Calculators

Every generic answer in this subject is wrong for almost everybody, because the only inputs that matter are yours: how many gallons you have, how many degrees you want, what you pay for energy and how much heat leaks out overnight. A page that says “expect $200 a month” cannot know any of those.

So these three calculators take your numbers and do the arithmetic in front of you. Each one prints the formula, every assumption and every intermediate step beside the result — not tucked into a disclosure at the bottom, but as the main event. If our arithmetic is wrong you should be able to catch it, and if our assumptions do not suit your equipment you should be able to change them. Both of those are the point.

What each one answers

The heating time calculator answers “I turned the heater on, when can we swim?” It takes your volume, the temperature rise you want and your heater’s rating, and returns hours. It also handles the trap that catches everyone: gas heaters are rated by fuel input and heat pumps by heat output, so the number on the box means different things and cannot be compared directly.

The running cost calculator answers “what will that cost me?” for all five heating options at once, so you can see the gap rather than one number in isolation. It ships with US average residential energy prices from the EIA and expects you to replace them with your own — your utility rate is on your bill, and it is the single input that changes the answer most.

The cover savings calculator answers the question nobody publishes: is a cover worth it for this pool? It works from the one measurement that tells you everything — how many degrees your pool drops overnight — applies the DOE evaporation share to work out how much of that a cover can address, and turns it into dollars a night and a payback period.

How to get the measurement they need

The cover calculator wants your overnight temperature drop, and it is worth taking two minutes to get it properly rather than guessing. Read the pool temperature at dusk, at least a foot down and away from the returns. Read it again at dawn, before the sun has been on the water. The difference is your overnight loss.

Take it on a still, clear night if you can, because that is the worst case and it is the one a cover helps most. And do not read it in the top few inches: on a sunny afternoon the surface layer of a pool can be several degrees warmer than the body of water beneath it, which is a real effect and, we think, the reason published solar-cover figures disagree so wildly.

What the calculators cannot do

They cannot model your weather. Cloud cover, humidity, wind and the clarity of the night sky all move a real pool around, and none of them are inputs here. What these tools give you is the energy arithmetic, which is the fixed part of the problem — the part that does not change with the forecast, and the part that every buying guide in this category leaves out.

Every constant they use comes from the methodology page, which lists each figure with the source it came from and the date it was read.