Electricity Facts and Calculations
Amperage/Amps (A) is the measure of current, which can be considered to be the volume in the circuit. The more Amps, the bigger the “pipe” (wire) has to be.
Voltage/Volts can be considered to be the pressure in the circuit: the higher pressure (voltage), the more power is forced through the wire. The common household voltage in North America is 110V/220V.
Resistance or impedance is the opposition to the pressure (voltage) and is measured in Ohms. The smaller the wire, the higher the resistance will be.
A Watt (W) is the measure of the amount of power that a device uses. This is an instantaneous value and in and of itself does not have time attached to it.
Watt hours or Kilowatt hours is the amount of electricity consumed in one hour. 1000 Watt hours = 1 Kilowatt hour (KWh), the common unit charged for on your power bill.
The relationship between these units:
Voltage multiplied by Amperage gives Watts. Volts X Amps = Watts
Voltage divided by Resistance gives Amperage. Volts / Ohms = Amps
Amperage multiplied by Resistance gives Voltage. Amps X Ohms = Volts
Watts multiplied by Hours gives Watt hours Watts X hours = Watt hours
Watt hours divided by 1000 gives Kilowatt hours Watts / 1000 = KWh
Most appliances and devices will have Voltage and either Amps or Watts stated. From that information you can use these formulas to get everything to a common unit of Watts and from that calculate Kilowatt hours (KWh) used per day.
Application to a renewable energy system:
The design of a renewable energy system requires calculating the amount of power you require. This is accomplished by making a list of your requirements based on the above information for all energy-using devices and systems in your house to come up with a total average Kilowatt hours per day. From that information we can determine how much energy production will be required as well as the size and capacity of the battery bank, inverters and other equipment. Note that a lot of equipment does not consume its rated amount of power all the time. For example, fridge compressors, water pumps and the like do not run 24/7, they cycle on and off as required. Appliances that run all the time, even though they may consume a low value in terms of Watts, can consume more Watt hours than larger, higher-Wattage equipment. A good example of this is a computer. A full size computer running 10 hours a day can easily consume 1.7 KWh per day, which requires about 850 Watts of solar modules. An energy efficient fridge might use about 1 KWh per day and thus only require about 500 Watts of solar modules.