Methodology & limitations

1. Load figures

For every selected device we use three inputs: running watts, startup watts and usage (duty cycle).

Total running load assumes all selected devices are on at the same moment:

Running load (W) = Σ running watts × quantity

Average load accounts for devices that cycle on and off. A refrigerator compressor, for example, might run around 40% of the time:

Average load (W) = Σ running watts × quantity × usage

Energy per 24 h (Wh) = Average load × 24

Startup peak assumes everything else is already running and one device is starting. It adds the single largest extra startup draw (startup watts − running watts):

Startup peak (W) = Running load + max(startup watts − running watts)

2. Usable energy and runtime

Not all of a battery’s rated capacity reaches your devices. Stations keep a reserve and the inverter loses energy converting DC battery power to 120 V AC. The calculator applies two adjustable factors:

Usable energy (Wh) = Capacity (Wh) × usable capacity (90%) × inverter efficiency (85%)

Estimated runtime (h) = Usable energy (Wh) ÷ Average load (W)

Worked example: a refrigerator (150 W, 40% usage) and a router (20 W, always on) on a 1,000 Wh station. Average load = 150 × 0.4 + 20 = 80 W. Usable energy = 1,000 × 0.90 × 0.85 = 765 Wh. Runtime ≈ 765 ÷ 80 ≈ 9.6 hours (about 9 h 34 min).

The defaults of 90% and 85% are general assumptions, not values for any specific product. You can change both under Advanced assumptions.

3. Rating checks

• Running load is compared with the station’s continuous AC output rating.
• Startup peak is compared with the surge rating (never treated as lower than the continuous rating).
• Each check is OK at or below 80% of the rating, near limit above80% and up to 100%, and exceeds above 100%. If either check exceeds, the calculator reports that the station likely cannot power the combination and does not show a runtime.

4. Example data

Every value below is an Estimate – a rounded, generic number chosen for illustration. None are measurements, and none come from a manufacturer’s specification. Real devices can differ greatly.

Power station presets

The presets are generic capacity classes, not real products. They exist only as a starting point.

5. Limitations

• Device draw varies. Size, age, model, settings and room temperature all change real consumption.
• Duty cycles are averages. A fridge runs more on a hot day, after the door is opened, or while cooling down food. Runtime can be noticeably shorter than estimated.
• Constant efficiency is a simplification. Inverter efficiency is usually lower at very light loads and can change with load level. Some stations also draw idle power when the AC outlets are on.
• Battery conditions matter. Cold temperatures, aging cells and the station’s own protection settings reduce usable energy.
• Surge is simplified. We assume one device starts at a time. Two motors starting together need more surge capacity. Surge ratings also have duration limits that vary by product.
• No recharging is modeled. Solar or vehicle charging during the outage would extend runtime.
• Plugs and outlets. A station may not have enough outlets, or the right type, even when power is sufficient. Some devices are better powered from DC/USB ports, which avoids inverter losses.
• Not for life-safety decisions. Do not use these estimates to plan power for medical devices without your doctor and equipment supplier.

6. Testing and future data

The formulas live in a separate, dependency-free module covered by automated tests (load sums, duty cycles, surge selection, efficiency factors, rating thresholds, input validation and formatting). The data model already distinguishes estimates from measured values and verified manufacturer specs, so real measurements and specific power station models can be added later – each clearly labeled with its source and date.