Apples to Apples [Exploring Solar Energy]

One of my favorite games to play is Apples to Apples. Each round works like this: You have 5 cards in your hand. Each card has a noun on it such as Coal, Sean Connery, Italy, My bedroom, or Cantaloupe. An Adjective card, such as Colorful, is played. Each player puts a noun card down that they think goes best with the adjective card. The judge then ranks the played cards by which he feels best fits the adjective. Whoever has the best card wins a point, and then the judgeship rotates. It is a great group game and involves a lot of trying to figure out how the judge thinks (in my family the Sean Connery card always wins).

Sometimes comparing the different costs of electricity generated by different sources can be like that, especially electricity generated from Photovoltaic (PV) solar cells.

Panneaux photovoltaïques
Creative Commons License photo credit: Trebosc

The price for putting a Solar PV array in your house includes the cost of the unit, the cost of the installation, and the cost of the maintenance. Once the system is installed, you are not paying for the energy it produces. When you see the cost of solar electricity compared to the cost of other sources of electricity, the cost of solar remains high. Why is that?

Well we have to count a number of factors into it to make a comparison. First is the cost of the PV array. They are expensive. It is a major up front investment for most people. Depending on which system you pay for it could be anywhere from $1,000 to $45,000. Some PV providers include installation and maintenance and some don’t. The system does end up paying for itself, but it takes time. PV arrays will usually have a lifetime of 20-30 years. That’s a long time to wait, but you just might have that house for 20-30 years.

Now we have to figure out how much electricity the PV is producing over its lifetime. Just because a PV array tells you a number, like 250 watts, does not mean that the PV array is always producing 250 watts of electricity. It does mean that it can produce up to that number. There are times when the PV array will produce no power, mainly night and particularly in bad weather. But, just because the sun is out does not mean that the PV array is producing at its maximum. The sun moves all day long (well, it looks like its moving), and it gives off different amounts of light depending on where it is in the sky. That’s why the measuring system called “peak sun hours” was invented.

During solar noon, when the sun is directly overhead of a given spot (1 meter squared), that spot receives 1,000 watts or 1 kilowatt. If the sun is overhead for an hour, that would be 1 peak sun hour or 1,000 watt hours per meter squared. Now that we have a standardized measurement we can figure out how much electricity our PV array is producing a day.

overhead it is sun
Creative Commons License photo credit: Torley

Once I figure out how many peak sun hours there are in my area, I can multiply that by how many watts the PV array produces. Let’s say I’m in Texas and I get 4.5 peak sun hours a day and I have a 250 watt PV array. That means I would get (250 X 4.5) 1125 watt hours a day. Notice also that those 1125 watt hours are spread over a 12 hour period (24 hours if you want to include night). So that means I would get an average of 93.75 watts per hour, although the actual amount I get would vary widely through out the day.

The PV array would be able to work 7 days a week. Remember the lifetime of the PV array is 20–30 years. That’s around 9,125 days. All that means is that the array will produce 10,265,625 watt hours over its lifetime (or 10,265.625 kilowatt hours). Now if the system originally cost $10,000, that would mean the electricity it created cost $1 a kilowatt hour. When the average cost of a kilowatt hour in Texas is 11 cents, solar energy can be quite high.

After saying all that, solar energy still has a bright future. Each year, the PV panels get more and more efficient, and the cost for producing them keep going down. In the future PV cells will be a corner stone of a smart electrical grid and smart houses.