By Emma Stenhouse, Published July 26, 2021
On average, solar panels designed for domestic use produce 250-400 watts, enough to power a household appliance like a refrigerator for an hour. To work out how much electricity a solar panel can produce in one day, you’ll need to multiply the wattage by the hours of sunlight.
The higher the wattage of each panel, the more electricity produced. By combining individual panels into a solar system, you can easily generate enough power to run your entire home.
The average American home uses 10,649 kilowatt-hours (kWh) per year, or 887 kWh per month. If you want a solar system to power your entire home year-round, you’ll need to install a system that can supply all of these energy needs.
The actual output of each individual solar panel will also depend on a range of factors including your location, local weather conditions, plus the angle and direction that the panels have been installed.
What Are Watts and Kilowatts?
To understand how much electricity a solar panel can produce, we first need to get comfortable with some units of power and energy.
If you’ve been reading about solar panels, you’ll have noticed some specific units being mentioned: watt (W) and kilowatt (kW), plus watt-hours (Wh) and kilowatt-hours (kWh). Watt and kilowatt are units of power, and indicate how much power a solar panel can provide; 1,000 watts (W) = 1 kilowatt (kW).
Watt-hour and kilowatt-hour are units of energy, and are used to show how much work (by work we mean running a light or an air conditioning unit) can be completed in one hour; 1,000 watt-hours (Wh) = 1 kilowatt-hour (kWh).
How Is Solar Panel Output Calculated?
The maximum or peak amount of electricity that can be produced by a solar panel is defined by its wattage. Remember this is measured under standard test conditions (STC) of 77 degrees Fahrenheit, 1 kW of solar radiation per square meter, and no wind. You’ll rarely find these conditions in nature, so expect your solar panel’s output to be a little less than this peak rating provided by the manufacturer.
Once you know the wattage of your solar panel, you can use the following calculation to work out how much electricity your solar panel can produce in one day:
Solar panels watts x average hours of sunlight = daily watt-hours
This calculation relies on you knowing (or being able to estimate) the number of sunlight hours your panel receives. You can either estimate this or use a solar calculator like the National Renewable Energy Lab’s solar resource maps. Let’s look at some examples:
Your solar panel has a rating of 250 watts, and your home receives six hours of sunshine per day. Multiply 250 x 6, and we can calculate that this panel can produce 1,500 Wh, or 1.5 kWh of electricity per day.
On a cloudy day, solar panels will only generate between 10% and 25% of their normal output. For the same 250-watt panel with six hours of cloudy weather, you may only get 0.15-0.37 kWh of electricity per day.
Upgrade to a 400-watt panel, and with the same amount of sunshine, you would now get 2,400 Wh, or 2.4 kWh of electricity per day. On a cloudy day, the electricity generated may only be 0.24-0.6 kWh per day.
For reference, the average American home uses 29.5 kWh per day. Install a solar power system with 20 panels of 250 watts each, and in the same six hours of sunshine, your system will generate 30 kWh, which is just enough to power the average home for one day.
Variables Affecting Solar Panel Output
In addition to the amount of sunlight received per day, there are other factors that affect the output of your solar panel or system.
Anything that builds up on the surface of your solar panel can affect the output. This can include dust, leaves, snow, or bird droppings. A clean solar panel can be 5% more efficient than a dirty and dusty panel.
Roof Direction and Angle
Solar panels are most efficient when directed in a south-east to south-west direction, at an angle of 20-50 degrees. Systems at other directions and angles can still work, but your outputs will be decreased.
Solar panels are very sensitive to shade, including trees, or a building next door. Minimal shading in the morning or evening is fine, but significant shading throughout the middle part of the day will significantly impact the amount of electricity a panel can produce.
The amount of sunshine and cloud cover will affect the amount of energy a solar panel can produce.
Time of year
Solar panels can produce electricity year-round, even on overcast days. Through summer, the days are longer which generates more output, but shorter days in winter mean your output will be lower over these months.
As solar panels age, their efficiency decreases at around 0.5% each year. The life cycle of the system is approximately 25 years before performance has decreased to the point a new system is needed.
The efficiency of solar panels is usually measured at 77 F, and temperatures above this can end up decreasing their efficiency. Solar panels can work well in cold weather, and can still generate power in snowy conditions, too.
How Much Electricity Does My Home Need?
One solar panel on its own isn’t going to create enough electricity to power your entire home, but a solar panel system can. To work out what size system you need, you’ll need to complete some basic calculations that we've covered in our article How Many Solar Panels Do You Need?
To fully power an average home using 11,000 kWh per year, a typical solar power system will need between 21-24 panels of 320 watts each. The exact number and wattage of panels, as well as the output they can produce, will depend on where you live and the setup of your specific system.
Types of Solar Panels and Output
There are three main types of solar panels used for domestic systems:
Monocrystalline. These are the most popular type of panel, made with pure silicon. They have an efficiency of 24.4%, with a moderate cost and a long lifespan.
Polycrystalline. These are made of silicon crystals that have been melted together. They have an efficiency of 19.9%, a low cost and a moderate lifespan.
Thin-film. Made with a variety of materials including small amounts of silicon, thin-film panels have an efficiency of 18.9%, with a high cost and a shorter lifespan.
The output of each type of panel will vary depending on the individual manufacturer, but will always be stated as a power rating in watts. The higher the watts, the higher the output. You may also see a kilowatt peak rating, which is the maximum power the panel can produce under the standard test conditions mentioned earlier.
Cost vs. Value
The solar market is very cost competitive, but some brands will offer you a more efficient system for a slightly higher investment. These systems will generate more electricity over the life of the system, so in the long run, they will create more value with consistent increased output.
Remember to look into federal tax credits and other incentives designed to reduce the cost of solar panel installation.