Pool Covers: The Highest-Return Item on a Pool

How much water does a pool actually use?

Less than most people assume, and almost all of it in one go.

The fill is the big number: a pool takes its full volume once, at the start of its life, and again only if it is ever drained for resurfacing. After that, the ongoing consumption is evaporation plus whatever leaks. There is no continuous draw.

That makes a pool very different from the landscape it sits in. A lawn consumes water every week, indefinitely, for as long as it exists. A covered pool, past its fill, is a comparatively modest ongoing user — and a pool that replaced lawn can easily be the lower-water choice of the two.

Which puts the whole question on evaporation. Reduce that and you have reduced essentially all of a pool’s ongoing water use, and its heat loss at the same time, because they are the same physical process.

What a cover saves

Valley Water — the water district covering Santa Clara County, over the hill from us — states that using a pool cover can prevent up to 95 per cent of pool water evaporation. That is the headline number, and it is the reason every water agency in California that discusses pools recommends one.

The heat follows the same path. Evaporation is a phase change and phase changes carry a great deal of energy, so the surface you cover is the heat you keep. On this coast, where the afternoon breeze drives evaporation hard, that is not a marginal saving — it is the largest single lever on a heating bill.

And then there is debris. A covered pool collects far less of what falls out of the trees, which means less filter loading, lower sanitiser demand and less time with a net. Under the Monterey pines that is a genuine weekly difference.

It is also required

California’s Energy Code requires a pool cover on an outdoor pool or spa fitted with a gas heater or a heat pump, and it makes furnishing that cover the pool contractor’s responsibility. Gilroy’s own swimming pool submittal guidelines state it plainly and cite the code section.

Separately, an approved safety cover is one of the seven drowning-prevention features listed in California’s Swimming Pool Safety Act, of which a permitted pool needs at least two. So a cover can do double duty: satisfy the energy requirement and count towards the safety requirement at the same time — which is worth knowing when you are working out how to meet the safety obligation without a fence line cutting the yard in half.

Note that not every cover is a safety cover. The requirement is specific about the standard, so a floating solar blanket does not do that job even though it is excellent at the evaporation one.

The three kinds of cover

They do overlapping but not identical jobs, and only one of them is a safety device.

Water rules come from your supplier, not from the state

One thing worth being clear about, because there is a lot of confused writing on it: there is no single rule across our service area about filling a pool. Valley Water states the position plainly — restrictions on pools, including refilling, new construction and pool covers, are set by individual water retailers and cities and vary across the county.

That is equally true around the bay. The supply picture differs sharply between the Monterey Peninsula, the Salinas Valley — where the groundwater subbasin is designated critically overdrafted by the state — the Pajaro Valley, and inland San Benito and south Santa Clara counties. Each is managed differently and each retailer sets its own rules.

So the practical step before you plan a pool is a short conversation with whoever bills you for water, asking what currently applies to your address. We will help you frame the question. And the answer does not change the recommendation: fit a cover.

The rest of the water checklist

• Find leaks earlyAfter evaporation, leaks are the only other way water leaves. A bucket test separates the two in a couple of days and costs nothing.
• Do not oversize the poolThe fill is the largest single use. A pool sized for how you will actually use it is the first conservation decision.
• Cartridge filters do not backwashSand and DE filters discharge water every time they are backwashed. A cartridge filter is cleaned by hand and sends none down the drain.
• Keep the level at mid-skimmerToo high and the skimmer stops working; too low and the pump takes air. Neither wastes water directly, both waste energy.
• Windbreaks workWind drives evaporation. A hedge, a wall or siting the pool in the lee of the house reduces water and heat loss for free, permanently.
• Mind the water featuresMoving water in air evaporates faster than still water. Run features on a schedule rather than continuously and you keep the sound without the loss.

Cover and water questions

Related reading

• Heating a pool on the coastThe other half of the same story
• Equipment and automationSpecifying the pad
• Pool maintenanceWhat a cover does for your service load
• Pool builder in SalinasA critically overdrafted basin
• Pool builder in WatsonvilleThe Pajaro Valley basin
• Pool repairLeak detection