I don't have a link but I do have some useful math that will greatly help a person figure out what they may be getting energy wise from a system based on basic measurements and readings using these three common volume and energy definitions.
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1 BTU, British thermal unit: A unit of heat equal to the amount of heat required to raise one pound of water one degree Fahrenheit at one atmosphere pressure.
One gallon of water weighs 8.34 pounds.
1 BTU/hour is equal to 3.415 watt/hours.
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The simplest way to determine that is to know your realistic flow rate and temperature change for a given stage of the system which works like this.
If your heat collectors are producing a 40 degree F temperature change from the incoming line to the outgoing line and your pump or gravity flow rate is estimated to be 5 gallons per minute you can factor that out to find how much heat you are capturing with the collector.
5 gallons of water weighs 5 * 8.34 = 41.7 pounds and it is gaining 40 degrees F in one minute which means 41.7 * 40 = 1668 BTU's per minute is being transfered to the water. That value may at first not should like a whole lot but consider that home heating appliances are rated in BTU's per hour that 1668 BTU value would need to be changed into a hourly heat rate, 60 * 1668 = 100,080 BTU.
The electrical energy equivalent to this 100,080 BTU's per hour the collector is capturing equates to 100,080 / 3.415 = 29,306 Watt hours or 29.306 Kilowatt hours of electricity.