What Size Generator Do I Need? Running Watts, Starting Watts and Sizing (2026 Guide)

What size generator do you need for power tools?

Short answer: add up the running watts of everything that will draw at once, then add the single largest starting surge on the list. For one 15-amp saw (1,800 running watts) plus a work light and chargers, that lands near 2,800–3,500 watts — a 4,000–4,500-watt-class generator with 3,500+ running watts covers it with headroom. A compressor changes the math: induction motors can briefly draw several times their running load at startup.

"What size generator do I need" is really two questions — how many watts does my equipment draw continuously, and how big is the worst-case starting spike — and every sizing mistake comes from ignoring the second one. This guide gives you the two-watts distinction, a tool draw table, a four-step sizing worksheet you can run on a napkin, and the two topics generator spec sheets skip: what OSHA actually requires for grounding a portable generator, and how the cord between the generator and the tool becomes the weakest link. Placement, exhaust and refueling are covered in depth in how to run a generator safely — this page stays on sizing and the electrical side.

Why this matters.
An undersized generator does not fail politely: it bogs under the starting surge, browns out every connected tool (which overheats their motors), and trips its breaker mid-cut. Oversizing wastes fuel and money. And the sizing decision is also a safety decision — the U.S. Consumer Product Safety Commission estimates that generators meeting the UL 2201 CO-mitigation standard would avert nearly 100% of generator CO deaths, against about 87% for PGMA G300 units — so the shortlist should start CO-shutoff-only before wattage enters it.

Running watts vs starting watts: the two numbers on every spec sheet

Running (rated) watts is what the generator sustains continuously. Starting (peak or surge) watts is what it can deliver for a few seconds while a motor spins up. Manufacturers headline the bigger number — a "4500W" generator is typically a 4,500-watt peak machine with roughly 3,400–3,800 running watts; the AMERISUN 4500W open frame inverter generator, 3800 running watts states both figures in its own name. Loads mirror the same split:

• Resistive loads — heaters, incandescent lamps — draw the same watts always. No surge.
• Universal-motor tools — circular saws, grinders, drills — surge modestly, roughly 1.5–2× running draw, and briefly.
• Induction-motor equipment — compressors, pumps, refrigerators — surge hard, commonly 2–6× running watts, and a compressor restarting against tank pressure is the worst case on most jobsites.

Tool draw table: planning figures

Watts = volts × amps, so a 120-volt tool's nameplate amps convert directly. Use the tool's own nameplate for the final math — these are planning ranges:

The four-step sizing worksheet

1. List what runs at the same time. Not everything you own — what draws concurrently. A saw and the vacuum its operator triggers together count together; two saws used by two crews count together; the charger bank counts always.
2. Add the running watts. Nameplate amps × 120 for each item, summed. This must land under the generator's running rating — not its headline peak.
3. Add the single largest starting surge. Take the one item with the hardest start — almost always the compressor — and add its surge increment on top of the running total. Motors that start one at a time only spike one at a time; if two motors genuinely start together, add both surges.
4. Buy 20–25% above the answer. Generators age, air density and altitude shave output, and continuous running near 100% load shortens engine life and worsens voltage regulation. Headroom is not waste; it is regulation quality.

Worked example: one cut station plus support loads

A framing crew wants one 15-amp miter saw, a pancake compressor for a trim nailer, an LED area light and a four-bay charger on one generator:

1. Concurrent list: saw (1,800 W), compressor (1,440 W running), light (150 W), chargers (600 W).
2. Running total: 1,800 + 1,440 + 150 + 600 = 3,990 W — if everything literally draws at once. Realistically the saw and compressor alternate, but sizing to the honest sum is what prevents brownouts.
3. Largest surge: the compressor. At a conservative 3×, its start adds roughly 2,900 W over its running figure — peak demand near 6,900 W if it kicks in mid-cut, or about 5,800 W with the saw idle.
4. Verdict: this is honestly a 6,500–7,500-watt-class job — e.g. the Westinghouse 6500 watt dual fuel home backup portable generator — or a workflow decision: keep the compressor on its own smaller unit, and a 4,500-peak-class inverter like the WEN 4500-watt inverter generator, RV-ready, quiet, portable runs the cut station alone comfortably.

Both routes are legitimate; what is not is buying the 4,500 and hoping. The full CO-shutoff shortlist lives in best generators with CO shutoff and the jobsite generators collection.

Inverter vs open-frame, and when a battery replaces the engine

Two spec-sheet forks matter at sizing time. Inverter generators produce cleaner power (low total harmonic distortion) that sensitive electronics tolerate, throttle down under light load to save fuel and noise, and dominate below ~7,000 watts. Open-frame conventional units deliver more watts per dollar and shrug off dirty duty, at the cost of noise and weight. For loads under about 1,800 watts with no hard motor starts — chargers, lights, small tools in short bursts — a portable power station sidesteps the engine entirely: zero CO, zero fuel, indoor-safe, with capacity measured in watt-hours (a 1,000 Wh station runs a 250 W load about four hours). The trade is refuel speed — a gas tank refills in minutes; a battery recharges in hours.

