Core concept
Rated capacity is the nominal battery size.
Menu
Usable Capacity vs Rated Capacity
A 1,000 Wh label is the starting point, not a promise that 1,000 Wh reaches your device.
Planning impact
Usable capacity is what remains after losses and reserve.
What to verify
Measured capacity records are stronger than calculator estimates when the source is clear.
Usable Wh = nominal Wh x efficiency x (1 - reserve) x battery health x temperature factor.
Rated Wh Is The Label
Portable power stations and solar generators usually advertise nominal battery watt-hours. That number is useful for comparison, but it is measured before delivery losses and user reserve.
Usable Wh Is The Planning Number
The calculator reduces nominal Wh by the output path efficiency, reserve kept unused, battery health, and temperature loss before dividing by the load.
Measured Data Beats Guessing
When a third-party source publishes delivered Wh for the same model and output path, use that record as a reality check and keep unknown test conditions visible.
Worked example and a published reality check
This calculation is hypothetical, not a test of a named station. Change one assumption at a time. Keep reserve and output path the same when comparing, and use input watts rather than an appliance’s output rating.
- Battery capacity (Wh): 1,000 Wh · Device wattage (W): 40 W
- Efficiency (%): 85% · Reserve kept unused (%): 10% · Duty cycle (%): 100%
- Battery health (%): 100% · Cold weather loss (%): 0% · Output path: AC inverter
Estimated runtime: 19.1h
1,000 × 0.85 × 0.90 ÷ 40 = 19.1
The same publisher reports different delivered energy for the same model at low and high AC loads. Read each Wh value together with its load and duration. The approximate watts cannot reconstruct the separately measured Wh and hours. Do not transfer the 380 W result to a router or CPAP setup.
EcoFlow DELTA 2
Third-party results apply to the listed model, output path and test load. The delta is against a fixed no-reserve planning formula, not an error score or a prediction at other loads. Missing test conditions remain unknown.
Swipe table sideways
| Test | Measured | default planning estimate | Vs estimate | Source |
|---|---|---|---|---|
| Runtime test (AC) Average test load: ~380 W test date unknown | 900 Wh 88% | 870 Wh | +30 Wh (+3%) | Outdoor Life Best Portable Power Stations of 2026 2026-01-09; accessed 2026-09-04 |
| Runtime test (AC) Average test load: ~34 W test date unknown | 690 Wh 67% | 870 Wh | -180 Wh (-21%) | Outdoor Life Best Portable Power Stations of 2026 2026-01-09; accessed 2026-09-04 |
Why rated Wh and delivered Wh differ
These are attributed third-party observations, not our hardware tests. Small continuous loads may deliver fewer Wh because an inverter remains active for longer. Efficiency also varies with load. Reported Wh and approximate watts × rounded hours need not agree; preserve the source's readings. Do not infer an inverter wattage from their difference.
Swipe table sideways
| Model | Below 100 W | 100 W or more | Difference at higher load |
|---|---|---|---|
| EcoFlow DELTA 2 outdoorlife.com | ~34 W · AC 690 Wh (67.4%) | ~380 W · AC 900 Wh (87.9%) | +210 Wh |
Same model, output and source series; temperature, battery endpoints and individual test dates remain unknown. These observations show a load association, not a controlled proof of its cause. Delivered Wh already include losses: do not subtract efficiency or overhead again.
Inverter draw: reported, with unresolved conditions
Outdoor Life attributes this AC inverter-and-port column to manufacturer replies. It is not an independently reproduced battery-side measurement. Load, interval, temperature and battery endpoints are unknown; these figures are not applied automatically in our calculator. Unavailable stays unknown. Recheck your exact setup before entering fixed overhead.
Swipe table sideways
| Model | Reported draw | Observation checked |
|---|---|---|
| EcoFlow DELTA 2 | 10 W · AC | Outdoor Life · 2026-09-30 |
Evidence and review
These sources support the definitions and planning method. Calculator results are still estimates, not measurements or guarantees.
