How we calculate
The planner does the same arithmetic an electrician would do on paper, using published manufacturer figures. Here is every rule, so you can check it.
1. Running watts
Add each appliance’s running watts × quantity. Defaults are the high end of the Honda and Champion wattage charts, on purpose, so you don’t undersize. Replace them with your nameplate values under “Edit watts and hours”.
running = Σ (running watts × quantity)
2. Peak watts
Motors (fridge compressors, pumps, fans, air conditioners, tools) draw a short burst when they start. We add only the largest single extra start, because motors rarely start at the same instant. Starting watts in our data are the total at start-up, so the extra is starting minus running.
peak = running + max (starting − running)
Some calculators add every surge together. That oversizes the generator, often by thousands of watts.
3. Recommended size
recommended = peak × (1 + 20%), rounded up to the next 500 W
The 20% margin comes from sizing guidance of 20–25% over peak demand (medium confidence). You can set it from 0 to 30%.
4. Verdicts
Generators
- Too small: rated (running) watts are below running × (1 + margin).
- Won’t start: running is covered, but the starting watts are below your peak. We name the appliance.
- Works, little headroom: covers the peak, but rated watts are under peak × (1 + margin).
- Comfortable: rated watts are at or above peak × (1 + margin).
- No 240V outlet / Check 240V: an appliance on your list needs 240V and the generator has 120V only, or we don’t know.
Power stations
- Too small: continuous output is below your running load.
- Won’t start: surge output is below your peak.
- Can’t confirm start: the maker doesn’t publish surge output and your list has a motor. We never treat an unpublished number as zero or guess it.
- Tight: continuous output covers the load, but not the margin. Fits: it covers both.
5. Runtime on a power station
runtime (h) = capacity (Wh) × 0.85 ÷ average load (W)
- 85% usable capacity covers inverter and battery losses (a common rule; medium confidence). EcoFlow’s own runtime examples imply 80%. You can set 75–95%.
- Fridges use daily energy by type: top freezer 1.1–1.4, side-by-side 1.6–1.9, French door 1.6–2.1, mini 0.6–1 kWh a day. Multiplying a fridge’s nameplate watts by its duty cycle overstates its use two to three times.
- Other appliances use running watts × the hours per day you set. Presets start with example hours; anything left at “all day” is counted as running nonstop.
- Results are ranges wherever the inputs are ranges.
6. Runtime on a generator
Fuel use isn’t linear with load, so we show only the runtimes a manufacturer publishes, usually at full and quarter load, plus the tank size. Between those two points we give a labelled range; we never extrapolate beyond them.
7. What we don’t do
- We don’t size whole-house standby generators. That needs a load calculation from your panel; ask a licensed electrician or a dealer.
- We don’t show prices, and no verdict depends on any commercial relationship (disclosure).
- Altitude reduces generator output, but the only figure we found (3.5% per 1,000 ft) is low confidence, so we mention it rather than calculate with it.
Every figure traces to a source. Data last checked 2026-09-26.