Pool Equipment
Sizing a heater: one calculation, and a warning about the answer
Working out how long a heater takes is straightforward arithmetic. The uncomfortable part is that the answer for small electric heaters is usually measured in days.
Heads up: we earn a commission if you buy through the links on this page, at no extra cost to you. It never decides the ranking — here is how we pick and here is the full disclosure. Prices are checked automatically; last verified October 8, 2026.
The short version
- One BTU raises one pound of water by one degree. A gallon weighs 8.34 pounds, so published gallons convert straight into an energy requirement.
- One kilowatt is 3,412 BTU per hour. That single conversion turns a heater’s published kW into a heating rate.
- Undersized heaters cost more, not less. They run continuously and never reach temperature, which is maximum hours for no result.
- All of this ignores losses, which dominate. An uncovered pool loses heat faster than a small heater adds it.
The calculation
Everything follows from two constants that do not vary: a US gallon of water weighs 8.34 pounds, and one BTU raises one pound of water by one degree Fahrenheit. Multiply out and you have the energy your pool needs.
Energy to raise a pool by 10 degrees F
- Pool volume
- 3,955 gallons
- Weight of one US gallon
- 8.34 lb
- Temperature rise wanted
- 10 °F
3,955 × 8.34 = 32,985 lb × 10 °F = 329,850 BTU
About 330,000 BTU to warm this pool by ten degrees
Turning that into hours
Now the heater. Electric units publish their draw in kilowatts, and one kilowatt delivers 3,412 BTU per hour. That gives you a heating rate, and dividing the requirement by the rate gives you a time.
How long a 3 kW heater takes on that pool
- Energy required
- 329,850 BTU
- Heater draw
- 3 kW
- BTU per kWh
- 3,412
3 kW × 3,412 = 10,236 BTU/h 329,850 ÷ 10,236 = 32 hours
About 32 hours of continuous running for ten degrees — before any losses
| Pool | BTU needed | 3 kW | 9 kW |
|---|---|---|---|
| 1,718 gal (12ft frame) | ≈ 143,000 | ≈ 14 h | ≈ 5 h |
| 3,955 gal (15ft) | ≈ 330,000 | ≈ 32 h | ≈ 11 h |
| 6,981 gal (18ft XTR) | ≈ 582,000 | ≈ 57 h | ≈ 19 h |
Why undersized is the expensive mistake
The instinct with a big number is to buy the cheaper, smaller heater and accept it will take longer. That reasoning fails here because of the loss term.
A properly sized heater reaches temperature and then cycles, spending most of its time off. An undersized one runs continuously, and if its output is close to the pool’s loss rate it approaches an asymptote below the temperature you wanted — maximum hours, maximum bill, no result. Our heater roundup leads with the published kW figure for exactly this reason.
The step that comes before sizing
Reduce the loss before you size the input. A bubble solar cover cuts evaporation dramatically while it is on, which lowers the number the heater has to fight. It is also inexpensive, and on many pools it produces enough of the benefit that no heater is needed at all.
Bubbles face down, cut to size, and buy a reel — a cover that is a struggle to remove stops being used by July, and an unused cover reduces nothing. Our solar cover roundup covers fit and thickness, and the solar heating guide covers the wider set of options.
The electrical constraint, which may decide it for you
The heater sizes in that table are not all pluggable. Electric pool heaters of any real capacity frequently need a dedicated circuit, and that is an electrician and a consumer-unit conversation rather than a purchase decision.
Establish what your supply will actually support before choosing a size, because discovering it afterwards can substantially change the effective cost. Our electrical safety guide covers what is and is not a homeowner job, and the answer for anything hard-wired is unambiguous.
What the arithmetic argues for
Three conclusions fall out of the table, and none of them is “buy a bigger heater” on its own.
Heat a smaller pool. Energy scales directly with gallons, so a 1,718-gallon pool is heatable in a way a 6,981-gallon one is not, on the same equipment and budget.
Cover it, always. The cover reduces the loss term, which is what makes any heater size work.
Target a modest rise. Ten degrees is a comfort-changing amount of temperature and it is four times cheaper than forty. Aiming for a specific number on a thermometer is usually more expensive than aiming for “warm enough to get in”.
