# How to Estimate Load on a Photovoltaic PV System

Updated on May 7, 2010

One of the keys to designing a photovoltaic system is understanding how much energy demand is required. The energy demand is also called the load on the PV system. The unit measurement for PV load is watt-hr per day.

Step 1. Take an inventory of all of the demand use electric which includes appliances, lights, and other electronics that the PV system will power. Demand use electronics only use power when turned on. Record the total number of each electronic device, the number of volts, and the amount of amps of each item. Estimate the number of hours per day each electronic device is used.

Step 2. Take an inventory of all of the phantom electric devices using power. Phantom electronics use power anytime they are plugged in to an outlet. Some examples of phantom power devices are clocks, computer equipment, televisions, and video games. Record the total number of each electronic device, the number of volts, and the amount of amps of each item. Most electrical devices except dryers typically run on 120 volts in the United States. Estimate the number of hours each device is actually used and not used.

Step 3. Calculate the number of watts by multiplying the number of both the demand use and phantom electric devices by the number of volts and the amount of amps of each device. An example of calculating the number of watts needed for 10 compact fluorescent lights is:

10 x 120 volts x 0.125 amps = 150 watts AC

Perform this calculation for all the electrical devices that the PV system will power.

Step 4. Calculate the average watt hours per day for each electrical device. Multiply the number of watts calculated in Step 3 by the number of days used each week and the number of hours used each day, then divide by 7 to get the average. Using the example in Step 3 assuming the lights are used for 5 hours per day, 7 days a week:

150 watts x 5 x 7 / 7 = 750 watt hours / day

Perform this calculation for all the electrical devices that the PV system will power. Total the number of all devices to get the average number of watt hours needed per day. This is the required load on the Photovoltaic (PV) system. A typical house will be in the range of 4000 to 9000 watt hours/day, depending on the size and number of people living in the home.

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