Noise belongs in the sizing decision too, because it decides where the generator can physically sit. Open-frame units commonly run in the mid-70s to mid-80s dBA at rated load — loud enough that OSHA's hearing-conservation math starts to matter for anyone working beside one all shift, a scale you can place on our decibel levels chart — while closed-case inverters in the 2,000–3,000-watt class advertise low-to-mid 50s dBA at quarter load, the difference between a unit that can sit near an occupied area and one that cannot. The manufacturer's dBA figure is measured at a stated distance and load, so compare like to like; the Westinghouse 2550 peak watt super quiet portable inverter generator is the pattern of the quiet small-inverter class. Distance does the rest: sound pressure falls roughly 6 dB per doubling of distance, which the mandatory 20-foot placement already buys you.

Grounding a portable generator: what OSHA actually requires

The persistent jobsite myth is that every portable generator needs a driven ground rod. OSHA's construction standard says otherwise for the common case: under 29 CFR 1926.404(f)(3)(i), the frame of a portable generator need not be grounded to a grounding electrode and may itself serve as the grounding electrode when both conditions hold:

• the generator supplies only equipment mounted on it and/or cord-and-plug equipment connected through receptacles on the generator itself, and
• the noncurrent-carrying metal parts of that equipment and the equipment grounding conductor terminals of the receptacles are bonded to the generator frame.

Plug tools into the generator's own outlets with proper three-wire cords and the fault path is complete through the frame — no rod. The picture changes when the generator feeds a structure's wiring: bonding and transfer-switch rules engage (a separately derived system's neutral must be bonded per 1926.404(f)(3)(iii) and the installation must isolate from the utility), which is transfer-switch territory covered in how to run a generator safely — along with why backfeeding through a dryer plug is never on the table.

The cord is part of the generator: sizing the last hundred feet

Because the generator sits 20 feet from any building and often farther from the work, the cord run is long by design — and every sizing gain evaporates if 8% of the voltage dies in thin copper. Two rules close the loop:

• Feed 15-amp tools through 12 AWG at 100 feet, 10 AWG past that. The arithmetic and the full chart are in the extension cord gauge chart; the cords themselves are in extension cords and cord reels.
• Use the 30-amp outlet only with a 30-amp cord. A generator's L5-30 locking receptacle delivers its rating through 10 AWG cordage with matching ends — adapting it down to a 15-amp household cord moves the bottleneck (and the heat) into the cord.

GFCI protection follows the cord too: on construction sites, receptacles not part of permanent wiring need GFCIs under 1926.404(b)(1)(ii) — the standard carves a narrow exception for receptacles mounted on generators of 5 kW or smaller with insulated circuit conductors, and an inline GFCI at the generator settles the question for any unit. The full protection rules are on OSHA extension cord requirements.

Why does my generator trip or bog? The troubleshooting section

• Trips the moment a specific tool starts: starting surge exceeds peak capacity. Re-run the worksheet with that tool's surge; stagger starts so only one motor spins up at a time.
• Bogs mid-cut, tools feel weak: running total exceeds the running rating — or the voltage is dying in an undersized cord. Check the cord gauge against the chart before blaming the engine.
• Trips only when the compressor kicks back in: the restart-against-pressure surge. Bleed-down leaks make it worse by forcing frequent restarts; fix the leak, or move the compressor to its own circuit or unit.
• Runs fine, then shuts down near a wall or in an enclosed bay: that is the CO shutoff doing its job — exhaust is accumulating. Do not restart in place; relocate the unit. Placement rules are owned by how to run a generator safely.
• Electronics glitch on an open-frame unit: harmonic distortion. Sensitive gear wants an inverter unit or a portable power station upstream.

Frequently asked questions

What size generator do I need to run power tools?

Sum the running watts of what draws concurrently, add the largest single starting surge, then buy 20–25% above that. One 15-amp saw plus lights and chargers lands in the 3,500–4,500-watt class; add a compressor and the honest answer is 6,500-watt class or a second unit. The worksheet above walks it step by step.

What is the difference between running watts and starting watts?

Running watts is continuous output; starting watts is a seconds-long reserve for motor start-up. Both appear on every legitimate spec sheet, and sizing against the headline peak number instead of the running rating is the single most common generator-buying mistake.

What size generator runs a circular saw?

A 13–15-amp saw draws 1,600–1,800 running watts with a brief start spike, so a unit with about 2,500+ running watts handles one saw with margin — the BILT HARD gas generator 2500W, 80cc 4 stroke quiet inverter generator class is the floor for saw work, sized for the saw alone.

