The bubble cover is behind the equipment pad, rolled into a stiff blue tube, which is where a lot of bubble covers end up. Next to it sits a gas heater, and somewhere in the house is the SoCalGas bill that made somebody ask what’s the cheapest way to heat a swimming pool. The honest answer starts with the tube, not the heater, and it’s the first thing we look at before anyone talks about heater repairs and equipment upgrades.

Heat is easy to buy. Keeping it is the hard part, and a Ventura County pool in spring loses heat faster than most heaters add it. So here is the cheapest way to heat a pool, ranked from cheapest to most expensive, using 2026 SCE and SoCalGas rates instead of national averages.

Why the cheapest heat is the heat you keep

The Department of Energy’s guidance on pool covers says evaporation is by far the largest source of energy loss from a pool. The numbers are lopsided. One pound of 80°F water that evaporates takes 1,048 BTU with it. Raising that same pound one degree takes 1 BTU.

A gallon of water weighs about 8.34 pounds, so every gallon that evaporates walks off with roughly 8,700 BTU. It also leaves its minerals behind, which is how Ventura County hard water ends up as white calcium scale on the tile line. (Two bills for one gallon. Evaporation is efficient that way.)

Wind makes it worse. A DOE fact sheet on outdoor pool energy says a 7 mph wind at the surface of the pool can increase energy consumption by 300%. Weather Spark’s climate data for Thousand Oaks puts the windier part of the year at November 1 through May 7, with average winds above 7.1 mph. April averages about 7.7 mph.

DOE also lists low humidity as a driver of evaporation, which is a polite way of describing a Santa Ana afternoon.

What’s the cheapest way to heat a swimming pool? All seven, ranked

Cost figures come from HomeGuide’s 2026 pool heater pricing unless noted. The cost per 100,000 BTU is our math, explained below the table.

The per-BTU math uses SCE’s June 1, 2026 rate advisory (an average residential rate of 34.4 cents a kWh), SoCalGas’s January 2026 rate alert ($2.08 a therm, bundled residential), and the 84% thermal efficiency Pentair’s gas heater brochure lists for the MasterTemp. (The federal minimum is 82%, per the Appliance Standards Awareness Project.) Your tariff will differ from the average. The order won’t.

Number 1: a $50 cover beats a $4,000 heater

The best independent data on this comes from California. In a 2015 study (reported in January 2016), the National Pool Industry Research Center at Cal Poly San Luis Obispo tested covers on real test pools. Bubble covers cut evaporation 94.9%, foam covers 95.9%, solid track covers 93.9%. The researchers note those are maximums, because the covers stayed on around the clock.

Which is the catch. A cover saves heat only while it’s on the water.

A clear bubble cover doesn’t block much sun, either. DOE figures an uncovered outdoor pool absorbs 75 to 85% of the solar energy that hits it, and a transparent bubble cover reduces that by only 5 to 15%. Opaque covers cost you 20 to 40%. So the clear cover can stay on during sunny days and the pool still gains heat under it.

DOE adds that a cover cuts make-up water 30 to 50% and chemical use 35 to 60%. And if you already own a heater, California’s Energy Code requires it: Section 110.4 of Title 24 says outdoor pools and spas with gas or electric heaters shall have a cover installed.

Numbers 2 and 3: solar rings and liquid covers, honestly

Solar rings did better than we expected. HomeGuide lists them at $35 to $50 each, or $170 to $200 for a six-pack. The same Cal Poly study tested floating solar disks and measured a 50.1% cut in evaporation. That is half a bubble cover for about the same money. The gaps between rings are open water, and on a breezy afternoon they drift toward the downwind wall anyway.

Liquid solar covers are a thin film you pour in, and it spreads across the surface. Heatsavr runs $329.95 for four 1-gallon jugs, dosed at 1 ounce a day per 400 square feet of surface. On a 400 square foot pool that’s 512 days of product, about 64 cents a day, or roughly $19 a month.

The label says it saves up to 50% of the evaporated water and energy. Cal Poly measured 8 to 17% across its runs, about 15% in the longer extension study, and suspected the wind and storms during those weeks dragged the liquid numbers down. A surface film has to stay in one piece to work. We have wind.

Number 4: solar panels, the cheapest heat you can add

Once the pool keeps its heat, solar is the cheapest way to add more. The Department of Energy’s page on solar swimming pool heaters puts a system at $2,500 to $4,000 to buy and install, with a payback of 1 to 7 years depending on local fuel costs and sun. They typically last 10 to 20 years. HomeGuide’s 2026 range runs higher, $2,500 to $6,500, with $10 to $25 a month to run the pump.