WattRunTime publishes source-based editorial calculations. We do not claim to have laboratory-tested these products or to provide medical or certified engineering review. Sources support specific statements; automated checks validate structure and arithmetic, not product truth. Third-party measurements name their publisher.
- DOE Explains...BatteriesU.S. Department of Energy · Government reference — Open source
- SUN Inverter technical specificationsVictron Energy · Technical manual — Open source
Try The Assumption
Open the calculator and change efficiency, reserve, health, or temperature to see why two stations with the same Wh label can plan differently.
Loading calculatorPreparing calculator
This tool didn't load
Something blocked the calculator from starting. Check your connection or reload the page to try again.
Power station model
Battery capacity (Wh): 1024 WhContinuous output rating (W): 1800 WSurge output rating (W): 2700 W
Estimated runtime13h 3m8h 10m – 16h 13mEstimated runtime: 13h 3m, 8h 10m – 16h 13m
Output path
Estimated runtime
Within the tracked power limits; compatibility still needs checking
This is an estimate. Real runtime changes with load, temperature, battery age, AC/DC output, and device behavior. Runtime is conditional on successful startup, compatible voltage/connectors and sufficient output. Verify the actual device setup before relying on these hours.
13h 3mEstimated runtime: 13h 3mThis setup has comfortable headroom for overnight use, assuming the wattage estimate is realistic.
Conservative8h 10m
Estimated13h 3m
Optimistic16h 13m
These are calculated sensitivity cases, not a statistical confidence interval. Conservative uses lower efficiency and battery health, more reserve/temperature loss and higher load; optimistic changes these assumptions in the opposite direction.
- This is an estimate. Real runtime changes with load, temperature, battery age, AC/DC output, and device behavior. Runtime is conditional on successful startup, compatible voltage/connectors and sufficient output. Verify the actual device setup before relying on these hours.
- Within the tracked power limits; compatibility still needs checking
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
- Usable energy
- 783 Wh
- 1024 Wh x 85% x 90% x 100% x 100%
- Running watts
- 60 W
- Average load
- 60 W
- Max surge
- n/a
- 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
- Output path
- AC inverter
- Efficiency (%)
- 85
- Reserve kept unused (%)
- 10
- Battery health (%)
- 100
- Cold weather loss (%)
- 0
This link contains the numeric values you entered, but not custom load names.
Source-Backed Next Steps
Smallest matching record (8h)Jackery Explorer 600 Plus
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.
Usable energy783 Wh1024 Wh x 85% x 90% x 100% x 100%
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. Runtime is conditional on successful startup, compatible voltage/connectors and sufficient output. Verify the actual device setup before relying on these hours.
- Within the tracked power limits; compatibility still needs checking
Share URL
Use With These Tools
Related Guides
Portable Power Station Runtime Calculator
Apply this guide's assumptions in the runtime calculator.
Battery Runtime Calculator
Turn battery capacity and voltage into a usable-energy plan.
Portable Power Station Size Calculator
Turn the required runtime into a capacity target with margin.
Portable Power Station Size Finder
Turn the required runtime into a capacity target with margin.
Measured Runtime and Capacity Library
Compare calculated estimates with attributed delivered-energy tests.
Efficiency guide
Inverter Efficiency Losses
AC is convenient, but the inverter changes battery energy into wall-style power and some energy is lost as heat.
Margin guide
Reserve and Battery Health
A runtime plan is safer when it leaves margin for age, cold, changing load, and measurement error.
FAQ
Learn why rated watt-hours are not the same as usable watt-hours after inverter losses, reserve, battery health, and temperature.
Is usable capacity always lower than rated capacity?
For real device output, yes. Reserve, conversion losses, battery health, cold weather, and device behavior reduce the energy available to the load.
Should I replace the calculator estimate with measured Wh?
Use measured Wh when it matches the model and output path. If the load, temperature, or test conditions are unknown, keep the result as a planning estimate.