PV Energy Production

Power and Energy Output

When discussing a solar electric system, we talk about production in terms of both 'power' and 'energy,' and it is important to understand the distinction between these two terms. Power, measured in watts (W) or kilowatts (kW), is instantaneous. At any given moment, the array will produce a certain amount of power, and the capacity of the system is rated based on the amount of power it would produce under 'peak sun conditions.' For instance, a 1 kW array getting lots of sunlight without overheating will produce 1,000 W of electric power. This is enough to run 77 energy-efficient light bulbs. However, since the amount of power the system is producing varies depending on the amount of sunlight it receives, it is important to be able to estimate and then measure the amount of energy produced over  a certain period of time.

Energy is the measure of power over time (Energy = Power x Time). For example, if a 14w Compact Flourescent Light was on for 2 hours, then we can say that light bulb used 28 watt-hours (Wh) of energy (14w x 2 hrs = 28 Wh). In general, we pay for the electricity we use in our homes and businesses based on kilowatt-hours used each month, one kilowatt-hour equals 1000 Wh. Installing a photovoltaic array reduces the amount of electrical energy that we need to take from the grid. An average U.S. home uses about 10,000 kWh per year . Under normal circumstances, a 1 kW PV array installed in Portland, OR will produce about 1,100 kWh per year. Therefore, this array would supply roughly 10-15% of an average U.S. household's annual electricity needs. It is also important to understand that in many other parts of the world the average electricity use is far less than in the U.S. And for many homes, by taking simple energy efficiency and conservation measures, annual electricity use can easily be half of the U.S. average, or less.

Estimating Energy Production

The most obvious variable that affects the amount of energy produced by a solar array is the amount of sunlight that hits the modules. On long summer days with bright sunshine, a photovoltaic system will naturally generate more energy, than a short cloudy winter day.  Different parts of the country gets different amounts of sunlight over the course of the year. To get an estimate of how much energy a system in your area would generate, you can use NREL PVWatts v.2 Viewer, an online calculator created by the National Renewable Energy Laboratory (NREL).

Geography and weather aren't the only factors that impact the amount of sunlight that hits the array, however. Another variable is the tilt and orientation of the system – whether the modules are set up in such a way as to absorb the most sunlight possible. In general, the ideal set-up for an array will be with the modules facing due south, tilted at an angle equal to the latitude of the site's location. One more important thing to keep in mind is the affect of shading. A photovoltaic system should be installed in a place that gets as little shade as possible from nearby buildings and other obstructions throughout the course of the day.

Another, often unconsidered factor is simply the cleanliness of the panels. Something as rudimentary as keeping the modules free from dirt makes a difference in how much energy a system produces. 

And finally, the temperature of the photovoltaic cells will affect their productivity. Solar modules become less efficient as they heat up.