
You can calculate power usage of any appliance by finding a label that shows how much current (in amps or A) and the voltage (in volts or V) it uses. The amount of voltage appliance draws can vary, depending on the appliance. Maximum voltage is what you usually find on the appliance’s info panel. For example, a clothes dryer has multiple power settings, depending on the temperature setting for a given load of clothes. So voltage draw depends on the dryer’s operating temperature. Higher voltage draw is required with higher temperature.
In some cases the label tells you how many watts (W) an appliance uses to operate. A watt is a product of amp and volt (W = A * V). In this case you don’t need to look for volts or amps. Such labels are usually found on the door or the door opening of the dishwasher, clothes dryer, on the back side of the iron… In case you are having a hard time finding it on the appliance itself, you can find it in the appliance’s operation manual. Use this formula to calculate how much power it uses in kilowatt-hours (KWHRs). One kilowatt-hour equals to 1000 watt-hours. For your estimation you will need to know how many hours (HRs) per month the appliance is running.
KWHRs = A * V / 1000 * HRs or
KWHRs = W / 1000 * HRs
Keep in mind that even if the appliance’s ON switch is on, it doesn’t mean that it draws the same amount of power all the time it stays on. For example, even though refrigerator is plugged into the wall all the time, it turns on and off automatically. So, when it is ‘ON’ (you hear a light buzzing sound coming from it), it draws the power according to its label, and when it is ‘OFF’, it draws a very insignificant amount of power.
Calculations for estimating power use by the water heater are more complicated, but if you are curious enough, then read on.
To find out how much power appliance is using due to its hot water use, you need to know how many gallons of water your appliance uses, the temperature of your water heater’s thermostat setting and the temperature of cold water in your house.
Use following calculation:
(# of gallons of water) * 8.33 * (hot water temp F – cold water temp F) / 3413 = #KWHRs used
To estimate how much money it costs to operate, find out how much your electric company charges per kilowatt-hour from your energy bill.
# KWHRs * $/KWHR = $
Example:
According to the label, my dishwasher uses 120 volts and 2.7 amps and say it takes approximately 1 hour to complete the wash cycle. My dishwasher is Energy Star, so it uses 4 gallons of hot water per cycle. Since I live in Florida, I’m assuming my cold water is 80 F and my water heater’s thermostat is set at 125 F. My electric company charges 10.5 cents per kilowatt-hour. So a load of washing dishes in my dishwasher costs me:
120 volts * 2.7 amps / 1000 * 1 hour = 0.324 KWHRs
(4 of gallons of water) * 8.33 * (125 F – 80 F) / 3413 = 0.439 KWHRs
(0.324 + 0.439) KWHRs * $0.105 = $0.08