Portable power station runtime is not printed on the battery label because it depends on what you connect, how often that equipment runs, and how much stored energy reaches the output. This calculator exposes those assumptions instead of hiding them behind a single optimistic number.

The core estimate is:

advertised watt-hours × battery availability × output efficiency × remaining reserve ÷ average watts = estimated hours

Use the result to compare plans, not as a promise from a battery or appliance manufacturer. A field test with the actual station, cable, settings, and load remains the best evidence.

Interactive estimate

Power station runtime and sizing

Battery assumptions
Load and inverter
Estimated runtime7.29 hours

729 usable output watt-hours at an average 100-watt load.

Battery needed for target1,097 Wh

Average load after cycling100 W

Continuous load is within the entered inverter limit. Startup surge was not evaluated.

Planning estimate only. Real results change with temperature, battery age, inverter behavior, waveform, startup duration, load cycling, cables, automatic shutoff, and the manufacturer’s reserve logic. Measure the actual equipment. Never use this calculator as the only backup plan for life-sustaining medical equipment.

Hands writing in a blank notebook beside a portable power station running a small fan on a desk.
One measured load, one honest note - runtime estimates start with what the device actually draws.

Understand every input

Advertised capacity is stored energy

Battery capacity is the manufacturer’s watt-hour rating. Watt-hours describe energy; watts describe the rate at which equipment uses or supplies energy. A station labeled 1,000 Wh and 1,800 W may store roughly 1,000 watt-hours while supporting up to 1,800 watts of continuous AC output. Those numbers answer different questions.

Enter the nameplate capacity, not a watt rating and not an amp-hour rating copied without converting voltage. If a separate battery attaches to the station, include it only when the manufacturer confirms that its capacity participates in the same system.

Battery availability accounts for internal limits and age

Not every nameplate watt-hour is necessarily available before the station shuts down. Battery-management limits, displayed state-of-charge calibration, cell age, temperature, and product design can change the amount delivered internally. Use a measured figure or credible independent test when available. The calculator’s default is an assumption, not a claim about every product.

Do not quietly lower this percentage to represent AC inverter losses too. That is what the separate output efficiency field is for. Keeping assumptions separate makes later field-test corrections much easier.

Output efficiency depends on the port and load

Energy is lost while the station converts battery voltage to AC or regulated DC output. AC conversion also consumes some power simply to keep the inverter on. A small router plugged into an AC outlet can therefore produce a different result from an efficient direct-DC setup, even when both power the same task.

Use the port and cable approved for the equipment. Never improvise voltage, polarity, grounding, or connectors. For a first plan without measurements, leave a conservative margin and record the assumption beside the result.

Reserve protects the plan from false precision

A reserve is energy you deliberately do not count. It covers forecast error, a delayed recharge, an additional phone call, or a battery display that drops faster near empty. A 10% reserve means the estimate stops treating the last 10% as available for the planned load. This is different from battery availability: one is a planning decision, the other is a system characteristic.

Running watts and duty cycle create average load

Enter the watts drawn while the equipment is running. For equipment that stays on, such as a modem, duty cycle is 100%. For equipment that cycles, average load can be approximated as:

running watts × percentage of time running = average watts

A refrigerator drawing 120 W for 30% of the hour averages 36 W in a simplified estimate. The real duty cycle changes with room temperature, door openings, food temperature, thermostat setting, ventilation, and the refrigerator’s condition. Measure a complete cycle long enough to capture those changes instead of guessing from one moment.

The U.S. Department of Energy recommends using nameplate wattage, an electricity-use monitor, or other appliance-specific information to estimate consumption. A nameplate maximum can be higher than normal running demand, so label data is a starting point rather than a universal measurement. (Source: U.S. Department of Energy, Estimating Appliance and Home Electronic Energy Use.)

Continuous and surge limits are compatibility gates

The inverter’s continuous watt limit must exceed the simultaneous running load. Motors, compressors, pumps, and some power supplies may briefly need much more power when starting. Enter a measured or manufacturer-specified startup surge and compare it with both the station’s surge rating and allowed surge duration.

The calculator can flag numbers that exceed the entered limits, but it cannot determine waveform compatibility, grounding requirements, startup duration, or whether a manufacturer permits the use. Passing the calculator is not the same as passing every compatibility check.

Worked example: refrigerator planning

Suppose a station has 1,024 Wh of advertised capacity. You assume 90% battery availability, 88% AC efficiency, and keep a 10% reserve. A refrigerator measures 130 W while its compressor runs and operates 35% of the time.

  1. Delivery factor: 0.90 × 0.88 × 0.90 = 0.7128.
  2. Planned output energy: 1,024 Wh × 0.7128 = about 730 Wh.
  3. Average load: 130 W × 0.35 = 45.5 W.
  4. Estimated runtime: 730 Wh ÷ 45.5 W = about 16 hours.

That estimate does not prove the refrigerator will start. If its measured startup surge is 1,000 W, the station must support that surge for the required duration. It also does not guarantee a fixed duty cycle during a hot room or frequent door openings. The safer plan combines a field test, a food thermometer, reduced door opening, and the decision thresholds in the power outage food safety guide.