High local energy rates are what shorten that payback, and neither SCE nor SoCalGas is cheap. Our climate helps too. DOE says unglazed plastic collectors, the cheap kind, suit pools used in above-freezing weather, which describes nearly every pool we service.

Sizing matters. DOE says collector area should equal 50 to 100% of the pool’s surface area, and its example of a northern California pool used 60 to 70% for a six to eight month season.

Two local limits. Weather Spark shows Thousand Oaks getting 14 hours 26 minutes of daylight on June 21 and 9 hours 52 minutes on December 21, so solar is a spring-through-fall tool. And shade kills it. The oak that keeps the patio cool is also shading the roof and the water.

Solar also needs the pump running while the sun is up. That’s the good hours: SCE’s TOU-D-4-9PM plan charges 58 cents a kWh on summer weekdays from 4 to 9 p.m. and 34 cents off-peak, so daytime pump hours are the cheap ones. If your pump is an old single speed, California’s variable speed pump rule is worth reading before you add run time.

Numbers 5 and 6: heat pump or gas, and why April decides it

The full matchup, speed and circuit amps included, is in our post on repairing or replacing a pool heater. Short version: at a COP of 5.0 a heat pump beats gas, $2.02 against $2.48 per 100,000 BTU. Real, but about 20%, not the landslide national comparisons suggest.

DOE says heat pumps work best with the air above the 45 to 50°F range and lists COPs from 3.0 to 7.0. Weather Spark has April lows in Thousand Oaks at 49 to 52°F. At a COP of 3.0, 100,000 BTU costs about $3.36, which is more than gas.

So run a heat pump in the afternoon, when the air is warmest. Gas doesn’t care what the air is doing, which is the whole point of gas.

80°F in April or a longer season? Different problems

This is where people overspend, buying for one goal and using it for the other.

Raising a 20,000 gallon pool 10 degrees takes about 1.67 million BTU. By our math that’s about $41 with gas, $34 with a heat pump at COP 5.0, and $168 with electric resistance. Then an April night, 50°F and breezy, pulls a good share of it back out of an uncovered pool, and you pay again tomorrow.

The DOE outdoor pool fact sheet adds that each degree of temperature rise can cost an additional 10%. So 84°F costs noticeably more to hold than 80°F.

If you want a longer season, start with a cover, then add solar. That combination stretches the warm months on both ends for almost nothing to run. If you want 80°F on a Saturday in April for a birthday party, buy gas (or keep the gas heater you have) and put the cover on the minute the party ends.

The bad ideas

Electric resistance heaters. DOE’s own comparison shows an electric resistance heater that costs $1,000 a year to run dropping to $200 with a heat pump. On SCE rates it’s about $10.08 per 100,000 BTU, roughly four times gas and five times a heat pump. California’s Energy Code already bars electric resistance pool heating except for listed, fully insulated package units (think portable spas) with R-6 covers or pools getting 60% of their heat from solar. The Appliance Standards Awareness Project says federal standards starting May 30, 2028 will effectively require heat pump technology for electric pool heaters.

Black trash bags. We couldn’t find a single test of this, so here’s what DOE’s cover data implies. An opaque cover cuts the sun the water absorbs by 20 to 40%. A black bag heats itself and the air over it, not the pool. Then it tears, and the pieces end up in the skimmer basket. (It’s never one piece.)

Solar rings as a heater. They’re a partial cover, not a heater. Judge them on evaporation, where they earned their 50%.

Questions we get at the equipment pad

Does a bubble cover actually heat the pool? Mostly it keeps heat. The sun still gets through, since DOE says a clear cover blocks only 5 to 15% of it, so a covered pool keeps gaining heat on sunny days.

My pool drops an inch a week. Evaporation or a leak? DOE figures a cover cuts make-up water 30 to 50%. If the cover goes on and the loss doesn’t change, stop blaming the sun and read up on how we find and fix pool leaks.

Will a cover mess up my chemistry? DOE says a cover cuts chemical use 35 to 60%. If your chemistry is off under a cover, it was off without one, and that’s what weekly pool maintenance is for.

Put the cover on tonight

Before you price a heater, do the free part. Find the cover (check behind the pad), trim it to fit, bubbles down, and put it on every night for two weeks. Watch the water temperature and the water level. That tells you how much heat you actually need to buy.

If the answer is still more heat, send us your pool size and what’s on your pad and we’ll tell you which rung of the ladder makes sense. We service Pentair, Jandy, Zodiac, Hayward and Sta-Rite heaters and pumps. Call (805) 586-2055.