$1000 Solar Water Heater -- Cost and Payback

This section provides an estimate for the cost of materials to make the solar water heating system, and also provides an estimate of energy saving, cost saving, and greenhouse gas emissions reduction.

Bottom line is that the system will offer a family of 3 a payback period ranging from 2.5 years to 3.4 years depending on the fuel. The system will also reduce CO2 emissions ranging from 2000 to 4000 lbs of CO2.

For a description of the system go here...

Cost Summary

The system used for this cost estimate:

Collector: 6 by 8 foot (48sf) with PEX-AL-PEX tubing and aluminum fins, polycarbonate glazing, and polyiso back insulation

Storage: Lumber framed, plywood walls, EPDM rubber lined, insulated, 160 gallon storage tank

Heat Exchanger: Pipe coil heat exchanger using a 300 ft coil of 1 inch diameter HPDE NSF pipe

Pump & Controls: Pump is TBD and Stecca differential controller

Plus connecting plumbing and fittings, ...

While its going to take some more performance gathering on the current system to be sure, I think that this system would be about right for a family of 3.

I don't attach any cost to your labor, as I know you love to do these kinds of projects :)

Cost Table

Item	Description	Unit Cost	Qty	Item Cost	Total cost
Collector
Frame lumber	2X4 and 2X6 lumber	$0.4 / ft	40 ft	$16
Backing board	1/2 inch CDX plywood	$0.6 /sf	48 sf	$29
PEX-AL-PEX tubing		$0.43 /ft	100 ft	$43
Glazing	SunTuff polycarbonate	$1.1 /sf	48 sf	$53
Absorber	0.018 aluminum soffit	$1 / sf	60 sf	$60
Back insulation	1.0 inch polyiso (R7)	$0.50 / sf	48 sf	$24
Paint, fasteners, glue, silicone,				$25
Total Collector					$250

Storage Tank and Heat Exchanger
Plywood	3/4 exterior plywood	$0.8 / sf	64 sf	$51
Tank frame lumber	2X4's	$0.4 / ft	40 ft	$16
EPDM lining	EPDM rubber pond liner	$0.7 /sf	~120 sf	$84
Insulation	2 inch polyiso note 1	$1 / sf	60 sf	$60
Heat Exchanger	300 ft 1 inch dia HDPE	$70		$70
Misc fasteners ...				$20
Total Storage					$301



Pump and Controls
Pump cost allowance	note 2			$100
Differential Controller & sensors	Stecca			$130
Wiring, tubing, fittings				$20
Total Pump/Controls					$250

Connection to House Plumbing
Pipe and fittings			$50		$50

Total system					$851

Note 1: Tank is lined inside with 2 inch polyiso (no thermal bridging)

Note 2: These are allowances, see the section on pumps and controller for more explanation.

I'm going by memory on some of this, and I probably left out some minor parts, and I priced stuff like plywood and insulation by the square foot so you would have to round up to full sheets which would cost more.

Still, it looks like the $1000 target can be met.

Economic Payback

I will be collecting more performance data as time goes on, but for now I am going to assume that this size collector would supply 60 gallons of hot water a day for a family of 3 with a 75% solar fraction. That is, the solar heating system will supply 75% of the hot water used with the backup heater supplying the rest. In any case, increasing the size of the collector to provide more hot water is not very expensive. In fact, one of the aims of the system is to reach a higher solar fraction by increasing collector and storage size, and optimizing the system for better winter collection, but I have only assumed a 75% solar fraction for the following.

The energy required to heat 60 gallons per day from 50F to 110F is 11 million BTU/year, or 3200 KWH per year. I will assume that the solar water heating will provide 75% of this, or 8.25 million BTU/year or 2400 KWH per year. I think that the system is likely to actually do better than this.

Energy Source	Unit Cost	Efficiency	Energy Saved	$ Saving	CO2 reduction
Electricity	12 c/KWH	0.9	2667 KWH	$320	4000 lbs
Propane	$2.50/gal	0.65	138 gallons	$345	1900 lbs
NG	$2 /therm	0.65	127 therms	$254	1900 lbs

So, for the conditions and energy prices assumed the payback period ranges from 2.5 years for propane up to up to 3.4 years for NG. Also bear in mind that you may qualify for some rebates or tax credits, and that as the price of fuel goes up the payback period, return on your investment, and peace of mind all improve.

You can also use this nice water heating calculator from www.InfinitePower.org to estimate payback.

It seems to come with even shorter paybacks.

It also shows the power of lowering the initial cost, and it allows you to enter your own fuel costs and/or anticipated future fuel costs.

Gary September 12, 2008, October 15, 2008, October 18, 2008