You’re at the kitchen window with your coffee, watching the rinse wand sweep down the siding. Then the thought hits: how many gallons was that, and whose water bill is it on? Here’s the math, who pays, and when local water rules come into play.

The simple math: gallons per minute times minutes of spray

Every pressure washer is rated in gallons per minute (GPM). The rating sits right in the product name or spec sheet. Multiply it by the minutes the trigger is actually pulled, and you have your gallons.

Trigger time is the part nobody can promise you in advance. It depends on the house, the crew and the dirt. So the table below uses one full hour of spraying as a round, generous example.

Machine type Example (spec page) Flow Gallons per hour of spray San Antonio per-gallon charges for that hour
Homeowner electric Greenworks 1800 PSI 1.1 GPM 66 about $0.20 to $1.08
Homeowner gas Expert Power Wash 3,200 PSI 2.5 GPM 150 about $0.45 to $2.45
Pro gas, direct drive Kleen-Rite / BE 4,000 PSI 4 GPM 240 about $0.72 to $3.92
Pro belt-drive skid APW 3500 PSI GX690 8 GPM 480 about $1.44 to $7.84

For scale, the EPA says standard showerheads use 2.5 gallons per minute, and showering adds up to nearly 40 gallons a day for the average family. So the 240-gallon hour is roughly a week of a family’s showers. Our electric or gas pressure washer guide explains what those GPM numbers mean for cleaning power.

What that water costs: one real utility’s rates

Rates vary a lot from town to town, so let’s use one real bill as the example: San Antonio’s water utility, which bills per 1,000 gallons in tiered blocks.

For 2026, the residential volume charge inside city limits starts at $0.907 per 1,000 gallons for the first 4,000 gallons and climbs to $5.669 above 20,000 gallons. A separate water supply fee also runs per 1,000 gallons ($1.631 in the first block, $10.194 above 20,000 on the utility’s posted schedule). Two smaller per-gallon fees add about $0.47 more.

Added up, those per-gallon charges run about $3.01 per 1,000 gallons in the lowest block and about $16.33 in the highest. That’s where the table’s ranges come from. Where you land depends on your household’s total use that month.

What about sewer? Many bills charge sewer based on water use, but not always on every gallon. San Antonio bases residential sewer on your average use over a 90-day winter window that begins after November 15. A summer house wash doesn’t touch it. A wash in that window can nudge the average up a little.

Drought pricing. During Stage 3 drought, San Antonio adds a $10.37 surcharge per 1,000 gallons, but only on residential use above 20,000 gallons a month. A 240-gallon wash is about 1% of that threshold.

So, who pays for the water?

You do, almost always. A house washing crew screws its supply hose onto your outdoor spigot, and the water runs through your meter. One washing company’s FAQ spells this out: its crew needs access to a water spigot, or water can be brought to the job for an additional fee. If a company hauls water in, expect that cost to show up in the quote.

Washers on one trade forum say bigger machines can outrun a house spigot, so they run a tank on the truck that refills from the spigot as they work. It’s still your water, just stored for a minute. One washer’s manual asks for a supply of at least 5 GPM at 20 PSI. The bucket test in our electric or gas guide checks that. The weight of that tank is covered in water tank weight, trailer brakes and DOT numbers.

What about using the neighbor’s tap? In one online discussion among pressure washers, a washer borrowed the neighbor’s spigot with permission. The neighbor then blamed a bill of several hundred dollars on the job. Replies pointed out the bill covered a whole billing period, not an hour of washing. Keep it simple: use your own spigot.

If a spike worries you, ask the estimator roughly how many minutes of spraying they expect and what GPM their machine runs. That’s a fair question for any house washing company.

Drought rules and local registration

Water rules can matter more than the water bill. In San Antonio, washing driveways, sidewalks and other impervious cover is banned during drought stages, with exceptions for health and safety. Siding isn’t on that list, but runoff from any wash can still count as water waste. Our guide to pressure washing during water restrictions covers what is usually allowed. Storm cleanup is a different case; see after a hurricane or flood.

San Antonio also requires anyone who uses a power washer commercially or for pay to register with the water utility. If you live there, asking a company whether it’s registered is a quick check. Elsewhere, look up your own water district’s drought stage and outdoor-use rules before you book.

The water is rarely the expensive part of a house wash. Labor, equipment and chemicals are, and our house washing cost guide breaks those prices down. For how we sourced these numbers, see our methodology.

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