Restroom Trailer Generators: Sizing, Fueling, and Who Owns It

In most cases the customer orders the generator for a construction site restroom trailer, or plugs the trailer into the GC's temporary power or an existing site generator. When that is not practical, the facilities vendor can supply one sized to the trailer load and handle the fueling. What actually decides whether the setup holds is not who owns the equipment. It is whether the generator is sized to the real circuit load and whether one named company fuels it on a logged schedule. Get those two right and the question of ownership is a line item. Get them wrong and you find out at 6 a.m. in January, with a dry tank and frozen supply lines.

Below: circuit counts per model, why the generator ordered "to be safe" is the one that fails, what a 24/7 run burns, and how to split supply, fuel, monitoring, and service between contracts.

Who supplies the generator: three arrangements that work

There are three ways a trailer gets power on a jobsite, and all three are normal. The right one depends on what is already on the site and who manages it. Pick one deliberately rather than inheriting whatever shows up on delivery day.

• Customer-supplied generator or GC power. The most common arrangement, and the one we recommend. The site orders a generator or already has temporary power or a larger generator feeding the office-trailer compound, and the restroom trailer becomes one more load on it. The vendor tells you the circuit count; your electrician provides the receptacles.
• Vendor-supplied generator, on request. When the customer would rather not order one, the facilities vendor brings a generator sized to the trailers it is delivering, sets it on the pad, runs the cords, and folds fueling and generator checks into the service schedule. One company owns the whole chain from fuel tank to flush.
• Shore power. A temp power pole or permanent service is close enough to the pad, and the trailer plugs straight in. No generator, no fuel, no wet-stacking. The limit is distance and the number of 20A receptacles available.

Long jobs often move between these, starting on a generator during mobilization and cutting over to shore power once temp service is energized. Schedule that cutover, because it changes who keeps the heat on.

Size to circuits, not to the number of trailers

Our trailers do not need a heavy electrical service. Power is multiple standard 20A/120V cords, and the count changes between summer and winter because the cold-weather systems draw. That makes sizing a counting exercise: add the circuits for every unit on the generator, for the season you are actually in, and work from that number instead of from a rule of thumb per trailer.

The table gives the circuit count per model. The last column is simple electrical arithmetic, not a measured draw: a 20A circuit at 120V tops out at 2.4 kW, and the electrical code's 80 percent rule for continuous loads puts the practical limit at 16 amps, or about 1.9 kW per circuit. Treat it as a ceiling; the running load is usually lower.

Model	Summer circuits (20A/120V)	Winter circuits (20A/120V)	Winter breaker ceiling (circuits x 2.4 kW)
2 Stall	1	2	4.8 kW
4 Stall 4 Door	1	3	7.2 kW
8 Stall 8 Door	3	5	12.0 kW
10 Stall 7 Door	3	5	12.0 kW
8 Stall 2 Door	3	4	9.6 kW
10 Stall 2 Door	2	4	9.6 kW
3 Stall ADA	2	4	9.6 kW
8 Station 8 Door Shower	3	4	9.6 kW
Laundry	4	4	9.6 kW

The winter count is the design point: a 4 Stall 4 Door goes from one circuit to three, a jump of three times, because heat is the biggest load on the unit. Even a site with two 8 Stall 8 Door units and a 10 Stall 7 Door totals fifteen winter circuits. That is a real load, but a knowable one, and far below what gets requisitioned when someone assumes "trailers need big power."

Why the oversized generator is the one that fails

Oversizing feels like insurance. On a diesel generator it is the opposite. Industry guidance, including the NFPA exercise standard for standby sets, puts the floor at roughly 30 percent of rated load, and most field techs quote 30 to 35 percent. Run lighter than that for weeks and the engine never reaches full operating temperature. Unburned fuel and carbon collect in the exhaust, injectors foul, the stack drips black, and the unit loses power or shuts down. That is wet-stacking, and on a restroom trailer load it is the most common way a perfectly good generator ends up on a service truck.

