Frequently asked questions

Add up the watt-hours your devices use over the time you need power, then add room for conversion losses and a reserve. That total is the battery capacity to shop for. Then check two power ratings: continuous output for everything running at once, and startup surge for motors and compressors.

For example, a refrigerator, Wi-Fi router, LED lights, phone, and CPAP through a 24-hour outage call for about 2,850 Wh of capacity, 270 W of continuous output, and 900 W of startup surge. The calculator runs the same math on your own devices and hours.

Multiply each device’s running watts by the hours it runs and by the share of that time it actually draws power. A refrigerator’s compressor runs about 35% of the time, so 150 W × 24 h × 35% ≈ 1,260 Wh a day.

Add your devices together, divide AC loads by the inverter’s efficiency (about 92%), add the inverter’s own idle draw, and divide by 0.95 because a battery never gives up its full rated capacity. Keep 20% on top for battery aging and estimation error.

Watts measure power at one moment: what a device draws and what a power station’s inverter can deliver. Watt-hours measure energy over time: what a device uses across hours and what the battery stores.

A 60 W laptop running for 4 hours uses 240 Wh. A power station needs enough watts to run everything at once and enough watt-hours to last as long as you need.

Motors and compressors draw two to five times their running power for a split second as they start. A refrigerator that runs at 150 W can need 750 W to start. If the power station’s surge rating can’t cover that moment, its inverter shuts off even though the running load is small.

The calculator adds the single largest startup draw on top of everything already running, the way generator sizing worksheets do, since motors rarely start at the same instant.

Runtime ≈ rated capacity × 0.95 usable × 0.92 inverter efficiency ÷ load in watts. A 1,000 Wh power station running a steady 100 W AC load lasts roughly 8 hours, a little less once the inverter’s own idle draw is counted.

Devices that cycle, like refrigerators and heaters, stretch runtime because they only draw power part of the time. Devices charged over USB-C or 12 V skip the inverter and its losses.

For a short outage, usually yes. For a full day, usually not. A typical refrigerator uses about 1,260 Wh a day with its compressor cycling. To run one for 24 hours with losses and a reserve covered, the calculator recommends about 1,900 Wh, 190 W continuous, and 830 W of surge for the compressor’s start.

An 8-hour outage needs about 630 Wh. Check the watts on your own refrigerator’s label, since models vary widely.

Loads that run for hours should stay under 80% of an inverter’s rating, the same rule electricians apply to continuous circuits. Batteries also lose capacity as they age and in the cold: about 90% of rated at 32–50 °F and 80% below freezing.

So the calculator sizes continuous output at 125% of what runs for hours and keeps 20% of battery in reserve, instead of recommending a unit that only just makes it.

No. It sizes the battery to cover your whole plan from a full charge, so you are covered on cloudy days and wherever you can’t recharge. Each power station’s solar input is shown as a spec for planning recharges.

Real models from a public product dataset, with their published capacity, continuous output, surge, outlets, weight, and price. Any unit that can’t finish your plan, carry your busiest moment, start your largest motor, or offer the outlets you need is ruled out.

The rest are ranked by how well they fit: headroom, being right-sized rather than oversized, weight for your use case, and battery chemistry. Price never decides the ranking, and only base units are recommended; add-on batteries appear as a spec.

They are planning estimates built from typical device wattages, the run times you enter, and documented assumptions about inverter losses, usable battery capacity, and temperature. Your own devices can draw more or less than the typical values.

For the closest estimate, edit each device’s watts to match its label, then confirm the recommended power station’s specifications with the manufacturer before you buy.

Yes. Add your own device with a name, its type of load, its running watts, and how many hours a day it runs. You can also change the watts, startup surge, hours, and quantity of any device from the catalog.

Yes. There’s nothing to pay, no account, and no sign-up. Watt48 doesn’t sell power stations; each recommendation links to the manufacturer’s official site.

Yes, in your browser only. The calculator keeps your use case, devices, and runtime plan in local storage so you can pick up where you left off. Nothing is sent to Watt48’s servers, and Start over in the calculator clears your selections.