Power Station Size Calculator Math: Watts, Hours, and Wh

For one constant load, estimate power station size with two equations:

Load energy (Wh) = average load watts x runtime hours

Required battery nameplate Wh
  = load energy / (conversion efficiency x usable battery fraction)

If a 45W fan must run for ten hours, the load needs 450Wh. Assuming 90 percent inverter efficiency and keeping 10 percent of battery nameplate capacity in reserve:

Load energy = 45W x 10h = 450Wh
Required battery = 450Wh / (0.90 x 0.90) = 556Wh

The result is above 500Wh, so the first upward shopping bucket is 700Wh. A product in that search range qualifies only if its actual capacity is at least 556Wh and its output specifications meet the load.

Use the power station size calculator to change watts, hours, days, extra charging energy, and usable battery percent. The sections below show how to choose defensible inputs.

What number should you use for watts?

Use average watts during the operating period, not whichever number is easiest to find.

Best inputs, from strongest to weakest:

1. Energy measured over a representative session, divided by session hours
2. Average or typical operating power published for the exact device and mode
3. A measured steady watt reading for a genuinely constant load
4. A nameplate or power-supply maximum used as a conservative screen

The U.S. Department of Energy defines the basic relationship between watts, time, and energy: watts multiplied by hours gives watt-hours. The difficult part is not the multiplication; it is representing a changing load with an honest average.

Convert measured kWh to average watts

If an outlet meter reports 0.36kWh over eight hours:

0.36kWh x 1000 = 360Wh
360Wh / 8h = 45W average

Use 45W and eight hours in the size calculation. This captures cycling and mode changes during that measured session better than one instant on the display.

Do not hide efficiency and reserve in one mystery buffer

Two separate things reduce the nameplate capacity available to an AC load:

• Conversion efficiency: energy lost while the inverter converts battery DC to AC, including operating overhead.
• Battery reserve: nameplate energy deliberately left unused for uncertainty, battery management behavior, or planning headroom.

State both. If the exact station publishes no usable-energy figure for your load, run more than one scenario instead of presenting a single efficiency assumption as fact.

For the 45W fan example:

Scenario	Efficiency	Reserve	Required nameplate Wh	First upward class
Favorable planning case	93%	5%	509Wh	700Wh
Worked planning case	90%	10%	556Wh	700Wh
More conservative case	85%	15%	623Wh	700Wh

Even the favorable result is above 500Wh, so none of these scenarios supports a 500Wh station. The exact 700Wh-class model must still provide at least the calculated capacity.

Upward-only size classes

After calculating required nameplate Wh, use the first bucket whose ceiling is not below the result:

<= 300Wh    -> 300Wh class
301-500Wh   -> 500Wh class
501-700Wh   -> 700Wh class
701-1000Wh  -> 1000Wh class
1001-1500Wh -> 1500Wh class
1501-2000Wh -> 2000Wh class
2001-3000Wh -> 3000Wh class

A 701Wh result goes to the 1000Wh bucket, not the 700Wh bucket. A 2001Wh result goes to 3000Wh. Above 3000Wh, investigate a larger or expandable system instead of labeling it 3000Wh.

These are shopping terms. Current official specifications show why exact verification remains necessary: EcoFlow lists the RIVER 2 at 256Wh despite its 300W output, and Jackery lists the Explorer 1000 v2 at 1070Wh with 1500W output. Product naming, capacity, and inverter power are not interchangeable.

Watts, watt-hours, and surge answer different questions

• Watts (W): how much power the load demands at a moment.
• Watt-hours (Wh): energy used over time.
• Continuous output (W): what the station can supply continuously.
• Startup or surge output (W): brief extra capability under manufacturer-defined conditions.

The capacity equation does not approve a load. After calculating Wh, verify that continuous output exceeds everything that may run at once and that startup behavior is adequate for motors, compressors, and power supplies.

When the constant-load formula is the wrong tool

Refrigerators and freezers

They cycle, and their energy changes with ambient temperature, door openings, thermostat settings, and condition. Use measured daily kWh or the EnergyGuide figure in the refrigerator and freezer outage calculator.

CPAP equipment

Pressure, mask leak, humidification, heated tubing, altitude, and power connection can change energy use. Use the exact device’s manufacturer guidance or measured energy in the CPAP runtime calculator.

Several outage loads

One-load math does not capture overlapping operation or a priority list. Use the outage backup load planner to add devices and calculate a combined plan.

Very small AC loads over many hours

Inverter idle or standby power can become a meaningful part of total energy. Measure the whole station-and-load combination when possible, or include documented idle watts in the load.

Include charging only when it is dependable

The size calculator can subtract energy expected from charging during the planning period. Enter that only when the source is available and supported by the exact station.

For solar, do not enter panel nameplate watts multiplied by daylight hours as guaranteed energy. Panel conditions, shade, temperature, orientation, controller behavior, wiring, and station input limits affect delivery. Model the charging side separately with the solar recharge time calculator.

Final calculation checklist

1. Measure energy over a representative session when the load varies.
2. Convert kWh to Wh if needed.
3. Calculate average watts or use measured session Wh directly.
4. State conversion efficiency and battery reserve separately.
5. Calculate required nameplate Wh.
6. Round up to the first non-smaller shopping class.
7. Verify the exact model’s Wh, continuous output, startup behavior, ports, and recharge limits.
8. Run a less favorable scenario if failure would matter.

Next action: enter the measured or documented load in the power station size calculator, then test the exact model in the runtime calculator before shopping.