Shade, latitude, clouds, the size of the solar panel, something called solar irradiance: It all plays a role.
Like many Americans, you might be asking yourself about making the switch to solar and whether or not it makes sense for you. Will solar panels keep your lights on during a blackout? (Probably not, unless you have a battery.) Will solar panels save you money over time? (How much and how quickly depends on your situation.) And how much electricity will a solar panel on my roof even make?
To answer that last question, we need to know about your solar panels, your home's location and anything that comes between your panel and the sun.
Any reputable solar installer will be able to give you an idea about how much electricity your solar system will be able to produce. Some are starting to guarantee certain levels of production. But if you want to dig into it yourself, here's everything you need to know about how much energy solar panels can produce for your home.
At the most basic level finding how much electricity a solar panel will produce is a simple matter of multiplying its size by how much sunlight it gets.
"To calculate how much output a solar panel generates, consider the wattage and solar irradiance the panel receives," said Neil Gallagher, vice president of Brighterway Solar, a Florida solar installer. "The wattage rating is the maximum electricity the solar panel can generate under ideal conditions. In contrast, solar irradiance is the amount of sunlight energy that reaches the panel's surface."
Residential solar panels commonly come with wattage ratings up to about 400 watts. The National Renewable Energy Laboratory provides solar irradiance maps that cover North and South America by year and month. You can also find maps for the entire planet.
"Solar irradiance can be easily determined using various online tools available. To calculate the electricity output of a solar panel, multiply its wattage rating by the solar irradiance. The resulting value will estimate the energy output in kilowatt-hours per day. However, it is vital to remember that the daily energy output can vary due to temperature, panel degradation over time, and system losses," Gallagher said.
NREL will also do the work for you with its PVWatts calculator. You just need to put in your address.
Several environmental factors can affect -- for better or for worse -- how much electricity solar panels produce. Here are the key factors that can affect the electricity production of a solar panel.
Weather conditions, specifically cloud cover, can significantly affect solar panel production. The density of cloud formations in the atmosphere, especially on overcast days, can reduce the solar irradiance reaching the panels, decreasing the energy captured by the solar panels. While solar panels can still generate electricity on cloudy days, energy production is less efficient and occurs at a reduced rate. This is particularly true when there are intermittent periods of cloud cover throughout the day.
Latitude tells us the north-south position of a point on the Earth's surface. It's measured in degrees and ranges from 0 degrees at the equator to 90 degrees at the North and South Poles. Locations closer to the equator receive more direct sunlight throughout the year, which leads to higher solar energy generation. Locations at higher latitudes experience reduced solar irradiance due to the sun's lower angle in the sky. This means that sunlight has to travel a longer path to reach the solar panels through more of the Earth's atmosphere, resulting in lower energy production. While solar panels have been installed and are operating from the equator to the poles, you have to take latitude into account if you want a precise idea of how much your solar panels will produce.
When a solar panel is partially or fully shaded, that shade creates areas of high resistance within the panel where heat accumulates, known as hotspots. Hotspots result in reduced efficiency, decreased power output and potential long-term damage to the affected solar cells or the entire panel. To maximize the production of solar panels, install them in areas with minimal shading.
In cases where shading is unavoidable, solar panel installers often mount panels fitted with bypass diodes or microinverters to mitigate the effects of shading. However, some startups manufacture solar panels capable of performing at near-optimal levels even in shaded conditions.
Heat can diminish the efficiency of solar panels. While solar panels will perform optimally in direct sunlight, if the temperature rises significantly, it can affect the performance of solar cells, reducing their electricity production capability. Experts have determined that solar panels operate efficiently in areas with moderate temperatures. Solar panels are rated on how much their efficiency changes per degree over 25 degrees Celsius (77 degrees Fahrenheit).
Depending on its specifications, your solar panel system might generate more electricity than you can consume during the day. However, when night falls and there is no sunlight, the panels won't produce enough electricity to meet your energy requirements. To address this imbalance, you'll draw energy from the grid, though you can install a battery to store some of your excess solar electricity to use later.
Solar batteries are an excellent backup solution for your solar system and your connection to the utility grid. They provide a reliable and resilient power supply for power outages or low solar panel production. However, battery systems can be expensive -- about $10,000 for a 10 kilowatt-hour module. Therefore, it is crucial you fully understand the financial implications of installing a battery unit in your home.
Gallagher of Brighterway Solar said it's essential to have your solar installer or vendor assess your electricity usage to determine whether it makes sense to incorporate a battery into your solar system. By examining your energy consumption, professionals can provide valuable insights into the feasibility and cost-effectiveness of installing a battery unit.
Most residential solar installations are put in without batteries, though the percentage of solar systems with batteries attached is increasing quickly (nearly 9% in 2021). California recently reworked its net metering program, making buying batteries a more attractive option. Whether or not a battery makes sense for you depends on how you're charged for energy, how your utility pays for your excess solar energy and your own energy usage.
"In certain regions, the local power company implements a 1-to-1 net metering credit system, where you receive payment at the same rate per kilowatt hour as the utility company charges you. If the credit from the electric company is lower than the rate they charge you on your bill, then investing in a battery might be a worthwhile consideration," said Gallagher.
Time-of-use rates, where you pay more for energy at different points during the day, can also make batteries a better financial choice, since you can use free stored energy instead of pricier energy directly from the grid.
Evaluating factors such as electricity consumption, net metering and time-of-use rates can help you determine whether incorporating a battery into your solar installation will be financially advantageous.