Calculation methodology

Watts vs watt-hours

A watt (W) is how fast a device draws power at a moment in time. A watt-hour (Wh) is an amount of energy — one watt sustained for one hour. A device's daily energy use is its wattage multiplied by the hours it runs per day, multiplied by how many of that device you have. The calculator adds this up across every device you list.

Backup period

The combined daily watt-hour total is multiplied by the number of days you want the power station to cover. That gives the raw energy requirement before any real-world adjustments.

Inverter efficiency

Converting a battery's DC power into household AC loses some energy as heat. The calculator divides the requirement by an inverter-efficiency figure — its default is 85% — so the battery is sized to still deliver enough after that loss. Running a device directly from a DC output skips this conversion, and you can raise or lower the percentage to match your setup.

Battery reserve

Draining a battery to empty on every cycle leaves no margin for a cold night, a cloudy day, or an under-estimated load, and is harder on the cells. The calculator keeps a portion of capacity unused — its default reserve is 20% — by dividing by the fraction that stays available.

Rounding to a size class

The adjusted figure is rounded up to the next common power station size so there is a real product to shop for. The reference sizes are 300 Wh, 500 Wh, 750 Wh, 1,000 Wh, 1,500 Wh, 2,000 Wh, 3,000 Wh, 5,000 Wh. A requirement above the largest of these is shown as 5,000 Wh+.

Runtime estimate

To estimate how long a specific power station would last, its usable energy — capacity multiplied by inverter efficiency and by the non-reserve fraction — is divided by the device's average power draw. This tracks stored energy only; it does not check whether the inverter can handle a motor's starting surge.

Solar derating

Solar panels almost never produce their rated wattage in the field — sun angle, heat, shade, cabling, and the charge controller all take a share. Solar estimates multiply the panel's nameplate rating by a real-world efficiency factor before using it. Example panel ratings referenced on the solar pages are 100 W, 200 W, 400 W.

Peak sun hours

A day's usable sunlight is expressed as “peak sun hours”: the number of hours of full-strength sun that would deliver the same total energy. Multiplying derated panel watts by peak sun hours gives a rough daily harvest in watt-hours, which in turn drives the charge-time and panel-size estimates.

Capacity-class recommendations

After the calculator produces a recommended capacity, the recommendation section below the result maps it to one of four shopping ranges:

• 500Wh class — up to 500 Wh
• 1,000Wh class — up to 1,000 Wh
• 2,000Wh class — up to 2,000 Wh
• 3,000Wh+ class — over 2,000 Wh

These are ranges to compare within, not endorsements, and they are listed in a fixed order.

Calculator sizing and affiliate product matching are separate. The size class above comes from your energy estimate alone. The product recommendation then starts at the smallest class that could hold that energy and searches upward for the first range that contains a listed unit which meets all of your known requirements: the unit's real battery capacity is at or above the recommended capacity, and — when the calculator supplies them — its manufacturer-rated continuous output and startup / surge output are at or above what the load needs. Each unit's specs are taken from the manufacturer or the exact Amazon listing the link resolves to; a “boost” or “lifting” mode that drives resistive loads harder is not counted as a surge rating.

An unknown specification is never treated as confirmed compatibility. If your recommended capacity is larger than every listed unit, or no unit in any class has a confirmed output rating high enough for the load, the section shows a neutral note — pointing to expandable or whole-home systems, or stating the continuous and surge watts to look for — instead of linking a product that may not fit. When the matching class sits above the size class from your energy estimate, the section says so and why.

What the estimates do not cover

The math follows energy, not whether a power station's inverter can start a motor's surge, whether a port supplies enough current, or whether the unit accepts your solar array's voltage. Always confirm usable capacity, continuous and surge output, and maximum charge and solar input against the devices you plan to run. For more on the project's scope, see the about page, or go back to the calculator.