How to Choose the Correct PV Array Size (Click here to return to Homeowners)

There are three factors that are important in sizing your PV array. The first is the amount of power that is used, the second is the amount of time that the power is used, and the third is how much sunlight is available in your area to be converted to electricity.

The first step in sizing the array is to determine how much power is needed and the best indicator is what has been used in the past. If you have not kept your utility bills, then you must contact the utility and request the usage in kilowatt-hours for the house or business for the last 12 or 24 months. The second factor to consider is the timing of usage of your power loads. The first level of analysis is to list all of the loads in the house and estimate the number of hours per day that each is used. LBA Renewable Energy supplies an Electrical Consumption Table to help to identify the loads and their typical usage and an Electrical Usage Form to record the loads and their usage times. A spreadsheet is available on our website at www.lba-renewable-energy.com for this purpose. The second level of analysis is to map your daily load usage to the time of day and a form named Time of day Electrical Usage is also on the website. This allows for times of peak usage to be calculated. The purpose of this form is twofold, first to identify periods of the day of maximum usage and second to determine any loads that are on 24 hours per day. These 24 hour loads are sometimes called parasitic because they usually don't do much besides maintain equipment in a ready state. Examples of parasitic loads are battery chargers, color TVs, computers, microwave ovens, and so forth.

A typical family might average 40 KW-hrs per day, however, one needs to subtract out the high energy demand items such as electric hot water, cooking, and air conditioning. Hot water is usually estimated to be between 33% and 40% of electrical usage and let us estimate that the other loads add up to 50%. Therefore, about 50% of our usage or approximately 20 KW-hrs per day could be provided by a solar PV system.

PV panels are rated in terms of the maximum amount of electricity generated by the panel in full sunlight for example 120 watts. Solar PV systems are sized in kilowatts, for example, 1, 2, 3, 4 or 5 KW systems. These systems would have 8, 16, 24, 32, or 40 solar panels. We will calculate the amount of electricity generated by a typical 1 KW system located in the Washington DC area and then scale it up to meet the requirements. The system produces 1 kilowatt for each hour of full sun that it receives. The National Renewable Energy Laboratory (http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/sum2/state.html ) for the Department of Energy has measured sunlight all over the country for many years and has come up with a relative measurement of sunlight called equivalent full sun hours. Sterling, VA has a yearly average of 4.7 hours per day that varies from 3.2 in December to 5.7 in June. If we take the yearly average then a 1 KW system can generate 4.7 KW-hrs per day, that is 1 KW times 4.7 hours. This number needs to be adjusted downwards for the energy efficiency of converting the direct current from the PV array to alternating current that the house uses. This efficiency number is at least 90% so the daily production is 4.7 times 0.90 or 4.23 KW-hrs per day for a 1 KW system.

Therefore, using average numbers, a 5 KW system would provide 5 times 4.23 or 21 KW-hrs per day, enough to meet the requirements of a typical family.

A 5 KW system is a large and costly system and many users do not wish to devote the roof space or dollar resources to such a large system. The cost of the solar panels is the majority of the system cost and therefore one may decide to start out with 2 or 3 KW of PV panels but 5 KW of inverter capability. In this case, the system may be expanded in the future when the cost of PV panels decreases or when wind power and fuel cells become cost competitive.

In summary, a typical system would be sized with an inverter in the range of 4 to 5 KW capability and with as many PV panels as can be afforded or can fit onto the available roof space.

Net Metering: What happens when the system is producing more electricity than is being used? The system is designed so that excess power is fed back into the utility grid and your electric meter will run backwards. The term for this capability is "net metering" and it means that the value of the electricity that you generate is the same as what you pay the power company when you buy electricity.

The bottom line is that you can never have excess capacity because of net metering all of your excess is sold back to the power company. This is a very good thing!