finalizing specifications

Choosing a grid-tied system has its costs. The main one is that when the utility power goes down, so does the solar electricity. This is for several reasons. First, "grid tied" means what it says, tied to the grid, not to the house. It serves to mitigate some or all of the use of grid power, but the house still runs off the grid. Second, the house cannot run off power from the panels alone (inverted, of course). This is because of the fact of brownouts caused by clouds passing in front of the sun, or surges in demand. So a storage system is required as a buffer. This consists of a battery bank, which I decided against for reasons detailed below.

Our contractor visited with the solar energy engineer on Tuesday. They measured the estimated average daily production based on the hours of sun that can be expected per day. This measurement consists of first, a device that records the sky exposure of the site. Based on the annually integrated solar track, shadowing from trees, adjacent buildings, even our TV aerial, cloud-cover data for our location, and other parameters, it computes a number that they claim is quite reliable. In our case, we will get an average of 5.4 h/day of production. This comes to 300 kW-h/month, which is just over half of our energy usage.

We will purchase 2 kW in panels, but with a 4 kW inverter, so if we choose to add capacity, it means only additional panels. The panels are 200 W models from Canadian Solar. The inverter is a Sunny Boy 4000US. The reason to forego the batteries that would have given us a standalone system is mostly cost. The batteries are expensive and short-lived. In fact, I calculated that whatever we save in electricity costs, we will spend in replacing battery cells, which have roughly 5-year lifetimes.

Our contractor is PR Green Tech Corp. Julio Correa, the owner, is friendly and attentive, and appears to be quite devoted to his avocation. I hope this will serve us into the future as technologies evolve.

restart

After most of a year of delay, I have restarted my solar home project.

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 11^th 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.