One number does all the work
Water stores about 1 BTU per pound per degree Fahrenheit, and a gallon weighs about 8.34 pounds. So a gallon of water is roughly 8.34 BTU per degree, and a tank of G gallons absorbing Q BTU rises by:
ΔT = Q ÷ (G × 8.34)
That is the whole page. A 64 square foot collector at 1,600 BTU per square foot per day and 45 per cent efficiency delivers 46,080 BTU. Put that in 80 gallons — 667 pounds — and it rises 69 degrees. Put it in 185 gallons and it rises 30. Nothing else changes; only the denominator moved.
This is why the storage-to-collector ratio is the number solar thermal people argue about. It is not a preference, it is the size of the buffer between a good day and a problem.
Sizing on the quiet day, not the average one
Most sizing goes the other way round: take a demand, divide by insolation and efficiency, get a collector area. That is the right question when you are choosing panels, and the solar pool heater sizing calculator works it that way for a pool. It is the wrong question once the panels exist, because the constraint has moved. The array collects the same heat whether or not anybody is home.
So the storage question is set by the smallest draw, not the typical one. A household that uses 60 gallons a day comfortably absorbs a good day of sun. The same household away for a long weekend absorbs none of it, and three days at 69 degrees a day from a 55 degree start arrives somewhere no one intended.
| Tank | Rise from 46,080 BTU | Days from 55°F to 160°F with no draw |
|---|---|---|
| 50 gal | 110.5°F | 0.95 |
| 80 gal | 69.1°F | 1.52 |
| 120 gal | 46.0°F | 2.28 |
| 185 gal | 29.9°F | 3.52 |
| 300 gal | 18.4°F | 5.70 |
The right-hand column is linear because the arithmetic is. Real collectors get less efficient as the tank heats up, so the real days are longer than these. The direction of that error is the safe one, which is why this page does not try to correct it.
Stagnation, named plainly
A collector in full sun with nothing taking heat away does not stop collecting. It heats up until it is losing as much as it gains, and for a decent flat plate that equilibrium is far above anything in the rest of the system. The pump stops when the power goes out, or when the controller decides the tank is already at its limit, or when a component fails, and none of those are unusual events.
What is at risk: seals, gaskets and any plastic in the loop; the heat transfer fluid, which can boil, be pushed out through a relief, or degrade and turn acidic if it is glycol; and anyone who opens a valve or a fitting afterwards, because the temperatures involved scald instantly. Evacuated tube collectors reach higher stagnation temperatures than flat plates.
How a system is protected — whether heat is dumped somewhere, whether the panel is drained, whether it is shaded, how the expansion and relief are arranged — is a design decision made by whoever specifies the system, and it is one of the main reasons solar thermal is not a component you simply plumb in. This page gives no procedure and none should be read into it. What it gives is the arithmetic that tells you how close to that condition your storage puts you.
What is not in the model
Standing losses. A hot tank loses heat to the room continuously, and a large tank held at low temperature most of the year loses steadily for nothing. Pipe runs lose too, and on a long run between a roof collector and a basement tank that is not a rounding error — the pipe insulation heat loss calculator puts a number on it.
Stratification. Real tanks are hotter at the top than the bottom, which is useful because the draw comes off the top, and it means the tank is not the uniform block this arithmetic treats it as. A tank with a coil in the lower third behaves differently again.
Expansion. Water gets bigger when it is heated and the loop has to have somewhere for that to go — the thermal expansion tank calculator covers the domestic side of that question.
And demand, which is its own study. For a household the draw pattern matters as much as the total; for a commercial kitchen it is a different animal entirely, which the commercial kitchen hot water demand calculator handles.
Questions people ask
How many gallons of storage per square foot of collector?
That is the ratio this page exists to let you work out from your own numbers rather than take from a rule. Run your collector area, your insolation and your efficiency, decide how much rise you will accept on a day with no draw, and the volume falls out. What drives the answer is how big your quiet days are: a household that always draws water can live with a small tank, and a weekend house cannot. The commonly quoted ratios come from typical residential systems in typical climates, and if any of your three inputs is unusual the ratio moves with it.
What is stagnation and why does it matter more than the average day?
Stagnation is a collector in full sun with nothing taking the heat away, which happens whenever the pump is off: a power cut, a controller that has already hit its limit, a failed sensor, or simply nobody home. The absorber keeps absorbing and its temperature climbs until losses balance gains, well above anything else in the system. That can cook seals and gaskets, boil or expel a heat transfer fluid, degrade glycol into something acidic, and scald badly if a fitting is opened afterwards. It matters more than the average day because averages never break anything. How a given system handles it is part of its design and not something to improvise.
Does collector efficiency really change with tank temperature?
Yes, and it is the main reason the linear arithmetic on this page is conservative. A collector loses heat to the outside in proportion to how far above ambient it sits, so a panel feeding a cold tank runs cool and efficient, and the same panel feeding a hot tank runs hot and loses more of what it gathers. That is also why the efficiency you enter should be a daily average at the temperature the panel will actually work at, not the peak number from a test near ambient. The practical consequence is that a bigger tank collects more heat as well as buffering it, because it keeps the panel cooler.
Can I use this to size a tank for space heating?
The arithmetic is the same, since it is only mass times specific heat, but the demand side is completely different. Space heating demand is enormous compared with domestic hot water, concentrated in the months with the least sun, and needs storage measured in thousands of gallons rather than hundreds to carry any useful time. It also runs at lower temperatures, which suits collectors well. If that is the plan, work out the building heat loss first with the heat loss calculator, because that number will tell you very quickly what scale of storage the idea implies.
Where does the 8.34 come from?
It is the weight of a gallon of water in pounds, close enough for this at ordinary temperatures. Water has a specific heat of about 1 BTU per pound per degree Fahrenheit, so a gallon takes about 8.34 BTU to raise one degree. It is worth memorising because it turns up everywhere: 100 gallons is 834 BTU per degree, and any heating question in gallons and degrees is one multiplication away. Water gets slightly lighter as it heats, so the figure drifts a little at high temperature, but not by enough to matter next to the uncertainty in an insolation or efficiency number.