How Many Solar Panels Do I Need?

Converting Kilowatt-Hours to Panels

Now another website comes into play: the California Solar Initiative’s EPBB calculator. This site was designed for solar installers, but you can use it as well. And if you understand this calculator, you’ll be a smart customer.

The website is

The page you will get is:

Here’s the same page, with the important text enlarged:

In the Site Specifications portion, three of the cells are already filled-in correctly. You can give the project any name you want. Then enter the zip code for the property you’re analyzing. (We’re using 94920.)

The next portion, PV Site Specifications, is intimidating, in that you need to know the exact hardware you’re interested in installing. Notice the drop-arrows at the far right of three of these cells — that means that there’s a drop-menu with selections. Warning: these menus are long — listing every panel (the site calls them “modules”) and every inverter approved for sale in California. While, for the purposes of this demo, we could just stick with the first choice in the dropdowns — 1SolTech and Ablerex — let’s use the drop-menus to select hardware that’s more likely to be proposed. Note: no solar salesperson is familiar with all the hardware listed. Instead, it’s typical for a sales representative to be knowledgeable about one or two familiar brands, and perhaps a discount brand.

For the panel, I’ll look for a Sharp. In the drop-menu, there are dozens of Sharp panels. So let’s choose one of the better sellers — a Sharp 235 watt.

The next piece of information asked for is the number of panels. At this point, I’ll mention that, one weakness of using this site to quantify the number of panels is that we’re forced to work backwards. What I mean is, we already know how much power, in kilowatt-hours-per-year, we want, but, with this site, we’re not going to get the kilowatt-hours-per-year UNTIL we select the type and number of panels. So you’re probably going to get stuck doing a few iterations. But this site is so powerful – incorporating weather data, etc., that it’s worth it. Plus – there’s a button at the bottom of the results page that allows you to return to this data-entry screen, and change just the cells needed, so iterations are not difficult.

ANYWAY — we need to select how many panels we’re proposing. I’m going to propose “16” since it’s a common number for medium installations, and since even numbers allow matching rows — which are more attractive.

Next there is a dropdown for Mounting Method, with the choices being the number of inches the panels are off the roof.

Heat reduces electrical production, so most installers will propose the higher standoffs (the supports that hold the panels off of the roof). Let’s leave this cell at >6” average standoff.

For the inverter, I could explain how to multiply the wattage of the panels by the number of panels, then how to derate that number, but instead I’ll suggest that we select Enphase inverters. Enphase inverters are atypical, in that, instead of a single inverter for the system, one inverter is attached next to each panel. The cost is always a bit more, but there are advantages, including a longer warranty. And for this exercise, Enphase is useful, in that, if we need to do a few iterations and change the number of panels, the inverter size calculation can be skipped — we’ll just increase/decrease the number of Enphase inverters to match the number of panels.

In the dropdown menu for inverters, I’ll select the Enphase model 215’s — their model with a 25-year warranty:

For Number of Inverters, I’ll enter “16” to match the number of panels:

Shading is complicated, so, for this exercise, let’s assume that this property is away from trees and tall buildings.

Array Tilt is asking for the slope of the panels, in degrees. By chance, the slope of most shingle roofs is almost ideal for solar panels. (Learn more about tilt and other factors influencing panel effectiveness at Common shingle roof slopes are 4:12 (18 degrees) and 5:12 (22 degrees). Few homeowners know their roof slope, but the number doesn’t make that much of a difference. For our calculation, I’ll use 18 degrees.

Array Azimuth is asking for the direction of the panels. South is the gold standard, but southeast and southwest are also very desirable. Let’s say our best roof is not perfectly south, but rather southwest. (We used the compass on our iPhone to determine that it faces 225 degrees true — not magnetic — south. So enter “225.”)

We’re done. Hit “go” and the calculator will produce two pages full of information. The results look like this:

These two pages are attached, as PDFs, here.

Let’s skip to the top of the second page:

That’s it — the number we wanted: Annual kWh. Per an official website, our 16 panels, installed at a southwest orientation with an 18 degree slope, will produce 5,577 kilowatt hours per year.

Let me throw a wrench into the calculation: though the CSI-EPBB is a government website that is used to calculate your state rebate, “actual results may vary.” And they do. This CSI-EPBB calculator derates (that is, “penalizes”) for factors including location, panel brand, orientation, etc., but it doesn’t derate for every single factor that gets in the way of optimal production. If it’s critical to reach a certain level of production, play it safe, and add an extra panel or two.

Back to our electrical usage:

We determined that we’d like a system to replace all of the tier-3 and tier-4 kilowatt-hours, and calculated that that would need to produce 5,412 kilowatt-hours of electricity per year. The 16 panels proposed in this first iteration meet our needs almost perfectly: 5,577 kWh = 103% of 5,412 kWh.

There are lots of other numbers on this printout, but the number you DON’T have here is the cost. That’s because these sites have no idea how much your local installer is charging for the hardware, and how much labor is costing him, and what his markup is. Let’s take a shot at that:

The Bid