This article is my effort to shine some light on the comparative difference and value of two ways of harvesting solar energy, Domestic Hot Water and Photovoltaics. Don’t hate the conclusions, math doesn’t lie.

The ability to heat water with the sun has been apparent to all of us since the first time we got burned with water out of the hose. This is a basic solar hot water collector. Add some low iron glass, an insulated box, a pump, and a tank, and you have a ready source of hot water every sunny day. Writing from my desk here in Florida, the Sunshine State, I know we see about five fully cloudy days a year. Current systems even have a highly efficient booster which will take up the slack on those cloudy days, you don’t have to skip your shower.

A current domestic solar hot water system won’t even be a noticeable change to your lifestyle. The time it will be noticed is in the average $65 reduction in your power bill, every month. What you won’t notice is that the reduction in greenhouse gas generation is the equivalent to taking a regular car off the road. The average system is about 40-60 square feet of panel.

The cost for this modern marvel of old technology is about $4,300 out of pocket. Tax benefits and grants are available reducing this cost to as little as $2,350. This means that this system will pay for itself in about three years for the average home. If you were to convert this back to electrical generation the average hot water heater for the average family consumes 18 kilowatt hours of power per day, 6,570 KwH per year. If we look at this over a ten year period this means that an initial investment of $2,350 generated 65,700 KwH of reduction in power usage. A rate of $0.035 per KwH, I challenge you to find any utility company who will charge you three and a half cents per kilowatt hour and further expect that utility rates will triple or more in the next ten years making the value to the homeowner even higher. I know that I would buy as much power as I could at that kind of rate!

Unfortunately Solar Domestic Hot Water isn’t exciting. Telling your friends about saving money by making your own hot water has nowhere near the appeal of telling them that you are trying to get off the grid and generate your own power. That you can even sell back excess power to the utility company and they have to pay you! Let’s explore this further.

Photovoltaic systems are based on silicon wafer panels which convert a small percentage of the sunlight that falls on them into direct current electricity. Yes, direct current, not alternating current that we use here. This means that the system must include an inverter, a specialty electronic component that changes the direct current electricity into alternating current for use in our homes. Inverters run about 40% of the system cost on average and wear out in about 8 years having to be replaced.

A PV system I can find online runs $26,000 for the whole system, $3,400 of this is for the inverter. With tax credits this is reduced to $18,200, other incentives are often available. This is a 4.8 KW system, meaning that the panels generate 4.8 kilowatts of power per hour in optimum conditions. Overheating, shading, sun angle, all diminish this output. Conversion to alternating current loses a portion of this power as well, the average inverter reduces power by about 30%, this loss is experienced as heat. Now we are down to 3.36 KwH from the system which you might get for an average of 4-5 hours per day year round.

Daily energy output from the system is then 5 hours times 3.36 KwH or 16.8 KwH per day, 6,132 KwH per year. (Note this is approximately what the Solar Hot Water system was replacing.) If we then apply the same math to this unit as the Solar Hot Water system, we get the following. Over ten years the PV system costs a minimum of $18,200 plus one replacement of the inverter at $4,300, of course the inverter will cost more then but this is already complicated enough. Total cost of $22,500 resulted in a generation of 61,320 KwH of power. The cost per KwH is $0.367, more than ten times the cost of the power replacement by the Solar Hot Water System. Additionally this system takes 350 square feet of space, 9 times the space of the Solar Hot Water System.

Unfortunately this example is pretty indicative of the state of PV technology. Unless the cost of power more than triples, or the components drop to 1/3 their current price, this technology cannot compete at the homeowner level. To further make matters worse, the idea of selling power back to the utility company is like giving it away. They only pay approximately half of the going rate for the power you send them which cost six times that much money to generate.

Sadly for those of us who are excited and want to do something about alternative energy by generating our own power, PV is a solution that doesn’t work out mathematically. Get a solar hot water system and enjoy every shower more. Then put the money you wanted to spend on PV into other efficiency improvements and you will come out far ahead. More on this in my next post, Efficiency Improvement Priorities.
Paul

Tags: Solar Hot Water Domestic-solar-wate R Solar-hot-water Solar-heating Solar-e

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