A:Answer A Watt Hour is a unit of measurement for power over a period of time (an hour), or in our case, a way of measuring capacity. One Watt hour is equal to one Watt of average power flow over an hour. One Watt over four hours would be four Watt Hours of power. As an example, a 100 Watt light bulb on a 400 Watt Hour battery would last, on paper, 4 hours.