Yesterday (October 2), I gave a presentation on home solar power at a local parochial school, "Nuestra Señora de Carmen", in Hatillo, PR. I was very impressed by the abilities and interests of this group of 11th graders. They asked a lot of interesting questions, and in this post, I will begin to address them. We start with anticipated power cost savings.
The simple calculation is to estimate how many kilowatt-hours per month my system will produce and how much electrical power will cost over the coming years. We are installing a 2kW system. We can reasonably expect to have full sun on the panels for 6 hours/day, 25 days per month, which adds up to 150 h/month. We currently pay 20¢ per kWh, so this will be worth $30/month. At this rate, we will reduce our electric bill by $360 a year, or $3600 over 10 years, and it will take about 20 years to pay off our ultimate construction cost, which is close to the 25-year 80% efficiency lifetime of the panels. (A total cost of $7200 includes the tax credit we expect to receive from the PR government.) In reality, it may take 25–30 years to pay off the system.
The above calculation makes a big assumption: that electricity costs will not go up. However, only a little over a year ago we were paying close to 30¢ per kWh, and this cost will increase in the coming years. The question is, by how much? This is a big problem to estimate, as electric power costs do not change with any predictable pattern. We can assume that these costs have bottomed out, given the state of the economy, the environment, and of known reserves of petroleum (see this link for estimates of future petroleum costs) (in PR there are currently no other sources of electricity). If electrical power averages 30¢ per kWh over the next 10 years, we will save $5400 rather than $3600.
I will do some more research on electricity costs. In future posts I will touch on changes in our plans, solar-thermal power, and wind power.