Transparent method

Power planner formulas, assumptions and limits

The planner is deterministic arithmetic over values you enter. It does not contain a hidden product database, weather feed or brand score.

1. Daily energy

device Wh/day = running W × quantity × hours/day × duty-cycle fraction

The planner adds each device result. The U.S. Department of Energy defines a kilowatt-hour as one kilowatt of power sustained for one hour, which is the same power-times-time relationship at a larger unit scale.

2. Running and startup power

running W = sum of every selected running load
startup W = running W + largest single load-group startup addition

The startup result assumes all listed devices may be running while one device row starts. It does not model waveforms, motor controls, inverter overload duration or several unrelated devices starting at exactly the same moment.

3. Rated battery target

rated Wh = daily Wh × target days × (1 + reserve)
           ÷ (usable-battery fraction × output-efficiency fraction)

Energy capacity and power capacity are different constraints. The Department of Energy likewise distinguishes stored energy capacity from the rate at which a storage system can release power. Every loss and reserve value here is editable because real products, ports, temperatures and loads vary.

4. Solar replenishment

solar Wh/day = panel W × peak-sun hours × total-solar-efficiency fraction

This is a screening model, not a location-aware PV simulation. NREL PVWatts uses solar resource data, system specifications and a more detailed loss model. Enter a conservative seasonal peak-sun-hours value, and include shading, angle, temperature, wiring and conversion effects in your total efficiency assumption.

5. Essential-load fallback

Plan B keeps only rows marked Essential and recalculates the same formulas. It does not decide what is essential for you and does not treat any medical device as guaranteed backup.

Primary references

Not covered

The planner does not size wire, breakers, transfer switches, grounding, grid connections or permanent installations. It does not predict battery aging, cold-weather derating, idle consumption, non-sinusoidal loads, exact compressor behavior or manufacturer protection logic. Confirm the exact product manual and use a qualified professional where electrical work or safety-critical service is involved.

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