How we calculate

Every number on WattLong comes from the formulas on this page and the data rules below. Each assumption carries a grade: CONFIRMED (read off a primary source), REPORTED (published by a credible source we rely on), or INFERRED (our own assumption, labeled as such). If a grade ever changes, it changes here first.

Runtime

AC loads: runtime hours = battery capacity (Wh) × 0.85 ÷ load (W). The 0.85 is the inverter-efficiency factor in Bluetti's own published runtime formula ("Estimated runtime = total capacity × depth of discharge × inverter efficiency ÷ load", with both factors given as reference values; Bluetti's AC300 FAQ uses the same structure). Grade: REPORTED. It is deliberately mid-range: real inverter efficiency varies with load, roughly 0.80–0.90, which is why the runtime calculator lets you move it between 0.75 and 0.95.

DC loads: runtime hours = capacity × 0.92 ÷ load. DC output skips the inverter, so losses are smaller. The 0.92 factor is a WattLong assumption. Grade: INFERRED.

No double depth-of-discharge penalty. Some calculators multiply by 0.85 and a separate 0.80–0.90 depth-of-discharge factor. We don't: the 0.85 is our single combined real-world factor. If you want a safety margin, the calculators offer an explicit 10% reserve toggle (result × 0.90) instead of a hidden second penalty. Grades: method REPORTED (mirrors Bluetti's formula structure), reserve INFERRED.

Duty cycle. Cycling loads (refrigerators, freezers) draw their running watts only part of the time. Energy math uses running watts × duty cycle; the default fridge/freezer duty cycle is 40%. Grade: INFERRED — a planning assumption, adjustable on the fridge calculator.

The start check (surge)

Before any runtime is printed, the load must pass two checks against the model's manufacturer-published ratings: total running watts ≤ continuous rating, and total running watts + the largest single (starting − running) difference ≤ surge rating. Starting watts come from the Generac/Home Depot sources on the wattage chart. If a load fails, the tool says "can't start that load" and prints no runtime — a runtime for a load that trips the inverter in the first second would be a false promise. Grades: ratings CONFIRMED/REPORTED per model page; method CONFIRMED (Generac's worksheet method).

Boost modes are not ratings. EcoFlow X-Boost, Bluetti Power Lifting, and Anker SurgePad let a station run certain (mostly resistive) loads above its continuous rating by reducing voltage. We store boost watts in a separate field, label them on the model page, and never use them in the start check or sizing math.

Generator sizing

Required continuous watts = (total running watts + the single highest additional starting watts) × 1.20. This is Generac's worksheet method — sum the running watts, add only the largest motor's starting surge (motors are assumed not to start simultaneously) — plus the 20% headroom Generac's sizing guidance recommends for safety margin and future loads. Grades: method CONFIRMED (Generac Wattage Worksheet), headroom REPORTED.

Solar charging time

Effective input watts = min(panel nameplate watts × 0.78, the model's manufacturer-rated maximum solar input). Hours = watt-hours needed ÷ effective input. The 0.78 derate accounts for heat, angle, haze, and wiring losses; it is a WattLong assumption, grade INFERRED, consistent with common PV derating practice (real-world output typically 75–85% of nameplate). Results are peak-sun-equivalent hours, not clock hours. Maximum solar inputs and voltage ranges are manufacturer specs, grade CONFIRMED/REPORTED per model page.

Data rules

• Primary sources only. Model specs come from manufacturer product pages and manuals (or dealer pages reproducing the manufacturer's spec sheet, graded REPORTED and named on the model page). Appliance watts come from Generac's worksheet/manuals and Home Depot's sizing guide.
• No invented fields. If a spec can't be verified, the field is omitted and the gap is recorded in our data notes — e.g., two EcoFlow models ship without an X-Boost figure here because the sources conflict.
• No averaged conflicts. When sources disagree on an appliance's watts, the chart shows the range and the note names the disagreement.
• Efficiency is ours, never theirs. The 0.85/0.92 factors are site assumptions applied uniformly; they are never stored as if they were a manufacturer's per-model spec.
• Dated and correctable. Specs were pulled 2026-10-19. Manufacturers revise products; if you spot a figure that no longer matches the manufacturer's page, tell us via the contact page — corrections are logged publicly on the corrections log, and fixed errors say what changed.

All results on WattLong are estimates for planning. Verify critical figures with the manufacturer and a licensed electrician before relying on them. See our terms.