Watts is the amount of instantaneous electrical power required to support a load, and is a product of the voltage and current. Watt hours is the amount of energy a load has consumed over time, and is a product of power and time.

As an example: a 13W light bulb that has been left on for 4 hours has used 52Wh.

Kilo Watt hours (kWh) is just 1000 Wh. So in the above example the 52Wh could also have been expressed as 0.052kWh.

Understanding the difference is extremely important in off-grid power systems. In order to properly size an inverter, one needs to know the maximum amount of power required by the load. In order to properly size the battery, one needs to know the amount of power required by the load and for how long.

As an example: you might have a 1hp motor that runs for just 5 minutes a day. 1hp is about 750W, and 5 minutes is about 0.083 hours, so the motor consumes approximately 62 Wh/day. Therefore; you need an inverter to support the 750W, and a battery to support the 62 Wh/day.

So when working on your system, keep your Watts and Watt hours straight.