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# Solar Power: Solar Power Cost Effective Calculation? (7/27/2011)

An average householder uses 160 litres day-1 hot water at a temperature of 55oC, and the specific heat of water is 4.1868 kJ kg-1oC-1 and the inlet temperature of the water to the house is 10°C. Estimate the total energy requirement for hot water supply. If a 2 m2 solar collector is installed to save electricity for hot water supply, the efficiency of the solar panel is 55%. Assuming the mean availability of solar energy throughout the year is 115 Wm-2, the cost of electricity is 5.87p kWh-1, the total cost of the installation is £2000, and it has a life span of 20 years, is the scheme cost effective at a 0% discount rate?

I got the total energy required to heat the water as 11.47GJ/Annum and that its not cost effective as the solar panles will provide 34.7% of the heating requirement which is equilivent to £1300 over 20 years, thus as it cost £2000 to install, its not worth it.

Is this correct? I feel i have done it wrong

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Want to Help July 27, 2011 at 9:50 am

Yes I think you may have done something wrong. The company that installed my solar panels helped us figure it out and in the end we realized it really would be worth it. We decided to take the plunge and do it and now 6 years later, we realize how much money we are saving ourselves. Also a good company should be able to get you some good rebates, which really helps a lot. I think you should look into it further. Good Luck!

roderick_young July 27, 2011 at 10:39 am

My calculations show the panels producing 7.25 GJ/year, and 11.00 GJ as the original electrical requirement. That means the panels would produce about 66% of the heating requirement (oddly, this is the complement of 34%).

1 kWh = 3.6 MJ, so the annual energies are

solar 2013 kWh/y
electric 3056 kWh/y

The value of the solar heating is therefore 11800p / year. I’m American, but if p are like cents, and pounds like dollars, then that’s £118 / year, or £2360 over 20 years, a bare win over the system price.

It sounds like these calculations are for a place that is right on the cusp of where solar makes sense. In Hawaii, electricity would be 3-4x that price, the system price 2/3 of that (simple panels – freezing not an issue), the insolation 2x that, and the water inlet temperature higher, making the energy requirement 2/3 of that.