The trap is seasonal. A set sized for the winter ceiling of three large trailers can sit below 30 percent for the entire summer, because the summer circuit count is lower and a mild-weather trailer rarely pulls its breaker limit. On a nine-month job that starts in April, that is five months of light loading before the load the generator was bought for ever shows up. The industry fix is a few hours at around 75 percent on a load bank to burn the deposits off, which is its own line item and visit.

The better answer is to size for the phase you are in. That can mean a smaller set through summer and a swap before the first hard freeze, or putting the trailers on a generator that also carries the office compound so the combined load stays in the healthy band. Our earlier post on pull-behind generators for restroom and shower trailers covers the equipment itself. The point here is narrower: the question is never "is it big enough," it is "is it loaded enough, in this month."

Fuel on a 24/7 run is a service line, not an assumption

A trailer generator does not shut off at the end of shift. Heat, water heater, and lights run through the night, so the generator runs 24/7 for the length of the job. Fuel burn on that runtime routinely exceeds the bid-time estimate, usually because somebody multiplied a burn rate by a ten-hour shift.

Published industry fuel charts make the scale clear. These are typical approximate figures for diesel sets, not Stahla data, and actual burn varies with engine, age, maintenance, and temperature:

Generator size (typical diesel)	Load	Approx. gal/hr	24 hours	7 days
20 kW	1/4	0.6	about 14 gal	about 100 gal
20 kW	1/2	0.9	about 22 gal	about 150 gal
30 kW	1/4	1.3	about 31 gal	about 220 gal
30 kW	1/2	1.8	about 43 gal	about 300 gal

The larger set at quarter load burns more than the smaller set at half load for the same work, which is the fuel cost of oversizing on top of the wet-stacking risk. And a week of runtime is hundreds of gallons, more than most towable sets carry onboard. That makes fueling a tracked service: a scheduled refuel, a logged tank reading every visit, a trigger level for an extra delivery, and remote tank monitoring on bigger sites. For an equipment-arm buyer, fuel should be its own itemized line next to pump-out, water fill, and restock.

Who owns refueling: vendor or the GC's fuel contractor

Most sites already have a fuel contractor running a truck for the excavators and light towers, and it is tempting to add the restroom generator to that route. It works only if the generator is actually on the driver's list, with a location, tank size, and frequency, and only if someone covers the weekends when the rest of the fleet sits idle.

The alternative is vendor fueling on service visits, tied to the technician already on the pad to pump out and restock. On a distribution-center shutdown we worked, that was literally the job: our technician stayed on site through the overnight window keeping generators fueled, as described in restroom trailers for distribution-center shutdowns. Either model is fine. What fails is splitting the work without writing it down. Use a matrix like this one:

Task	Vendor-fueled setup	GC fuel contractor setup	Put in writing
Supply and place the generator	Facilities vendor	GC or its equipment arm	Size, pad location, cord runs
Size to the circuit load	Facilities vendor	GC electrician, using vendor circuit counts	Summer and winter circuit totals
Refuel	Facilities vendor, on service visits	GC's fuel contractor route	Frequency, weekend and holiday coverage
Tank level log	Service technician each visit	Fuel driver plus site staff	Trigger level for an extra delivery
Winter fuel treatment	Facilities vendor	Fuel contractor	Winter blend or anti-gel from first freeze
Generator maintenance (oil, filters, load bank)	Generator owner	Generator owner	Service hours interval
Trailer service (pump-out, water fill, restock)	Facilities vendor	Facilities vendor	Cadence per phase
After-hours power loss call	Facilities vendor	GC first, then vendor for trailer checks	Name and phone number, not a department

The last row is the one that gets skipped. When a generator dies at 2 a.m., the first call needs to know whether they are fixing power or checking plumbing. Write the name.

Shore power vs generator: when to plug in instead

Shore power is almost always the better answer when it is available: no fuel to haul, no generator to maintain, no wet-stacking, no overnight run-dry risk. The trade-off is placement. The trailer has to sit within cord distance of a temp power source with enough 20A circuits, which is not always where the crew needs the facilities.

Before delivery, confirm the connection with whoever runs site power. How many 20A/120V receptacles are available at the pole, how far is the run, and is that count good for the winter number or just the summer one? A mismatched or bare-wire connection on delivery day means an unplanned electrician and a lost day, and it is one of the more avoidable failures on this list.

