Watt-Hours, Amp-Hours, and Voltage: Understanding Portable Power Station Specifications

Volume I  ·  May 2026  ·  860 words

Portable power station specifications use units that are inconsistently applied across manufacturers, creating confusion and enabling misleading comparisons. This article explains the three fundamental electrical units — watt-hours, amp-hours, and voltage — and how to convert between them to make valid comparisons between products.

The Fundamental Relationship

Watt-hours (Wh) = Amp-hours (Ah) × Voltage (V)

A battery rated at 20 Ah at 14.4 V stores 288 Wh. A battery rated at 20 Ah at 25.6 V stores 512 Wh — 78% more energy — despite having the same amp-hour rating. Amp-hours are meaningless without voltage. This is the most common specification pitfall: a manufacturer advertises "20,000 mAh" (20 Ah) without stating the voltage, making direct comparison impossible.

Nominal vs. Usable Capacity

The watt-hour rating on the box is the nominal capacity — the total energy stored in the cells when new, measured at a standard discharge rate (typically 0.2C). The usable capacity is lower, for three reasons:

A realistic estimate of usable AC capacity: nominal capacity × 0.85 (BMS and inverter losses) minus inverter idle consumption. For a 288 Wh unit with 8 W idle draw over 8 hours: (288 × 0.85) − 64 = 181 usable Wh — 63% of the nominal rating. This is the number that matters, not the marketing number.

Voltage Configurations

Portable power stations use different internal battery voltages, which affects compatibility with external DC devices:

The internal battery voltage is not something most users need to think about — the unit manages it internally. It becomes relevant when connecting external batteries (expansion packs must match voltage) or when using the DC output for voltage-sensitive devices.

Marketing Pitfalls

The mAh Trap

Manufacturers of small power banks (and some budget power stations) use milliamp-hours (mAh) at the cell voltage — typically 3.7 V for lithium-ion cells. A "20,000 mAh" power bank stores 20 Ah × 3.7 V = 74 Wh. A power station rated at 288 Wh stores nearly 4× as much energy, but both could be described as "20,000 mAh" if the power station's amp-hour rating is measured at 14.4 V. Always convert to watt-hours for comparison.

The Peak vs. Continuous Power Trap

A unit advertised as "1,200 W" may be rated for 600 W continuous with 1,200 W surge. The surge rating — typically sustainable for < 1 second — is useful for starting motors but misleading if presented as the primary specification. The continuous rating is what the unit can sustain indefinitely. Verify which number you're looking at.

The Solar Input Trap

A unit with a "200 W solar input" may only accept 200 W under specific voltage and current conditions. A 200 W panel with Vmp of 20 V and Imp of 10 A, connected to a controller with a 10 A input limit, delivers 200 W. A 200 W panel with Vmp of 36 V and Imp of 5.56 A, connected to a controller with a 30 V input limit, may be current-limited and deliver less. Match panel specifications to the controller's input range.

Quick Conversion Reference

Example: a 50 W CPAP machine running for 8 hours consumes 400 Wh. With a 768 Wh power station (622 usable Wh), runtime is 622 ÷ 50 = 12.4 hours — sufficient for one night with margin. Adding a humidifier (+35 W) increases load to 85 W; runtime drops to 622 ÷ 85 = 7.3 hours — marginal for a full night. This is why we recommend disabling humidifiers during battery operation, as detailed in our CPAP guide.