Can a generator run an air compressor?

Yes, if it covers the compressor's restart surge — induction motors restarting against tank pressure briefly draw several times their running watts. A 1,500-running-watt pancake compressor can legitimately demand 4,000+ watts for a second; that surge, not the running figure, sizes the generator.

How many watts does a jobsite need in total?

Small crews running one cut station, lights and chargers typically plan around 3,500–4,500 watts; adding a compressor or a second concurrent saw pushes toward 6,500–7,500. Beyond roughly 8,000 watts of genuine concurrent load, dual units or a larger unit like the WEN 14500-watt 120V 240V tri-fuel generator for gas, propane and natural gas class enters the conversation.

Does a portable generator need a ground rod?

Not in the common configuration. Under OSHA 1926.404(f)(3)(i), a portable generator feeding only cord-and-plug equipment through its own receptacles, with frame-bonded components, may use its frame as the grounding electrode. A rod becomes relevant when the generator feeds building wiring as a separately derived system — transfer-switch territory.

Do generator receptacles need GFCI protection?

On construction sites, receptacles not part of a building's permanent wiring generally require GFCI under 1926.404(b)(1)(ii); receptacles mounted on generators of 5 kW or less with insulated circuit conductors can fall under the standard's exception. An inline GFCI at the generator covers you on any unit — the full rule is on OSHA extension cord requirements.

What is an inverter generator and do I need one?

An inverter generator rectifies and re-synthesizes its output, producing low-distortion power that laptops, diagnostic gear and modern tool chargers tolerate, while throttling to match load — quieter and more fuel-efficient at partial draw. For tool-only duty an open-frame unit works; for mixed tool-and-electronics duty the inverter premium pays for itself.

Generator or portable power station for the jobsite?

Engine generators win on sustained high loads and instant refueling; battery stations win on zero CO (indoor-safe), zero noise and zero maintenance for loads under about 1,800 watts without hard motor starts. Many crews run both: a generator for the saws, a station like the Anker SOLIX C1000 Gen 2 portable power station, 1024Wh LiFePO4 for chargers and lights inside the envelope.

How far should a generator be from a building?

At least 20 feet, exhaust pointed away from doors, windows and vents — the placement guidance FEMA, NFPA and the U.S. Fire Administration converge on, and the reason CO shutoff is the entry ticket on our best generators with CO shutoff shortlist. Placement, rain protection and refueling are covered start to finish in how to run a generator safely.

What does a generator CO shutoff actually do?

A CO sensor on the unit shuts the engine down when exhaust accumulates around it — the scenario that kills. CPSC analysis credits units meeting UL 2201 with averting nearly 100% of generator CO deaths and PGMA G300 units with about 87%, which is why the certification — not the marketing phrase — is what to verify before wattage even enters the decision.

What size extension cord does a generator need?

Size it like any cord: nameplate amps and real run length against the extension cord gauge chart — 12 AWG for 15-amp tools at 100 feet, 10 AWG with matching locking ends to use a 30-amp generator outlet at its rating.

Can I run two tools on one generator at the same time?

If their combined running watts fit under the running rating and only one motor starts at a time, yes. Two 15-amp saws cutting simultaneously need roughly 3,600 running watts plus one start surge — comfortably inside a 4,500-peak/3,800-running unit, tightly inside anything smaller.

Why does my generator surge or hunt at idle?

Cyclic surging under light load is usually fuel delivery (stale fuel, partially clogged carburetor) or an eco-throttle interacting with a load that pulses. Rule out the load first by disconnecting everything; a unit that hunts unloaded needs fuel-system service, not a bigger frame.

How long can a portable generator run continuously?

Most jobsite units run 8–13 hours per tank at half load — the spec sheet states runtime at 25% or 50% load. The practical limits are refueling (engine off, cooled — never hot-fuel) and oil-change intervals measured in tens of hours, both covered in how to run a generator safely.

Do watt-hours on a power station mean the same as watts?

No — watts is rate, watt-hours is quantity. A 1,024 Wh station delivering 500 W lasts about two hours; its separate output rating (say 1,000 W continuous / 2,000 W peak) plays the same role as a generator's running and starting watts. Both numbers appear in every listing in our portable power stations collection.

Further reading on this site

• Jobsite generators — the CO-shutoff lineup this worksheet sizes against.
• Best generators with CO shutoff — ranked picks with the certification checks.
• How to run a generator safely — placement, CO, backfeed, refueling and storm operation.
• Extension cord gauge chart — the copper between the generator and the tool.
• OSHA extension cord requirements — GFCI and cord rules for the whole temporary-power chain.
• Portable power stations — the zero-CO alternative for light loads.
• Jobsite power and electrical — the full temporary-power hub.