How we calculate
Every assumption the calculators make, why we chose it, and when you should change it.
By Zubair Abid. Updated .
In short
All tools use standard electrical relationships (W, Wh, V, Ah) plus a few conservative real-world factors: 85% inverter efficiency, 92% DC efficiency, 10% battery reserve, 50% usable capacity for lead-acid, 90% for lithium, 65% power bank delivery and a half-speed charge above 80%. You can change most of these in the tools.
The core relationships
Energy (Wh) = Power (W) × Time (h)
Energy (Wh) = Charge (Ah) × Voltage (V)
Everything else builds on these two lines. Runtime is usable energy divided by power; charging time is energy needed divided by charging power; cost is energy in kWh multiplied by price.
Default values and why we use them
| Assumption | Default | Why |
|---|---|---|
| Inverter efficiency (AC output) | 85% | Typical for portable power stations and home inverters across normal loads; lower at very light loads. |
| DC output efficiency (USB, 12 V) | 92% | DC-DC conversion loses less than DC-AC inversion. |
| Battery reserve | 10% | Most battery systems cut off before 0% to protect the cells. |
| Lead-acid usable capacity | 50% | Discharging deeper sharply shortens lead-acid battery life. |
| Lithium / LiFePO4 usable capacity | 90% | Lithium chemistries tolerate deep discharge; the battery management system keeps a small buffer. |
| Power bank delivery | 65% | Voltage step-up, the phone's own charging circuit, cable losses and heat. |
| Charging efficiency | 85% | Heat and conversion losses during charging. |
| Charge taper above 80% | Half speed | Constant-voltage charging slows as the battery fills. |
| Solar system losses | 25% | Heat, dust, shading, wiring, controller and inverter combined. |
Each default sits in the middle of the realistic range. Where the tool lets you change it, the hint text tells you when to go higher or lower.
Device wattages
Preset device wattages are typical averages for common products, shown in the tools with a '~' and described as typical. Real devices vary widely; a measurement with a plug-in energy meter is always better. Appliances with compressors (fridges, freezers) are listed at their average draw across a day, not their running peak.
Portable power station models only appear in the runtime calculator once their capacity and output have been checked against the manufacturer's published specifications.
Rules and regulations
The airline checker follows the US FAA and TSA thresholds for lithium-ion batteries (up to 100 Wh, 101 to 160 Wh with airline approval, over 160 Wh forbidden, spare batteries in carry-on only), which most airlines worldwide also use. Airlines may apply stricter rules, and IATA guidance for 2026 points towards a 100 Wh cap for power banks, so the tool always tells you to confirm with your airline.
Where a rule can change, the page shows its last-updated date. If you notice that a rule has changed, please let us know.
What the calculators don't model
- Battery ageing. A battery at 80% health holds about 80% of its rated energy. Reduce the capacity you enter for older batteries.
- Temperature. Lithium batteries deliver less energy in the cold; lead-acid batteries age faster in the heat.
- Peukert effect. Lead-acid batteries deliver less than their rated Ah at high discharge rates. The UPS tool warns you when the load is high relative to the battery.
- Idle draw. Inverters and power stations use a few watts just by being switched on, which matters for very small loads over long periods.
How to check a result yourself
Take the capacity in Wh, multiply by the efficiency and usable share shown on the tool page, and divide by the load in watts. For example, 1,000 Wh × 0.85 × 0.9 ÷ 100 W = 7.65 hours. If your figure matches the tool, the inputs were read correctly; if it doesn't, check units first (Wh vs mAh, W vs kW), because that's where most mistakes happen.
Rounding and precision
Results are rounded to a sensible precision: minutes for runtime, one decimal for watt-hours, whole numbers for PPI. Showing more digits would suggest more certainty than the inputs support.