How ManykWh Your House Uses, and What That Means for Solar System Size
Every solar conversation should start with one number. It is on your electric bill, and most people have never looked at it. It is the kilowatt hours you used last month.
I built a free tool that sizes solar for a house from its address. The first thing I tell people is to learn their own usage before anyone tries to sell them a system. Here is how that number turns into panels.
kW and kWh are not the same thing
A kilowatts a rate. It is how fast power is being made or used right now. A kilowatthour is an amount. It is one kilowatt running for one hour.
Your billcharges you for kilowatt hours. A solar system is sold in kilowatts. The wholesizing job is turning the first number into the second.
Readtwelve months, not one
Pull up afull year of bills. Write down the highest month and the lowest month.
A typicalAmerican home uses somewhere around 875 kWh a month. A hot climate houserunning two AC units can hit 1,500 to 2,500 kWh in July. If your bill swingsfrom 600 kWh in spring to 2,000 in August, you have an AC house. The systemshould be sized with the summer in mind and not just the average.
How big asolar system does my house need?
Take yourmonthly kWh and divide by 30 to get a daily number. Divide that by your peaksun hours. The answer is roughly the kilowatts of panels you need beforelosses.
Here is themath on a typical home. 875 kWh a month is about 29 kWh a day. Use 5 peak sunhours as the example. 29 divided by 5 is about 5.8 kW.
A modernresidential panel is about 400 to 440 watts. At 400 watts, 5.8 kW is about 15panels. Real systems lose a little to heat, wiring and dust. That is why mosthomes land between 15 and 25 panels.
Sun hoursmatter. North Carolina averages about 4.5 to 5 peak sun hours a day. The desertgets 6 or more. The same house needs fewer panels where the sun is stronger.
Do notsize it to zero on paper
Mostutilities now pay only a few cents for the power you send back to the grid.Then they sell it back to you at full price at night.
So a systembuilt to make every kWh you use on paper is not always the best deal. Size itto what you use. If you want to cover the evening too, price a battery. Abattery lets you keep your afternoon power for the night instead of selling itcheap.
Then checkwhat your roof can hold
Here iswhere math meets the house. Your usage says you need 20 panels. Your roof mighthold 14 on the faces that get sun.
Direction,shade, vents and fire setbacks all eat space. South is best. West is closebehind. A big tree on the south side can take a quarter of your production.Nobody can eyeball the panel count from the driveway.
If your rooffits 16 panels and you use 1,800 kWh a month, solar will cut your bill. It willnot erase it. Anyone who tells you otherwise is selling.
Get thenumber before the salesperson
That is whyI built my tool. You type in your address. It reads the roof from satelliteimagery and lays out the panels on the faces that make sense. It shows system size, estimated production and a cost breakdown. It also shows your staterebates and incentives, payback and 25 year savings.
No phone number. Nobody calls you. It is free and homeowners never pay. Installers pay to be on the site. If you want bids after you see the numbers, you ask, and no more than two licensed installers get the request.
The estimate is not the quote. The installer still comes out, inspects the roof and confirms it. But you walk into that meeting knowing your kWh, your system size and what it should cost. You can try it at solarroofprint.com in a few minutes.
Know your usage. Know your roof. Then put your address into Solar Roofprint and see what your own kilowatt hours turn into before anyone sets up a laptop at your kitchen table.