Worked example: communications load

A modem and router drawing 22 W continuously average 22 W. With the same 730 planned watt-hours, the estimate is roughly 33 hours. Add two 15 Wh phone charges and the plan changes: those charges consume about 30 Wh before conversion losses, reducing the energy left for the network.

List every simultaneous load. A light, radio, phone, medical accessory, and internet equipment do not each get to use the entire battery capacity independently. Build one load schedule or run separate scenarios that reserve energy for each priority.

Size a station from your target runtime

The calculator also works backward. It divides the energy needed at the output by your combined delivery factor to estimate advertised battery capacity.

For an average 100 W load over eight hours, the equipment needs 800 Wh at the output. With 90% battery availability, 90% output efficiency, and a 10% reserve, the delivery factor is 72.9%. The required advertised capacity is therefore about 1,098 Wh, before allowing for uncertainty or future degradation.

Do not buy from that number alone. First reduce unnecessary loads, decide what can operate intermittently, and determine how the station will recharge. The complete home backup power guide compares load reduction, portable batteries, generators, and layered systems.

Runtime is only one compatibility check

Confirm all of these before connecting a load:

  • continuous inverter output is above the simultaneous load;
  • surge output and duration support motor or compressor startup;
  • the output waveform is suitable for the equipment;
  • ports, cables, voltage, and grounding follow both manuals;
  • charging and operation remain within temperature and ventilation limits;
  • the system can be replenished during the planned outage duration.

Portable power stations do not produce engine exhaust at the point of use, but they do not make unsafe cords, overloaded circuits, damaged batteries, or incompatible equipment safe. If a generator is part of the recharge plan, follow the generator and carbon monoxide safety guide; never run an engine in a home, garage, basement, shed, or other enclosed space.

Why the field result may be shorter

Several effects compound rather than replace one another:

  • a cold or aged battery may deliver less energy;
  • a lightly loaded AC inverter may be inefficient relative to the small load;
  • a refrigerator may cycle more often than expected;
  • automatic shutoff may interrupt a very small or intermittent load;
  • a poor cable can waste energy or prevent charging;
  • simultaneous devices can increase both running load and startup demand;
  • the displayed percentage may not decline linearly;
  • charging other batteries adds their own conversion losses.

The estimate can also be longer than the field result if you entered a label maximum that the device rarely reaches. That is why measured watt-hours over time are more useful than an instantaneous watt reading.

Measure the real load safely

For an ordinary plug-in appliance within the monitor’s rating, an electricity-use monitor can record watts and accumulated kilowatt-hours. Read and follow both manuals. Do not use a consumer plug monitor on hardwired equipment, damaged outlets, wet locations, loads beyond its rating, or equipment whose manufacturer prohibits it. Ask a qualified electrician when the connection or measurement method is uncertain.

Measure long enough to capture a representative operating cycle. Record ambient temperature, appliance settings, door openings, connected accessories, starting percentage, ending percentage, and elapsed time. For a refrigerator, a short test immediately after plugging it in may mostly capture startup and cooling, while an overly quiet period may miss normal cycling.

For medical equipment, obtain required power specifications and approved backup procedures from the care team and equipment manufacturer. A consumer estimate must not become the only continuity plan.

Common calculation mistakes

  • Confusing watts with watt-hours: watts are power at a moment; watt-hours are energy over time.
  • Counting capacity twice: applying a tested AC-output result and then subtracting a second assumed inverter loss understates runtime.
  • Ignoring all other loads: the battery powers the combined simultaneous demand.
  • Using zero reserve: the output looks precise but leaves no room for uncertainty.
  • Treating surge watts as continuous watts: both limits matter, but they describe different behavior.
  • Assuming solar nameplate output all day: weather, angle, shade, controller limits, temperature, and daylight duration change harvest.
  • Planning only the first discharge: a multi-day outage requires a safe, realistic recharge cycle.
  • Relying on marketing maximums: maximum runtime often refers to a different load, output, or power-saving mode.

Test before relying on the result

  1. Inspect the station, ports, cables, and load according to their manuals.
  2. Charge the station fully and let equipment reach its normal operating state.
  3. Connect the exact intended load in a controlled, ventilated, dry setting.
  4. Confirm that startup succeeds without warnings, tripped protection, hot connectors, or abnormal behavior.
  5. Record starting percentage, output watts, temperature, elapsed time, cycling behavior, and ending percentage.
  6. Stop before the test consumes energy reserved for an actual outage.
  7. Compare measured energy and runtime with the calculator, then replace assumptions with evidence.
  8. Repeat after major equipment changes and during scheduled preparedness reviews.

A complete test does more than validate arithmetic. It reveals awkward cable placement, noisy fans, bright displays, inaccessible outlets, unexpected sleep modes, and whether household members can operate the system without improvising in the dark. Pair the result with the broader power outage preparedness plan.

Apply the result to the actual decision

Source reviewed

  • U.S. Department of Energy: Estimating Appliance and Home Electronic Energy Use

Source reviewed July 13, 2026.