Tuesday, January 27, 2009

Solar Energy Questions & Answers

The ten photovoltaic panels are on the right, the four solar thermal panels (for domestic hot water) are on the left.

How much power do the PV panels generate?
Each of the 10 PV panels has a rated output of 195 Watts. The highest instantaneous peak output that I've seen so far is around 1800 Watts. The estimated annual production is 2200 KWH, or about 1/3 of our average annual usage.  
Jan 2010 update: Our 2009 actual PV total was 2251 KWH.

Does the power company buy power from you? If so, is it at the going rate that other suppliers provide? They buy power from us at the retail kWh rate. So if we pay $0.16/kwh, that's what we get per kWh generated, because that's what we save. We also get an additional subsidy for the kWh we produce paid by people who have signed up to pay extra for clean energy. That's and additional $0.04 cents per kWh.

What did you need to do to connect the output of the panels to the grid? National Grid installed a new meter at no charge. Only the meter module was swapped.

Does your meter track when you supply power? Does the rate your are paid vary with time of day, peak times (assuming they pay you).
The PV system only measures the total power produced by the panels. I keep track of the total manually. National Grid rates are flat, so there are no special rates for peak usage periods.

Do you store excess power in batteries?
No, the excess power flows back onto the grid. We have "net metering", where we pay for our net usage. Excess power generated on one day reduces our total kWh usage for the month.

For the solar thermal panels, do you use the hot water to heat the house, in addition to providing hot water? We originally planned for another heat exchanger to get heat for the house from the solar panels, but we decided to save money and not install it right now. That would have added around $2000 for the additional plumbing costs (I'm not sure why it was so expensive, but that's what the heating subcontractor quoted) and we decided it wasn't worth it. We may add it later, if we can get it done for less. In the winter, the solar panels won't produce all of our hot water, so it's not as clear a benefit to use the excess to heat the house also.

What are the payback periods?

  • The payback on the solar hot water system is around 7 years. This payback period is based on a 5% average annual increase in energy costs.
  • The PV system will generate about 1/3 of the 6500 kW-hours we consume each year. The payback time on this system is around 18 years, when annual energy cost increases are factored in. The PV panels themselves are guaranteed for 25 years. Beyond 25 years, the power generated may be slightly less, but they should continue to work.
For the PV panels, I'm not thinking strictly in terms of payback. The way I look at it, I know we'll need electricity, and after the payback period I'll be even and the atmosphere will have less carbon dioxide than if I didn't do this.

Electricity to use and a planet that we can all live on, what else can you ask for?

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