On campus-scale jobs, early trailers run on generators because nothing is energized; as temp power reaches each building, units move to shore power and generators come off rent. Our guide to remote sites with no water, sewer, or power covers the full off-grid stack for sites where that cutover never comes.

What fails: the generator that runs dry overnight in January

The failure that matters most is predictable. A generator runs dry on a Saturday night in January. The heat goes off. By Sunday morning the supply lines, water heater, and exposed fittings have frozen, and Monday's crew finds a trailer that will not flush. The missing diesel cost a fraction of the thaw and the lost shift.

Cold-weather protection only works while it has power. HeatLock is our cold-weather system for keeping the plumbing chain protected through freezing conditions, and like any heated system it depends on the generator staying on. That is why the winter circuit counts in the table are higher and why the operating rule in winter is to never shut a trailer generator down to save fuel overnight. The Ellendale data center deployment, North Dakota in January, is what that discipline looks like on a real site.

The other failures cluster around the same few causes:

• Gelled fuel. Summer diesel left in a tank into the first hard freeze clouds, then gels and plugs the filter. Winter blend or anti-gel additive from the first freeze onward prevents it.
• The holiday weekend. The fuel route skips the site because nothing else is running. Three days later the tank is empty.
• A wet-stacked summer set that finally gets loaded in November and cannot carry it.
• Unreachable generators. A fuel truck that cannot get to the pad after a thaw is the same as no fuel at all.

None of this needs special equipment to prevent. It needs the matrix above, filled in, before mobilization, and an escalation path for when it breaks anyway. On the Montgomery County decon deployment, fuel ran every day or two on a round-the-clock generator, and when it still ran dry on a Saturday the fix was a direct escalation to the fuel vendor. More winter detail is in our write-up on winter-proofing restroom and shower trailers on a jobsite.

Frequently asked questions

What size generator does a restroom trailer need?

Size it to the circuit count, not to the number of trailers. Our units run on standard 20A/120V circuits: an 8 Stall 8 Door takes three in summer and five in winter, and a 4 Stall 4 Door takes one and three. Add up the winter circuits for everything on the generator, and choose a set that stays above roughly 30 percent load in the season you are in.

Does Stahla supply the generator, or do we?

Usually you do. We recommend the customer order the generator, or connect the trailers to your temporary power or site generator using the circuit counts we provide. If you would rather we handle it, we can supply a generator sized to the trailers we deliver and fuel it on service visits. Either way, write the arrangement down before delivery.

Who is responsible for refueling the generator?

Whoever the agreement names, and it should name one company. If the facilities vendor fuels, it happens on service visits; if your fuel contractor fuels, the generator needs to be on that route with weekend and holiday coverage. Unassigned fueling is the most common cause of an overnight power loss.

How much fuel does a trailer generator burn in a week?

Typically hundreds of gallons on a 24/7 run. Industry charts put a 20 kW diesel set at about 0.6 to 0.9 gallons per hour at quarter to half load, which is roughly 100 to 150 gallons a week. Actual burn depends on the engine, the load, and the temperature, so log tank readings rather than relying on the estimate.

Why can't we just order a bigger generator to be safe?

Because a lightly loaded diesel wet-stacks. Below roughly 30 to 35 percent of rated load, fuel and carbon build up in the exhaust and the engine loses power or shuts down, and the larger set also burns more fuel for the same work. Right-sizing is the safe option.

Can we shut the generator off at night to save fuel?

Not in freezing weather. The trailer's heat and freeze protection, including HeatLock, need power through the night, and a generator shut off on a cold evening is a frozen line by morning. In mild weather it is still better left running so the trailer is ready for early shifts.

Bring the circuit count to the conversation

Put the numbers on one page: which trailers, how many winter circuits, which months the job runs, and who already runs fuel on the site. The generator size, the fueling owner, and the shore-power cutover date usually fall out of that.

We work construction sites nationwide, from shops in Nebraska, Kansas City, and Denver plus vetted partners, and we are happy to run that arithmetic on your site. Send the unit mix and the schedule through a quote request, or reach us directly if you want to talk through the responsibility split before anything is ordered.