>Power, Energy and the Loads List
Power, Energy, and the Loads List
Preparation for any off-grid electric system (that is, one that's not connected to the local utility grid) begins with assessment and calculation of your energy needs. The primary tool used for this task is a Loads List.
A loads list is simply a tally of all electrical loads that will be used in the completed system. Everything from lights, to television sets, to hairdryers, to cell phone chargers must be included on your list.
The loads list provides two key pieces of information:
1. The total Energy needed, in kilowatt-hours, per day and/or per month
2. The total Power needed by all electrical devices at any given moment, in Watts.
The difference between Power, expressed in Watts, and Energy, expressed in Watt-hours, is a very important one. Power is an instantaneous measure of electrical supply; it's a rate, not a quantity. For example, a 15-Watt compact fluorescent lamp (CFL) requires an electrical power supply of 15 Watts to operate (often it is said to "draw" 15 Watts). It does not "consume" 15 Watts, since the Watt is not a measure of quantity. For quantities of electrical energy we use the Watt-hour, abbreviated Wh, and its big brother the kilowatt-hour (kWh) which is simply 1,000 Watt-hours. You may be familiar with the kilowatt-hour from your electric utility bills; it's the energy unit your bill is based on. As its name implies, the Watt-hour expresses "power across time", and can be calculated simply by multiplying the power figure (W) by the time figure, in hours; hence the name, Watt-hour. So our 15-Watt CFL uses 15 Watt-hours each hour.
