So often people ask me how to begin figuring out how much money a PV solar panel system is going to cost. Come on, it is by far the most common question asked on this website and it has been for over two years now which is how long the site has been around. So, let’s take a moment and talk about how you can figure out your solar system’s expected costs once again. The math is really quite simple.

**Steps for Calculating Solar Costs to Meet Electric Needs:**

Step 1: Figure out how many kilo watt hours or KWH’s of electricity your house uses on an annual basis.

Step 2: Take that annual number and divide it by 365 days to figure out your average daily kwh use for your home. Hopefully this number will be something that is less than 50kwh per day. If you are using more than that, you are consuming more electricity than the average household which is fine, but you should also be thinking of ways to conserve energy if possible.

Step 3: Now, we need to calculate the average hours of sunlight you have in your local area per day each year. You can use the solar sunlight data page to figure this out.

Step 4: Now that you have some of the main math figures, we can begin calculating exactly how many solar panels you can expect to 100% eliminate your electric bill.

Start with your daily kwh usage number. We will call that “X”

Next, let’s call your daily sunlight figure “Y”

So, X/Y= Average KWH Use Divided By Sunlight hours.

That new number is going to be “Z”. Which represents how many kwh the solar system must produce each hour it is working.

Let’s assume for example that Z= 10 kwh. That means the system must produce 10 kwh each hour that it has sunlight each day. That would create enough electricity to eliminate your electric bill and allow you to run on solar alone. But still, we need to figure out how many solar panels that would be.

Step 5: If we need to produce 10kwh (10,000 watt hours) each hour we still need to know how many watts each solar panel can produce. If each 200 watt PV solar panel produces 160 watts of electricity after energy loss is factored in, we can do the math of 10,000/160 to figure out how many solar panels we might need. This would equal 62 solar panels.

NOTE: Your numbers are likely to be different because your home’s sunlight hours and also your homes annual electricity use numbers are different. But, the formula above is what you need to use to gain an understanding of how many solar panels are required to eliminate your energy bill. If you have any questions, please feel free to contact adam@longtermsolar.com

there must be a mistake in this calc. somewhere I always get ~ 20 panels of 250 watts each (ignoring loses) . I think it is in the hours of sunlight. My house uses 9000 wkh per year or 24 kwh per day and it 5 hrs of sun (average) that is 5 kw or 5000 watts and at 250 watts per panel = 20 panels but you are correct I should figure in the lost of 20% to make it 24 panels required.

Hey Marc,

I think your math is pretty spot on. With the approximate need of 5kw each hour of good sunlight for you, and with factoring in the energy loss from DC to AC, yeah, about 24 of the 250 watt solar panels should come really close to meeting your needs.

Please realize that these conditions and sunlight hours are going to assume that you have ZERO shading. Any form of shade on the solar panels will decrease the output of the panels so make sure your panels are angled properly towards the sun and with no shading.

Either way, only 9000 kwh use annually means you are way below the national average which is a BIG PLUS for you!

Good luck!