Rooftop Solar refers to privately owned solar energy systems on residences.
A Common Misunderstanding About Rooftop Solar
Solar panels on your roof do not power your house directly, otherwise you would have no power at night, right? All of your electricity comes from the power company. Your solar panels pay off the bill during the day. The energy generated by your solar panels is accounted for by your electric meter and is credited to your bill.
A Sense of Scale
- Solar panels produce 200-230 watts of power per square meter.
- It takes 16-20 solar panels to power the average home.
- About 5 solar panels are needed to power one wall outlet.
- One square mile of solar panels would power 58,000 homes.
Grid Tied Solar
When you have power lines to your house and you pay an electric bill, you are “grid tied”. All of your electricity comes in from the power company on the grid. Have you ever wondered how solar panels keep your lights on at night? They don’t. The grid does.
On sunny days, solar panels pay for part, or all, of your electric bill. Think of it like a credit card. You buy power from the grid, and the solar panels pay for it. If you have enough solar panels, over time they will pay for all of your electricity.
Net Metering
When you generate more electricity than you use, it is available to the grid for use by anyone. Your meter runs backwards when you over-produce, so you end up paying only for the “net” amount you buy from the grid. Think of it like a credit card. You charge things like buying electricity at night, then pay it off by producing excess electricity the next day. Because the amount of energy you produce is not tracked, it is a dollar for dollar exchange for every kilowatt you produce. This is net metering.
The Electrical Grid
Our current grid was designed to run with large central power plants distributing energy outward. Security measures were designed in to prevent the back-feeding of electricity from unknown sources.
Too Much Rooftop Solar Is Beginning To Be a Problem
As more and more homes install solar, it creates a problem for the power companies. In many cases, rooftop solar creates more energy than the home uses, for instance on a sunny day when the house is empty.
Electric utilities are beginning to limit the size of system you can install on your roof. This is not because they don’t want you to make your own power, it is because they have no way of managing the excess power when too much rooftop solar electricity is back-fed into the grid.
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Solar Insolation
Solar Insolation is the term for the amount of solar energy reaching the surface of the earth.
- The average amount of solar energy reaching the surface of the earth is 850 watts (1.1 hp) per square yard.
- Solar panels convert 15%-20% of that into electricity = 150-190 watts (0.21 – 0.25 hp) per sq yd.
- Single panels of 325 watts (0.38 hp) are available.
Some utilities install an additional meter and separately track the excess you produce. They then credit your account at a very reduced rate. It’s like a really high interest credit card. If you buy $10 worth, you have to pay $30 to get back to zero. This is not net metering.
Off Grid Solar
In remote parts of the world, solar energy can provide electricity where there are no power lines. The energy must be stored in batteries for use at night. Batteries are very expensive. In the event of several days of no sun, you will need a large bank of batteries. For an average house, it may cost $10,000 or more for a battery big enough for one day.
How Many Solar Panels Do I Need?
A solar installer will do this math for you, and there are websites that enable you to calculate it yourself. An adequate system for an average home would be 12-24 panels or 3 kW to 6 kW. A rooftop solar system should cost $2,500-$3,000 per kilowatt.
If you want to understand the math and check your own estimate, here’s how.
- Gather your electric bills from the last 12 months. If you don’t have them, ask your power company for them, or find them in your online payment account.
- Add up the monthly number of kilowatt hours you paid for over the year.
- Average homes consume 8,000-10,000 kWh per year.
- The average annual bill can range from $1,000 to $2,500
How much solar energy do I get per day where I live?
This map will tell you the average amount of solar energy per day in your region. The range is 4.00 to 5.75 kWh per day. Solar panels will convert about 15% of that sunshine into electricity.
Here is the calculation for the number of panels you’ll need to generate 100% of your annual electricity needs.
Set The Variables.
- 10,000 kWh consumed annually
- 4.5 kWh per day per square meter
- Solar panel efficiency = 15%.
- Area per solar panel = 1.7 square meters
Calculation
- 4.5 kWh X 365 days = 1,642 kWh. This is the total amount of solar energy per square meter that reaches this site per year.
- 1,642 X 15% = 246kWh. This is the total amount of energy that a one square meter solar panel can convert to electricity per year at this site.
- 10,000 kWh ÷ 246 kWh = 41. This is area of solar panels in square meters that are needed to generate 10,000 kWh in ayear year.
- 41 ÷ 1.7 sqm = 24. The average area of a solar panel is 1.7 square meters.
Individual Panel Output Required
- 246 kwh per yr/sqm ÷ 365 = .673 kWh per day/sqm ÷ 4.5 hours/day = .15 kWh per sqm
- .15 kWh X 1.7 sqm solar panel area = .25 kW or 250 watts.