Whether rooftop solar is a sensible investment depends on:
• Details of grid power tariffs in your locale — in progressive jurisdictions, you may be paying very high rates as part of a conservation/soak-the-rich scheme to charge much more for higher use, and subsidize lifeline use (very small houses, no AC, few appliances.)
• Site characteristics, including local shading by nearby trees or buildings, the practical tilt your roof allows. Few roofs are optimally aligned with the correct E-W roofline and angle so an optimal array of panels can collect maximum solar energy. Closely-spaced panels on flatter roofs will shade each other unless tilt is decreased and spacing between panels increased.
• Weather: clouds and fog, dust and rain cut down insolation (amount of solar energy hitting the surface), and dust or snow on the surface of solar panels either must be cleaned off or lower production accepted.
Online calculators that take solar angles and local climate records into account can give you a rough idea of how much power a rooftop solar installation will produce. One of the best is the National Renewable Energy Lab’s PVWatts Calculator. First look up the approximate latitude of your site, then look up the optimal tilt of panels for that latitude here. If your roof is ideally aligned and angled, you can use the optimum fixed tilt for your location, or substitute an angle required by your roof or siting issues. I’ll go through using the calculator for the site we used near Palm Springs, which has almost ideal solar conditions, then repeat for the same house in Seattle, which has fewer cloud-free hours and lower solar intensity even under clear conditions since at higher latitudes the sun is lower in the sky and solar radiation has to travel through more atmosphere to reach the panels. Not to spoil the surprise, but the combination of lower grid rates and much less power production from panels due to much less sun will demonstrate that rooftop solar is not cost-effective in Seattle now, and won’t be until grid rates rise and panel prices fall substantially from here.
The Palm Springs Case
First, enter the site address so the calculator can pick up the nearest climate and solar data:
Then you enter some details of the planned installation — in this case, 66 panels generating 360W each for about 24KW total power, “Premium” type (we used the current industry-leading Sunpower X-Series panels.) “Azimuth” is set to 180° for exact south-facing alignment, “Tilt” at 15° as installed, and “Average Cost of Electricity” (from the grid) at 20c/KWh — as we’ll see later, utilities in progressive areas have complex tariff schedules that make this number hard to calculate, and even more complex ToU (Time of Use) and moment-by-moment charging schemes are on the way. Since we’re simplifying this just to get an idea if solar comes close to being cost-effective, I’ve selected 20c as an average cost for high users in Palm Springs — actual peak rates are much higher.
For our own project, a flat roof with limited area meant keeping the tilt angle of the panels less than the optimal 28° — higher angles would have rows of panels shading the next row, so a lower angle (15°) was chosen.
The outcome is shown below: about 40,000 KWh/Year generated, saving about $8,000 a year in grid power bills. Since this installation cost about $65,000 after subtracting the Federal tax credit, it is expected to return about 12% of its cost yearly and pay for itself in about 8 years — neglecting some minor cleaning and maintenance costs and assuming little degradation in production as the panels age, which is close to correct for these premium panels, which are guaranteed for 25 years. This is one of the highest-return investments you can make, under these almost-ideal conditions. Note less costly thin-film panels cost less but also degrade faster.
The Seattle Case
Now we set the calculator up for a similar installation in Seattle:
Now to give the Seattle case the optimal tilt angle of 39° is entered — this works well if the Seattle house has a south-facing roof at a 39° angle to start. Few real Seattle houses will be so ideal for rooftop solar., and most will have local obstacles like trees and nearby obstacles like hills and other roofs cutting down on insolation. But we’re supposing the best imaginable house:
Results: only 27,000 KWh/year generated, and because local power averages out to 12c/KWh (on the low end of costs in the US), grid power costs saved is only $3,200/year. Ignoring other costs, the rate of return on investment is 4.9%, and payback period 20 years — but the maintenance costs and cost of money invested, with loans for 20-year periods in the 4% range, means the panels will be nearing the end of their guarantee and producing less than we assumed. The investment is marginal at best, close to break-even even after the 30% tax credit.
Large areas of the US have unsuitable weather, lower grid costs, or a limited supply of houses with appropriately aligned roofs for solar installation. So when you see solar installs in those areas, it’s a result of government spending foolishly on trophy installations that make no sense, or the desire by a few consumers to sport a trendy symbol. If you look around at your neighborhood and see few or no solar panels on roofs, that means you are likely to be disappointed in the return on your investment in solar. Designing in future solar during homebuilding, on the other hand, can make sense in much larger areas of the country — having the right roof and house orientation may well be a wise choice for when panels are even cheaper and electric rates have risen further in your area.
The widespread hype for solar, including the large number of scams and fly-by-night, high-pressure solar sales companies active recently, has victimized some consumers. “Aspirational” solar purchased by wealthy homeowners because they want to signal their enlightened attitudes is just another conspicuous consumption good, like Teslas. Rooftop solar is another complicated system to maintain and is a bad investment unless it returns its costs quickly.