Duty cycle
There are some loads that have a high power rating that are continuously connected to a power source, but that only actively consume their rated power during a certain percentage of each hour, like refrigerators. The percentage of time that an appliance operates during an hour or a day is referred to as its duty cycle. Most appliances that have a duty cycle only operate when a certain environmental condition is met like a high temperature inside of a refrigerator or a low quantity of water inside of a tank, therefore manufacturers typically do not provide an estimate of duty cycle. For a refrigerator it depends upon how the ambient temperature, how frequently the door is opened and what is being stored in inside. For off-grid system design the duty cycle of an appliance is very important, but it can be difficult to obtain an accurate value without performing a test of the appliance in the actual location. It is therefore common practice to simply use a conservative estimate.
Example 1: A refrigerator has a compressor that is rated at 160W. The appliance has been tested in a similar location and had a duty cycle of 30%. How much energy will the refrigerator consume on average during a 24 hour period?
- Energy consumption = Power rating × hours × duty cycle
- Energy consumption = 160W × 24 hours × .3 (30% duty cycle) = 1152Wh
