See the assumptions.
Understand the result.
This calculator estimates energy for directly connected AC appliances without recharging during use. It runs in your browser. A result is a conditional estimate, not a tested runtime or confirmation of electrical compatibility.
The calculation
First, add each appliance’s average watts multiplied by its quantity. All selected appliances are treated as in use throughout the planning window. Cycling appliances use a long-term average; the calculator does not simulate their cycles.
Battery draw (W) = appliance average watts ÷ AC efficiency + station idle watts
Usable fraction = battery health × (starting charge − reserve)
Required nominal Wh = battery draw × hours ÷ usable fraction
Runtime hours = nominal Wh × usable fraction ÷ battery draw
Why there is a range
The lower capacity estimate combines the lower appliance draw, higher efficiency and lower idle draw. The upper capacity estimate uses the opposite assumptions. For runtime, higher draw gives the shorter estimate. These are scenarios, not a statistical confidence interval.
Defaults assume AC efficiency of 80–90%, station idle draw of 5–15 W, a full starting charge, a 10% reserve and a new battery. These are editable planning assumptions. They are not universal manufacturer specifications. Capacity results are rounded up to the next 10 Wh for display.
Appliance profiles
Sources reviewed 2026-09-14. Manufacturer guide examples are not measurements of your appliances. LED and laptop starting ranges are explicitly identified assumptions.
Refrigerator
The manufacturer guide gives 1–4 kWh/day for household refrigerators. This broad daily-average range does not predict compressor timing in a short outage. Your model may fall outside it. Read the source.
Wi-Fi + modem
A 10–20 W combined router/modem estimate from the manufacturer guide. Internet availability also depends on your provider during an outage. Read the source.
LED light
An assumed 8–12 W LED bulb, not a verified model. Replace it with your bulb’s actual wattage when known.
Small fan
The guide gives a 50 W fan example. This is a reference example, not a range covering every fan. Read the source.
Laptop
An assumed 30–65 W average during use and charging. Gaming laptops and other chargers may draw more. The adapter maximum is not measured consumption.
Annual labels and measurements
Annual kWh converts to long-term average watts as annual kWh × 1,000 ÷ 8,760. Use an annual appliance figure, not a whole-home bill. A label or measured average does not establish running or starting power. Enter those separately when known.
What this version does not establish
- Startup duration, outlet compatibility or operation of a particular appliance.
- Solar recharging, direct DC loads, changing schedules, temperature effects or fridge cycle timing.
- Whole-home wiring, transfer-switch design or medical-device backup.
- Current prices, availability, an exhaustive product ranking or guaranteed savings.
How products are shown
Reference models are listed by capacity. The comparison separately checks estimated energy and known continuous output. Missing running or startup specifications stay unknown. We never treat an affiliate commission as evidence of suitability.