How Much is 80 Gallons of Hot Water Worth?

Winter is finally giving way to spring, and the sun is now clearing more of the trees that have been shading the solar panels.  The result is that for two straight days in a row, the system has produced 80 gallons of 160 F water.  That leads me to this question, how much is that really worth.  Time to break out Excel.

So the water starts out (at this time of year) at 52 F and is heated to 160 F, for a delta T of 108 F.  The 80 gallons of water weighs 668 lbs and given that a BTU is the energy needed to raise 1 pound of water by 1 degree, this totals 668*108=72,144 BTU.    But what does that really mean?

If that water had not been heated by the sun, it would need to be heated by home heating oil burning in the furnace.  Let's assume that the furnace is only 80% efficient, so in stead of 72,144 BTU, we would need 90,126 BTU from the fuel.  Looking up on the internet, we see that there is 139,000 BTU in one gallon of home heating oil.  Thus we saved 0.65 gallons/day or (at $4/gallon) $2.59/day.

Well that hardly seems worth the thousands of dollars that I spent on the solar hot water system.  But before we conclude that, let's look at how much this adds up to over the course of the year.

Let's assume that half the time, the system will produce a tank full of hot water.  That is 182.5 days per year.  So, I will save 118 gallons/year or $473/year. 

At that rate, it will take a depressing 17 years to pay back the $8000 of out of pocket expenses (after federal 30% tax credit.)  However, there are two more things to consider.  First, it may be possible for me to shut off the furnace during the summer which might double the savings.  Second, perhaps over the next 10 or 20 years, oil might cost a lot more than $4/gallon, accelerating the payback.  That may make facing the future just a little bit more bearable.