If the goal is a longer season rather than a hot pool, our season extension guide starts with the free measures and reaches the heater last, deliberately.
What to buy
Scores are our own reading of published specifications. They are not test results — we have not tested these products and we do not claim to.
Extending the season, at a price
Electric Above-Ground Pool Heater
Various
It works, and it will be the largest line on your pool electricity bill by a distance. Buy it knowing the running cost, not the purchase price.
Key specs
- Power draw
- Published in kW — this is the number that matters, not the price
- Circuit requirement
- Frequently needs a dedicated circuit — check before buying
- Rated pool size
- Published per model
How it scoresour reading of published specs, not a test result
- Sizing fit
- 7.0
- Running cost
- 2.5
- Maintenance
- 7.0
- Build
- 7.0
- Value
- 5.0
Overall 6.0 / 10
What works
- Genuinely extends a swim season by weeks at each end
- Electric units are simpler to install than gas and need no fuel supply
- Manufacturers do publish kW draw, unlike pump makers
What doesn't
- The running cost dwarfs every other item on this site
- Often needs electrical work, which is a cost nobody quotes you up front
- Almost pointless without a solar cover to hold the heat you paid for
Skip it if: An uncovered pool. Heating water and then letting it evaporate overnight is the most expensive thing you can do with a pool.
What it costs to run: Do this arithmetic first. A 3 kW heater running 6 hours a day for 30 days is 540 kWh; at the EIA's June 2026 US residential average of 18.34 cents per kWh that is about $99 for one month. Compare that to the cost of a solar cover, which is a one-time purchase, before you decide.
More than you need yet: Real capability a first-pool family cannot use yet. Buy cheaper and put the difference toward chemicals and a cover.
Specs from: EIA Electric Power Monthly, Table 5.6.A
The single highest-return accessory here
Bubble Solar Cover for Above-Ground Pools
Various
It cuts evaporation, and evaporation is simultaneously your water bill, your chemical bill and your heat loss. Nothing else on this site reduces three costs at once.
#ad how this is funded · price as of October 8, 2026
Key specs
- Construction
- Bubble sheet, thickness published in mil
- Thickness
- Higher mil lasts longer — 12 mil and 16 mil are common
- Fit
- Cut to size; buy oversize and trim
- Orientation
- Bubbles face down, toward the water
How it scoresour reading of published specs, not a test result
- Retention
- 9.0
- Durability
- 7.0
- Handling
- 6.0
- Running cost
- 9.8
- Value
- 9.5
Overall 9.0 / 10
What works
- Reduces evaporation, which is the mechanism behind the water, chemical and heat savings all at once
- Cheap relative to everything it saves
- Can be cut to fit any pool shape, so sizing is forgiving
What doesn't
- Awkward to get on and off without a reel, and awkward is how covers end up unused
- Degrades in UV like every vinyl product
- Not a safety cover and will not support weight — this is important
Skip it if: Being a safety barrier. A solar cover looks solid and is not; a child or pet on it goes through and under.
What it costs to run: The one accessory with a clear payback. Every gallon that does not evaporate is a gallon you do not replace at the EPA WaterSense national average of $6.06 per 1,000 gallons, and it is chlorine you do not re-dose. If you buy one thing from this hub, buy this.
Several seasons: Buy-once. Built and warranted to come back out of the box for a few years, if you store it properly.
Specs from: EPA WaterSense — cost per 1,000 gallons
Cutting the electricity bill without cutting filtration
Outdoor Pool Pump Timer
Various
A weatherproof outdoor timer is the highest-return equipment purchase on this site. Most people run the pump too long because remembering to switch it off is a chore.
#ad how this is funded · price as of October 8, 2026
Key specs
- Type
- Outdoor-rated mechanical or digital timer
- Required rating
- Must be rated for the pump's amperage and for outdoor use
- Must not defeat
- The pump's GFCI protection
How it scoresour reading of published specs, not a test result
- Sizing fit
- 8.5
- Running cost
- 9.8
- Maintenance
- 9.0
- Build
- 7.5
- Value
- 9.5
Overall 9.0 / 10
What works
- Turns your filtration from a guess into a schedule
- Pays for itself within a single season on most pools
- Lets you shift running hours to off-peak if your utility has time-of-use rates
What doesn't
- Must be genuinely outdoor-rated and correctly amperage-matched, which cheap indoor timers are not
- Never bypass or defeat the pump's GFCI to fit one
- Mechanical timers drift and need resetting after a power cut
Skip it if: Fixing an undersized pump. Running the wrong pump on a schedule just makes the schedule longer.
