WattRunTime.com
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Battery Runtime Calculator

Use this page for raw battery labels, power banks, DIY packs, and non-station batteries where voltage and usable watt-hours matter before the runtime estimate.

Battery pack runtime formula

Use this page when the label is a raw battery rating, power bank, DIY pack, or non-station battery and you need to turn capacity into usable hours.

1. Convert the label to Wh

Runtime math needs watt-hours. If the label only gives mAh or Ah, convert it with the pack voltage before comparing loads.

2. Apply usable-capacity losses

Keep reserve, inverter efficiency, battery health, and temperature loss visible instead of treating nominal Wh as fully usable.

3. Divide by average load

Use measured watts when possible. For cycling loads, use duty cycle so a compressor or thermostat load is not treated as steady draw.

Raw-battery formula path

This page is different from the station runtime calculator because it starts with the battery label, not a verified station model.

  1. Wh = volts × amp-hours, or Wh = volts × milliamp-hours ÷ 1,000.
  2. Usable Wh = nominal Wh × efficiency × battery health × temperature factor × reserve factor.
  3. Runtime hours = usable Wh ÷ average load watts.

Raw battery examples

Power bank labels

A 20,000 mAh power bank is incomplete until you know whether the label uses cell voltage, usually around 3.7 V, or USB output voltage. Convert the label first, then apply USB or inverter losses.

DIY 12 V / 24 V packs

For a 12.8 V 50 Ah LFP pack, nominal energy is about 640 Wh before reserve and conversion losses. A 24 V pack with the same Ah rating stores roughly twice the Wh.

Missing voltage

If a label only says mAh and does not name voltage, the runtime estimate is not trustworthy. Find nominal pack voltage or measure the pack before comparing loads.

Use the station calculator instead when

  • You are choosing a named portable power station model.
  • AC output, surge rating, solar input, or product sources matter.
  • You need source-linked model assumptions rather than raw Wh math.

Calculator

Start with a quick estimate, then switch to Advanced if you need multiple devices, startup surge, battery health, or cold-weather loss.

Loading calculatorPreparing calculator
Power station model
Battery capacity (Wh): 1024 WhContinuous output rating (W): 1800 WSurge output rating (W): 2700 W
EcoFlow DELTA 2
Estimated runtime13h 3m8h 10m – 16h 13mEstimated runtime: 13h 3m, 8h 10m – 16h 13m
Output path

Estimated runtime

13h 3mEstimated runtime: 13h 3m

This setup has comfortable headroom for overnight use, assuming the wattage estimate is realistic.

Conservative8h 10m
Estimated13h 3m
Optimistic16h 13m

Conservative assumes harder conditions; optimistic assumes favorable conditions.

  • This is an estimate. Real runtime changes with load, temperature, battery age, AC/DC output, and device behavior.
Usable energy
783 Wh
Average load
60 W
Fixed station overhead
Unknown (not included)
Running watts
60 W
Max surge
n/a

Show your work

Rated Wh is reduced by efficiency, reserve, battery health, and temperature before it is divided by average load.

Rated battery
1024 Wh
Battery capacity (Wh): 1024 Wh
Usable energy
783 Wh
1024 Wh x 85% x 90% x 100% x 100%
Average load
60 W
60 W running, 60 W average after duty cycle and quantity
Fixed station overhead
Unknown (not included)
60 W / 85% = 70.6 W battery draw; fixed overhead is unknown and excluded
Estimated runtime
13h 3m
783 Wh / 60 W = 13h 3m
  • 1024 Wh nominal battery capacity
  • AC inverter output path
  • 85% conversion efficiency
  • 10% reserve kept unused
  • 60 W average load from 1 load
  • 100% battery health
  • 0% temperature loss
  • Fixed station overhead unknown; not included

This link contains the numeric values you entered, but not custom load names.

Source-Backed Next Steps

WattRunTime.com

WattRunTime estimate

Estimated runtime

13h 3mThis setup has comfortable headroom for overnight use, assuming the wattage estimate is realistic.
Usable energy783 Wh
Average load60 W
Fixed station overheadUnknown (not included)
Running watts60 W
Max surgen/a

Runtime range

Conservative8h 10m
Estimated13h 3m
Optimistic16h 13m

Show your work

Rated Wh is reduced by efficiency, reserve, battery health, and temperature before it is divided by average load.

Rated battery1024 WhBattery capacity (Wh): 1024 Wh
Usable energy783 Wh1024 Wh x 85% x 90% x 100% x 100%
Average load60 W60 W running, 60 W average after duty cycle and quantity
Fixed station overheadUnknown (not included)60 W / 85% = 70.6 W battery draw; fixed overhead is unknown and excluded
Estimated runtime13h 3m783 Wh / 60 W = 13h 3m

Assumptions

  • 1024 Wh nominal battery capacity
  • AC inverter output path
  • 85% conversion efficiency
  • 10% reserve kept unused
  • 60 W average load from 1 load
  • 100% battery health
  • 0% temperature loss
  • Fixed station overhead unknown; not included

Warnings

  • This is an estimate. Real runtime changes with load, temperature, battery age, AC/DC output, and device behavior.

Best for

Turn a battery label into a conservative runtime estimate before choosing a named station or load plan.

Runtime worked example

A 1,024 Wh station at 85% AC efficiency with a 10% reserve leaves about 783 Wh usable. At a steady 60 W load, that works out to 13h 3m.

Change first

  • Change watts first when you have a watt-meter reading; change efficiency when switching between AC and DC output paths.
  • Check startup surge for motors, compressors, and pumps.
  • Use measured watts for outage or medical-adjacent planning.
This calculator gives a planning estimate. Treat the result as a starting point, then refine it with measured watts, the device manual, and the station's current output limits.

Verify your watts

  • Check the device label for running watts or input amps and volts.
  • Use a plug-in watt meter for AC loads when the result matters.
  • For cycling loads, measure long enough to capture on/off behavior.

Open wattage library

Check output path

  • AC inverter loads usually lose more energy than direct DC loads.
  • Keep reserve for cold weather, battery age, and load changes.
  • Do not treat label capacity as fully usable watt-hours.

Read the method

Related Guides

Use these explainers when an assumption changes the runtime result.

Related Calculators

Use the related tools when charging, sizing, surge, or power path changes the answer.

FAQ

These answers keep the estimate grounded in assumptions instead of pretending runtime is exact.

How do you calculate portable power station runtime?

Use usable watt-hours divided by average load watts. For AC loads, a practical first-pass formula is battery Wh x efficiency x reserve factor divided by device watts.

Why is real runtime lower than the label capacity?

Battery label capacity is nominal. Inverter losses, DC conversion losses, reserve settings, cold weather, battery age, and changing device loads all reduce real runtime.

Is a 1000Wh station really one kilowatt-hour usable?

Not for AC loads. A 1000Wh station at 85% efficiency with a 10% reserve gives about 765Wh usable for the load.

What efficiency should I use for AC devices?

Use 80% to 90% for most AC inverter loads unless you have measured data. The default calculator value is 85%.

Should I reserve 20% battery?

A 10% to 20% reserve is a practical buffer for uncertainty, startup spikes, and avoiding a completely empty station during an outage.

Why should I measure with a watt meter?

Device labels are often maximum ratings. A watt meter shows real average draw and makes runtime estimates much more reliable.