How Much Electricity Does a Solar Panel Produce?

Solar energy has become a powerful solution, not only to fight climate change but to reduce the cost of electricity. Many Americans are taking advantage of rooftop solar systems to help them become more energy independent in producing their own electricity and bringing down the cost of their utility bills. Furthering your knowledge of how much electricity a panel or a whole system can make is essential in understanding the dynamics of how much you can save by installing solar.

Of Watts and Kilowatts

The potential of a solar panel to produce energy is expressed in Watt (W). It’s a measure of power, similar to horsepower on a motor. The frequently used Kilowatt (KW) reflects 1,000 Watts. The actual energy derived from a solar panel is noted in kilowatt-hours (KWh). This number indicates the KW produced in one hour. It is also used on the consumption side; for example, with 2 KWh of energy, you can run your dishwasher for one load. Your refrigerator will only consume about 2 kWh per day. An average household in the U.S. consumes about 29 kWh per day.

Panel size

We most commonly see two different panel sizes in residential settings, the 72-cell, and the 60-cell panels, at approx. 39 x 77 in. and 39 x 66 in., respectively. The larger panels house more solar cells and, therefore, generally produce around 20% more energy.

Panel efficiency

You will often see solar panels advertised with their efficiency. Basically, this means how powerful they are per square inch. More efficiency means more watts produced with the same amount of cells. So a 60-cell panel that can produce 300 W is less efficient than a 60-cell panel producing 350 W. Higher efficiency regularly means a higher price tag. You will have to figure out whether the higher efficiency is worth it. The trade-off is space; if there is plenty, then lower efficiency panels work just fine.

The bottom line – panel wattage

At the end of the day, it’s most relevant to know how much energy each panel can produce. For simplification, there is an industry standard for measuring the panel wattage, called STC, making it easier to compare panels from different manufacturers. Most recently, 60-cell panels range from about 335 – 385 W, 72-cell panels from about 405 – 480 W. The higher nomination in each segment indicates a more efficient panel.

How is energy measured?

Taking an average 60-cell panel with a nominal 360W, exposing it perpendicular to the Southern California sun on a bright day, we should be seeing 360 W coming from that panel each hour, adding up to 2.16 KWh in a day. Just about enough to run the fridge and a few lights. By getting 10 of these panels, you can produce up to 21.6 KWh in ideal conditions, enough to run the AC on a warm summer day.

Factors influencing solar energy production

Unfortunately, we rarely face ideal situations like that. The energy generation of solar panels fluctuates substantially based on various external factors. Taking those into account is the specialty of most any solar installer, which should be running calculations for your specific situation. You will often need more than just one opinion to fully understand how much a system or a single panel can generate on your home.

Hours of Sunlight

The most crucial aspect of generating solar energy is sunlight for apparent reasons. It is not a secret that much more of that is available in the Southern States, such as California, Arizona, or Texas, making solar systems more productive. But solar has excellent potential further North as well. While Southern Arizona is blessed with about 6 hours of perfect sunlight on an average day, Boston is looking at around 4 hours of horizontal irradiance.

Shading

Close-by buildings and trees may throw shades onto your panels for certain times of day and in certain seasons. The sun’s angle can play a significant role, so changes throughout the year are the norm. Shading is a complex topic. To determine energy production of shaded areas, extensive modeling may be necessary. In general terms, shading in the middle of the day is worse than in the morning and evening since peak production happens mid-day. Shading also affects generation significantly in summer, when the panels are in high production mode.

Positioning, Orientation, Angle

Another factor influencing production is the positioning of the panel. An ideal angle would position the panel perpendicular to the sun. Most roofs with slopes between 30-40 degrees work well for this purpose. Energy losses due to sub-optimal angling are often minimal.

More relevant is the location of the panels on the roof. Best results are achieved by positioning the panels on the south-facing part of the roof. Some panels may have to be located on the west side or the east side if space is tight. The production of those panels will be about 15% lower. Having panels on the north-facing side of a roof is generally not recommended, as the energy loss is around 30%. Shading may further complicate the positioning of the panels.

Production loss over time

Solar panels lose some of their power over time. The most commonly quoted annual production loss is 0.5%. After ten years, your solar system will have lost about 5% of its production capacity. A single 360 W panel goes down to around 342 W after 10 years and approx. 324 W after 20 years. The total lifespan of solar panels is commonly estimated to be 25-30 years. After which, they can be re-purposed for other uses or recycled to manufacture new panels.

Concrete examples with energy production numbers

Again, the actual production of your panel or system will depend heavily on the external factors at your location. We have put together a few examples that give more insight into the environmental effects on energy generation.

Production loss over time; perfect Southern California conditions, without shading

A sunny location like Southern CA, AZ, NM; marginal shading; panels partially facing West or East

A north-east location like New York or Boston; partial shading; panels partially facing West or East

Conclusions

While the average peak sun hours in southern CA or AZ are around 6 per day, they drop to only 4 further north. The productivity of solar panels is therefore 1/3 less in those areas. Solar maps (https://www.nrel.gov/gis/solar-resource-maps.html) can help you determine the peak sun hours at your location.

Have a specialist help you figure out the best system design and positioning of the panels, based on shading if applicable. After all, that’s how you can optimize your system’s energy production.