voltamperehours

Volt-ampere-hour (VAh) is a unit of electrical energy defined as the product of voltage, current, and time, using apparent power (volt-amperes) rather than real power (watts). In essence, VAh equals the integral of voltage times current over a period of time. If voltage and current are constant, VAh = V × I × h.

Because volt-amperes equal watts only for a unity power factor, the relationship between VAh and energy in watt-hours (Wh) depends on power factor. The real energy delivered is Wh = VAh × pf, where pf is the average power factor (0 to 1). Equivalently, VAh = Wh / pf. This means a device with 1000 VAh of apparent energy may deliver less real energy if its power factor is below 1.

VAh is commonly used to rate energy storage devices and AC power systems, such as batteries and uninterruptible power supplies (UPS), because it communicates capacity in terms of apparent energy independent of phase angle. It is not the SI unit of energy; the SI energy unit is the joule, and 1 Wh equals 3600 J. A VAh does not represent a fixed amount of usable energy unless the power factor is known.

Limitations include dependence on the power factor and waveform quality, which can make VAh a less direct measure of usable energy than Wh. When comparing capacities, it is important to consider pf to convert VAh to an equivalent Wh value for practical energy availability.