Net-Zero, a house that produces all the energy that it uses
In 2012, we built the first certified Passive House in the state of CT. Since a Passive House is an ultra energy efficient house that requires very little energy to heat and cool, we knew that through the purchase of a solar PV system, we'd have the opportunity to also be a net-zero house, i.e. one that produces all the energy it uses. The idea of our home having a zero carbon footprint and zero energy bills was very appealing, but would the expense of the solar panels be worth it?
Solar panels are a financial investment
I spent much of my 22 year career in the financial services industry analyzing fixed income investments. When you purchase a solar PV system you make an investment upfront to pay for the system and you receive investment returns over the next 25 years in the form of savings on your electricity bill. The cashflows here are very similar to that of an annuity or long-term bond. Furthermore, the savings are after tax savings, so it's comparable to a tax-free annuity or a long-term municipal bond. It makes sense then to analyze the investment in a solar system the same way you would a financial investment, by discounting future cashflows and computing a yield on the investment.
One complication is that we don't know exactly what the future savings will be. In order to estimate the future savings we need to make some assumptions. Solar providers are pretty good at estimating the amount of electricity a system will generate over its life. Using the average annual generation provided is a reasonable place to start. But, the future price of electricity is much more difficult to estimate. The best we can do here is to look at history and analyze the yield of the investment using several different assumptions for the energy inflation.
When I bought my system in 2012 the net cost after state rebates and federal tax credits was about $3 per kW for our 10.78 kW system. I created a spreadsheet (borrowed from somewhere on the internet) and figured out that the yield on my system was about 1.1% plus whatever I assumed for annual electricity inflation. If I assumed electricity inflation was 4% the yield was 5.1%. If assumed 6%, the yield was 7.1%. If I assumed 1%, the yield was 2.1%. Historically, I think that energy inflation has averaged around 5% per year for the last few decades. Back in 2012 yields on long-dated municipal bonds were in the 2%-3% range. The investment in solar PV seemed like it would return more even under very conservative assumptions for electricity inflation. When I bought my system our utility charged $0.14 per kWh. Now they charge $0.17, an increase of more than 23% over just 3 years. Unless there is a persistent and significant decline in electricity prices going forward, this will have been an excellent financial investment. It's a bet that I'm happy I made.
Prices have dropped dramitically and investment yields have gone up dramatically
Prices for solar PV panels have decreased dramatically over the last 3 years. Additionally, our town is part of CT's Solarize program enabling residents to get bulk purchase discounts. With the Solarize Harwinton pricing residents can get a solar PV system installed for less than $2.00 per kW. Even if you have a partially shaded site and you assume zero electricity price inflation, a solar system through this program today yields 5.8% and again, that's an after tax return. If you add in modest electricity price inflation of 4%, the yields are closer to 10%. If you have a site without a severe shade problem with modest energy inflation you could see yields close to 12% and again, these are after tax yields.
In conclusion, solar PV panels seem like an amazing investment today. Where else can you get after tax returns of 6%, 10% or 12% for 25 years? Stocks? Bonds? I don't think so. And, as a bonus you get to do something good for the environment.
I've attached the spreadsheet I used for the analysis if anyone's interested in playing around with it.