Runtime Methodology and Trust Policy

The site is useful only if the assumptions are visible. This page explains the runtime formula, source labels, unknown fields, and recommendation policy.

Runtime Formula

The calculator starts from nominal battery capacity, then applies the losses and assumptions that usually separate a label number from field runtime.

Usable energy

Usable Wh = nominal Wh x efficiency x reserve factor x battery health x temperature factor.

Runtime

Runtime hours = usable Wh / average load W. Average load can include duty cycle, so a refrigerator that runs part-time is different from a constant resistive load.

Range

Conservative and optimistic ranges show planning uncertainty around the estimate instead of hiding it behind a single precise-looking number.

Fixed station overhead

If you know the station’s battery-side fixed draw, add it to the device load divided by output efficiency. Available battery energy includes reserve, battery health and temperature loss. Divide that energy by the total battery-side draw. Do not apply output efficiency to the fixed draw a second time.

Example: 1,000 Wh, 20 W continuous AC load, 85% efficiency, 10% reserve, 100% battery health and no temperature loss. Excluding unknown overhead gives 38.3 h. Entering an illustrative 10 W fixed draw gives 26.8 h. This is a calculated example, not a product test.

Unknown overhead is excluded, not confirmed to be zero. It can shorten runtime, especially at low loads. Use evidence for the exact output state and model, or a measurement of your setup.

Measured delivered Wh already include losses from that test. Do not subtract efficiency or fixed draw again unless the measurement method explicitly requires it.

Planning Assumptions

Shared defaults keep calculator results consistent across runtime, solar, sizing, and emergency tools.

Output efficiency

Default conversion efficiency is 85% for AC inverter output, 90% for DC and 92% for USB-C. Starlink DC barrel and 12 V step-up paths use 93% and 88% planning defaults.

Reserve and sizing buffer

Reserve remains user-controlled. Size finder and emergency planner add a 15% sizing buffer after efficiency and reserve; the buffer does not replace the reserve.

Solar derate

Solar tools default to 75% derate, the Starlink field kit to 70% and outage planning to 65%. Replace these assumptions with measured output or a site-specific PVWatts estimate when available.

Uncertainty ranges

The main runtime calculator and model-device pages vary efficiency, reserve, battery health, temperature loss and load to calculate sensitivity cases. Simpler planning tools use 75% and 120% runtime factors; solar yield uses 80% to 120%. Recharge ranges are inverted: a conservative recharge takes longer. These are planning cases, not statistical confidence intervals.

Source Confidence

Every model record can carry verified, estimated, or unknown confidence. The label should be visible wherever specs affect a result, and it is based on both source class and required spec completeness.

Verified

Required planning specs are present and backed by a manufacturer, manual, or official support source.

Estimated

The record is source-linked but has required spec gaps, relies on a retailer or editorial-only source, or needs stronger attribution before higher-trust recommendations.

Unknown

The site does not infer a precise spec when the available sources are missing or ambiguous.

Recommendation Policy

Model suggestions should come after the calculation. A station can be suggested only when the source-backed record clears capacity, output, and surge checks for the selected use case.

No active affiliate ranking is used in this phase. If commercial links are added later, they should be disclosed near the recommendation and must not change the calculation result.

Review Cadence And Corrections

This methodology is re-checked whenever planning assumptions, formulas, or confidence rules change, and station data is re-verified in dated passes recorded in the model data changelog.

Errors reported through the contact page are checked against the original sources before any published value changes.

Last reviewed: 2026-10-25.

FAQ

Use these policies when expanding into new tools, guides, and buying pages.

Is the runtime estimate a guarantee?

No. It is a planning estimate based on the inputs, source-linked specs, efficiency, reserve, battery health, temperature, and load behavior.

Why does WattRunTime.com keep unknown fields visible?

Unknown specs are safer than guessed specs. If a manufacturer page does not expose a precise port, surge, or input rating, the field stays unknown until a better source is found.

Do recommendations change because of affiliate status?

No affiliate ranking is active in the current phase. Future commercial links should be disclosed and kept separate from calculation logic.