divide the battery by it to get days on your schedule.

2. Can the station power it?

Battery size decides how long; the inverter decides whether it runs at all. We check three limits:

• Continuous output. The total running watts of everything that can be on at the same moment. By default, devices used under 2 hours a day (microwave, kettle, toaster…) count as occasional, and only the largest of them is added to the always-on devices. Choose “Everything at once” for the worst case.
• Surge. The running load plus the largest extra starting power of any one motor or compressor (we assume motors don't start at the exact same instant). When a manufacturer doesn't publish a surge rating, we compare against the continuous rating and flag it.
• Voltage. Devices that need 240 V can only run on models with split-phase 240 V output.

Results are marked OK below 80% of a limit, Near limit from 80% to 100%, and Too much above it.

3. What size do I need?

We then keep models that meet the required battery size, continuous output, surge and voltage, and sort them smallest battery first. Models that are too small alone but accept extra batteries are listed as “fits with extra battery”.

4. Solar charging

Real panels rarely produce their rated watts. Heat, sun angle, haze and wiring losses usually leave about 70–80%, and we use 75%. That output is then capped by the power station's solar input: its maximum watts (per input, when it has several), and its maximum current at your array's operating voltage.

We also compare your array's open-circuit voltage with the input's maximum and warn you when it's exceeded, which can damage the station. “Days” assume the peak sun hours you enter: the number of hours of full-strength sunlight equivalent your location gets per day.

5. Generator vs power station costs

We add each option's price to its running cost for your hours without power a year, over the years you choose.

A generator's gallons per hour come from the maker's run time per tank at a quarter load and at rated load, joined by a straight line for your average load. Below a quarter load we keep the quarter-load figure, because makers don't publish lower loads and a generator burns fuel just to keep running. For a power station, we assume 90% of the electricity from the outlet ends up in the battery. The calculator lists every default and its source.

6. Where the data comes from

• Power stations: each manufacturer's US product pages, spec sheets and user manuals, as of September 2026. Every model page lists its sources. When two official sources disagree, we use the current US product page and note the conflict.
• AI: we use AI for code, spec gathering and first drafts. We're checking the specs by hand and rewriting the drafts. See How we use AI.
• Generators and prices: Honda's spec pages for run time per tank and list prices, and the U.S. Energy Information Administration for gas and electricity prices and hours without power. Sources are on the generator vs power station calculator.
• Appliances: ENERGY STAR product data, the U.S. Department of Energy, manufacturer specifications (e.g. Starlink) and generator makers' wattage charts (Honda, Champion, Generac) for starting watts. Sources are on the appliance chart.

7. What an estimate can't know

• Your exact devices. Wattage varies a lot between models. A plug-in watt meter gives the best numbers.
• Battery age and temperature. Capacity drops as batteries age and in the cold; a margin of 10–20% covers typical cases.
• Inverter idle draw and battery reserve. These differ by model and aren't usually published.
• How long a surge lasts. Surge ratings are rarely specified by duration, so a borderline result may or may not start your device.

Treat every result as a planning estimate, not a guarantee, especially for medical equipment. If you spot an error in our data, email [email protected] and we'll correct it.

Keep reading

• Running Watts vs Starting Watts
• How to Size a Power Station for a Power Outage
• Series vs Parallel: Wiring Solar Panels to a Power Station