What it costs to run: The arithmetic that justifies it: at the EIA's June 2026 US residential average of 18.34 cents per kWh, trimming a 400 W pump from 12 hours to 8 hours a day across a 120-day season saves 192 kWh, or about $35. That is more than the timer costs.
Several seasons: Buy-once. Built and warranted to come back out of the box for a few years, if you store it properly.
Specs from: EIA Electric Power Monthly, Table 5.6.A
Pools up to about 6,000 gallons
Intex Krystal Clear C1000 Cartridge Filter Pump
Intex
The workhorse replacement pump for mid-size frame pools. 1,000 GPH pump flow, 750 GPH through the system once the cartridge is in the way — and that second number is the one your turnover math needs.
#ad how this is funded · price as of October 8, 2026
Key specs
- Pump flow rate
- 1,000 GPH
- System flow rate
- 750 GPH
- Rated pool size
- Up to 6,000 gallons
- Cartridge
- Type A
- Wattage
- Not published by Intex
How it scoresour reading of published specs, not a test result
- Sizing fit
- 9.0
- Running cost
- 7.0
- Maintenance
- 7.5
- Build
- 7.5
- Value
- 8.5
Overall 8.3 / 10
What works
- Intex publishes both pump flow and system flow, which is more honesty than most of this category offers
- Type A cartridges are the most widely stocked filter consumable in the US
- Sized correctly for the 12ft-15ft frame pools most families own
What doesn't
- Wattage is not published anywhere on the Intex site, so you cannot cost it before purchase
- Cartridge-only, so there is a consumable every few weeks in a busy pool
- GFCI cord length dictates where the pool can go more than people expect
Skip it if: Pools over about 6,000 gallons. Above that you are asking it to run all day to achieve one turnover, which is the expensive way to filter.
What it costs to run: Do the turnover math before you buy: 750 GPH system flow moves 4,000 gallons in 5.3 hours. Two turnovers a day is therefore about 11 hours of running. At the EIA's June 2026 US residential average of 18.34 cents per kWh, every 100 watts of pump draw running 11 hours a day for a 120-day season costs about $24. Find your wattage on the pump label and multiply.
Several seasons: Buy-once. Built and warranted to come back out of the box for a few years, if you store it properly.
Specs from: Intex — Krystal Clear cartridge filter pumps, EIA Electric Power Monthly, Table 5.6.A — US residential average price
Questions people actually ask
How do I work out what size pool heater I need?
How long does it take to heat an above-ground pool?
Is a smaller pool heater cheaper to run?
Do I need a solar cover if I have a heater?
Sources
Read next
- Equipment, sized to your pool and costed by the wattPumps, filters, vacuums, heaters and ladders - matched to your gallons and priced by what they draw.
- Pool heaters, priced by the wattHeater running costs computed at national average rates, and why a cover has to come first for a heater to make any sense.
- Solar heating: three products, two of which mostly stop lossesSolar covers, rings and panels compared on what they actually do, what each costs to run, and the order to buy them in.
- The electrical side, which is the one part of a pool you should not improviseGFCI protection, extension cords, timers and heater circuits - what is a homeowner job around a pool and what is not.
- Heating a pool, priced properlyThe physics of heating a pool turned into dollars, why a cover changes the answer more than the heater does, and when heating is worth it.
- Adding weeks to the season, starting with the cheapest thing firstCover, placement and timing before hardware, plus the kilowatt arithmetic that shows what running an electric heater actually costs.
- Above-ground pools for adults who want to swim, not sitThe published gallons converted into actual inches of water, so you can see which pools an adult can swim in before you buy one.
- Solar pool heating: collector area is the whole storySolar panels, mats and rings compared on published collector area, pump compatibility and what each maker will actually claim in writing.