Sizing Methodology

The core formula

Every runtime estimate on ChargeCostLab uses the same formula:

Runtime (hours) = (Rated Wh × 0.85) / Device Watts

Where Rated Wh is the manufacturer-specified battery capacity and Device Watts is the appliance's typical continuous draw in watts.

What the 0.85 factor represents

The 0.85 combined efficiency factor accounts for two real-world losses that occur between the battery's rated capacity and the energy delivered to your appliance:

1. Usable battery capacity (~85–92% of rated)

Lithium-ion and LiFePO4 batteries are managed by a Battery Management System (BMS) that prevents a full 0-to-100% discharge to protect cell health and longevity. Additionally, battery capacity decreases in cold temperatures and as the pack ages. In practice, most portable power stations deliver 85–92% of their rated Wh under normal conditions, per manufacturer specification sheets and standard battery engineering references.

2. AC inverter conversion losses (~85–88%)

Converting DC battery power to AC household current is not 100% efficient. Published inverter datasheets for the stations referenced on this site show typical AC efficiency of 85–88% at moderate loads. Light loads (below 10% of rated inverter output) can fall below 85%; heavy loads near the inverter's rated output can exceed 88%.

Combined: approximately 88% battery × 88% inverter = ~77% overall, or approximately 85% using a single conservative factor. We use 0.85 as the conservative single factor to avoid overpromising runtime.

Appliance wattage sources

Appliance wattage figures used in ChargeCostLab articles are sourced from:

• The U.S. Department of Energy appliance energy estimator (energy.gov/energysaver), which provides typical wattage ranges for common household appliances.
• Energy Star certified product data (energystar.gov), for categories with standardized testing data (refrigerators, televisions, room air conditioners).
• Manufacturer product specifications for specific appliance models cited in each article.

Where a range is given, the figure used in runtime calculations is stated explicitly in the article — we do not cherry-pick the lowest-draw end of a range to inflate runtime estimates.

Power station specification sources

All power station specifications (rated Wh, inverter output watts, battery chemistry) are read from the manufacturer's current product page and cited in each article. Specifications change when products are updated — if you find a discrepancy between an article's stated spec and the current manufacturer page, it reflects a product update after our publish date.

What this methodology does not include

ChargeCostLab does not currently conduct hands-on runtime tests. The analyst does not own a fleet of power stations and does not measure actual runtime with a kill-a-watt meter or similar equipment. All estimates are derived from published specifications and the formula above. This is disclosed clearly in each article's methodology block.

The practical implication: our runtime estimates are consistent and traceable, but they may differ from measured real-world performance for your specific appliance and environment. Variables not captured in the formula include: compressor cycling duty cycle for refrigerators, exact battery temperature, specific appliance efficiency variation within a product category, and load profile changes over time.

Surge/starting watts

For appliances with compressor motors (refrigerators, air conditioners, pumps), the starting surge is stated separately from running watts and sourced from manufacturer technical documentation or published engineering references. We recommend the power station's continuous inverter output exceed this starting surge — not just the running wattage — for reliable operation.

Corrections

If you find a factual error in a calculation or a specification that has changed, please contact us at [email protected]. We review and correct articles promptly when errors are identified.