Temporary power on a commercial build is not one load that grows steadily. It rises in steps, falls in places, and changes in character as the work moves from concrete to structure to interior finishes. A generator chosen for the first month can be wrong for the sixth, in either direction. This page follows the load through the phases so you can plan which unit serves each stretch and when to change it.
Safety rules follow the load too. Temporary receptacles on a construction site need ground-fault protection, as set out in OSHA 1926.404, and that requirement applies at every phase, not only the first. Plan the distribution gear to match each phase’s layout so the protection travels with the work.
Slab and site work: few loads, long hours
The early phase tends to have the lightest electrical load on the job. Site trailers, lighting, small tools and pumps make up most of it, and heavy equipment typically runs on its own fuel. What the phase lacks in size it makes up for in duration, because the trailer and lighting run whether or not crews are on site. A small unit matched to those loads usually serves better than a large one running far below its capacity.
Placement matters here because the building footprint is still changing. A unit set where the structure will rise has to move later, so choose a spot that stays clear through the next phase if you can.
Structure and shell: loads spread out
Once the frame goes up, the work spreads across the site and up the building. Welders, saws, hoists and temporary lighting appear in more places, and the loads that matter are the ones running at the same time. Cable runs get longer, and voltage drop begins to deserve attention. This is often where a single central unit with distribution gear starts to make more sense than several small ones.
The sizing guide shows how to total simultaneous loads, which is the number that sizes the unit. Rebuild that total at the start of each phase rather than carrying forward the one from the previous phase. The bid side of this planning is covered in our page on what a temporary power line item should include in a commercial bid.
Rough-in to finishes: the peak
Interior work usually brings the highest and most varied load. Many trades work in the same space, tools multiply, and temporary lighting and heating or cooling for the crews may be added. The table below sketches how the load changes by phase. It describes patterns, not figures, since every building differs.
| Phase | Load pattern | What to plan |
|---|---|---|
| Slab and site | Light, steady, long hours | A right-sized small unit, placed out of the building footprint |
| Structure and shell | Spreading, intermittent | Central unit with distribution, longer cable runs |
| Rough-in and finishes | Highest, many trades at once | Larger capacity or added units, tested distribution |
| Commissioning and punch list | Falling, with test spikes | Step down in size, plan for start-up loads |
When the load outgrows a single unit, you can add capacity instead of replacing it. Our page on paralleling generators explains how units combine to serve larger loads, which can suit a phase that peaks and then recedes.
Commissioning and punch list: smaller, but not simpler
As permanent systems come online, the temporary load shrinks, but it does not become trivial. Start-up testing can draw sharp spikes, and the schedule is tight because everything left depends on the last items finishing. A unit that was correct for the peak may now be much larger than the load needs. Stepping down saves cost, as long as the smaller unit can still handle the start-up loads on the test list.
The date permanent power becomes available is the other variable, and it can move. Build the exit plan with that uncertainty in mind, and confirm the current date with the utility and the electrical contractor. Keeping a short extension option in the rental terms costs less than a gap in power. For what a lost day of site power does to a commercial schedule, see our page on the cost of a site power outage.
Checking the plan at each handoff
The list below is a short check to run when the job moves from one phase to the next. It turns the timeline into a repeatable routine, so power planning does not depend on someone remembering.
- Rebuild the simultaneous load list for the incoming phase
- Confirm the current unit’s size still fits, or decide whether to step up or down
- Re-check where the unit sits against the next phase’s footprint
- Review distribution and ground-fault protection for the new layout
- Confirm fuel plan and run hours with the new shift pattern
- Check the date permanent power is expected and the rental end date
Planning the rentals with Stag
Stag Power Rentals supplies diesel generators from 20 kW to 2000 kW out of its Dickinson, TX yard, and the team can help map a rental plan across phases. Bring the schedule, the phase list and the loads you expect in each, and ask about sizes, shift billing and options for changing units mid-project. The power distribution page covers the gear that connects units to the work. For the broader view of construction support, see our construction power overview, and when you are ready, request a quote.
Frequently Asked Questions
Does a commercial build need the same generator from start to finish?
Not usually. The load starts light, peaks during interior work, and falls during commissioning. Matching the unit to each phase can reduce cost and avoid running a large unit at a small load.
When does temporary power demand peak on a commercial project?
Often during rough-in and finishes, when many trades work at once and tools, lighting and climate control for crews all run together. The exact peak depends on the building and the schedule. A simultaneous load list for each phase shows where yours falls.
Can I add generator capacity instead of swapping units?
In many cases, yes. Paralleling lets units share a load, which suits a peak that later recedes. Ask the rental team whether it fits your load and equipment.
How do I plan for a permanent power date that moves?
Confirm the current date with the utility and electrical contractor, and keep an extension option in the rental terms. That way a slip does not leave a gap. Recheck the date at each phase handoff.
What should I send with a quote request for a phased project?
Send the schedule, the phase list, the expected loads and voltage in each phase, and the shift pattern. If you know when permanent power is expected, include it. That lets the rental team plan sizes across the whole timeline.