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We always advise speaking with at least a few certified solar installers to understand how all the factors will affect solar panel output for your system.
Solar panels indicate how much power they intend to produce under ideal conditions, otherwise known as the maximum power rating.
But how much electricity your solar panels produce depends on several factors.
Because the seasons and weather conditions affect the amount of sunlight hitting your roof, and the amount of sunlight also varies on the time day, you can&#;t use just the solar panel ratings to predict how much power you&#;ll get. However, your location will allow you to do some math and determine how well a solar panel works where you are.
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A kilowatt-hour is a basic unit of energy, which is equal to power ( watts) times time (hour). Your electric bills show how the average number of kWh you use per month.
For example, a 50 Watt light bulb left on for one hour would be 50 Watt hours, and 20 50 watt light bulbs running for one hour would be 1 kilowatt-hour (kWh). According to the U.S. Energy Information Administration, the average monthly electricity consumption for a residential utility customer is about 903 kWh per month.
Divide your average monthly usage by 30 days in a month to get your daily usage. If you&#;re going by the national average, then you should be using about 30 kWh per day. Next, figure out the average amount of sunlight you get per day. The US ranges from about 4 hours &#; 6 hours of sunlight per day, on average, see the below map. Let&#;s estimate you get about five hours per day to generate that 30 kWh you use. So the kWh divided by the hours of sun equals the kW needed. Or, 30 kWh / 5 hours of sun = 6 kW of AC output needed to cover 100% of your energy usage.
This depends in part on the amount of electricity you want to offset with solar power as well as the question &#;how much energy does a solar panel produce&#;, so in order to get more specific let&#;s talk about the actual number of solar panels.
Related: How many solar panels do I need?
Typically, a modern solar panel produces between 250 to 270 watts of peak power (e.g. 250Wp DC) in controlled conditions. This is called the &#;nameplate rating&#;, and solar panel wattage varies based on the size and efficiency of your panel. There are plenty of solar calculators, and the brand of solar system you choose probably offers one. That said, there is a simple equation to calculate the amount of kilowatt-hours (kWh) your solar panel system will produce.
So now that we know you need to produce about 6kW of AC output, we can work backwards to figure out how many solar panels you need. Solar panels produce direct current (DC), and your home runs on alternating current (AC).
Yep, like the band, AC/DC.
Because of physics, there are losses in converting the energy from the sun into DC power, and turning the DC power into AC power. This ratio of AC to DC is called the &#;derate factor&#;, and is typically about .8. This means you convert about 80% of the DC power into AC power.
This continues to improve ever so slightly, but the losses are unavoidable&#; because of physics! So you take the AC amount you need: 6kW and divide by .8 (6kW/.8 = 7.5kW DC). This means that you&#;ll need 30 250Wp solar panels or 27-28 270Wp panels.
Finally, let&#;s find out how much you can save per month on average from your monthly electric bill!
Let&#;s plug it all in:
On average, your solar system is going to lose some energy due to wiring, power, inverter efficiency, so you actually end up using 80% of your solar system&#;s capacity.
To figure out how many kilowatt-hours (kWh) your solar panel system puts out per year, you need to multiply the size of your system in kW DC times the .8 derate factor times the number of hours of sun. So if you have a 7.5 kW DC system working an average of 5 hours per day, 365 days a year, it&#;ll result in 10,950 kWh in a year.
If you divide your expected 10,950 kWh of annual production by 12, you&#;ll see that your system will offset about 912 kWh per month from your monthly electric bill, which can translate to $100 or more (in California this would save you about $250) per month depending on how much you pay per kWh!
So to break this down into simple math that you can do:
AC rating = Average kWh per month / 30 days / average sun hours per day
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example: 903 kWh per month / 30 days / 5 hours = 6.02 kW AC
DC rating = AC rating / derate factor (.8 is conservative, but a range would be .8 &#; .85)
example: 6.02 kW AC / .8 = 7.53 kW DC
Number of panels = DC rating / Panel Rating (e.g. 25o W) *note this is important b/c panels are rated in watts, and the systems are rated in kilowatts ( watts). So a 7.53 kW system = Watts and a 250 watt panel = .250 kW
example: 7.53 kW x / 250 watt = 30.12 panels, so roughly 30 250 panels (30 x 250W = Watts = 7.5 kW)
NOTE: to get your average usage, preferably add up your last 12 months usage and divide by 12. In a pinch, the last 6 months can be a close approximation, but a year&#;s worth of data is far better.
Have you calculated how much your solar system will produce? Tell us in the comments!
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Image Credit: via FlickR under a Creative Commons license, By NREL [Public domain], via Wikimedia Commons
You can calculate how many solar panels you need by dividing your yearly electricity usage by your area's production ratio and then dividing that number by the power output of your solar panels.
To put it simply:
Number of panels = system size/production ratio/panel wattage
For example, 17 to 25 panels = 10,791 kWh / 1.1 or 1.6 / 400 W
Let's break that down a bit:
Calculating how many solar panels you'll need to meet your energy needs depends on several factors. The easiest way to find out how many panels you'll need is to use our Solar Calculator. When you put in your address and estimated monthly power bill, we'll do all of the math for you so that you can make an informed decision. If you'd like to do those calculations, we've explained our formula below to help.
The formula we used to estimate the number of solar panels you need to power your home depends on these critical factors. Here are the assumptions we made and how we did our math:
Your annual electricity usage is the energy you use in your home over a year. Measured in kilowatt-hours (kWh), this number is influenced by the appliances in your home that use electricity and how often you use them. Refrigerators, air conditioning units, small kitchen appliances, lights, chargers, and more all use electricity.
According to the U.S. Energy Information Administration (EIA), the average American household uses 10,791 kWh of electricity per year (or about 900 kWh per month), so we'll use that number as the ideal solar panel system or solar array size, which would mean you could offset 100% of your electricity usage and utility bill with solar panels (in practice, it's not this neat, but bear with us here). To get a more exact number based on how much energy you use, check last year's electric bills to determine your average electricity consumption. Once you have that number, plug it into the equations below.
Also known as a solar panel's power rating, panel wattage is the electricity output of a specific solar panel under ideal conditions. Wattage is measured in watts (W), and most solar panels fall in the 300 &#; 400+ W of power range. We'll use 400-watt panels in these calculations because 390-400 W is the most quoted capacity range on the EnergySage Marketplace, according to our latest Intel Report.
A solar panel system's production ratio is its estimated energy output over time (kWh) to its actual system size (W). These numbers are rarely 1:1 &#; depending on how many hours of sunlight your system will get (primarily based on your geographic location), your production ratio will change accordingly.
For example, a 10 kW system that produces 14 MWh (14,000 kWh) of electricity in a year has a production ratio of 1.4 (14/10 = 1.4) &#; this is an entirely realistic production ratio to see out in the real world. In the U.S., production ratios are usually between 1.1 and 1.6, so we'll use those two numbers as the high and low estimates for our calculation.
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