What happens if I overload a portable generator?

The generator's circuit breaker trips, cutting power to all connected devices. If the breaker fails to trip (rare but possible on cheap units), the engine bogs down, voltage drops below safe levels, and connected electronics risk damage from brownout conditions. Sustained overloading causes the engine to overheat, which can warp the cylinder head or damage the alternator windings. Some inverter generators have automatic shutdown at 110% load to prevent this, but you should never rely on that as a design strategy.

Should I use running watts or starting watts to size my generator?

You need both. The generator's rated (continuous) watts must exceed your total running watts. The generator's peak (surge) watts must exceed your running watts plus the highest starting watts of any one appliance, plus your safety margin on that peak: 20% by default, the margin Champion's tailgating guide applies. Generator makers advertise both ratings. This calculator gives you both figures, your running watts and your peak, and the peak with your margin on top, so you can match them against the generator's spec sheet. Makers do not all read the peak the same way. Champion's tailgating guide adds only the 300 W a mini-fridge draws above its running watts: its 1,700 W of running watts with a mini-fridge of 100 W running and 400 W starting give its peak of 2,000 W, where this page gives 2,100 W. With its 20% margin Champion prints 2,400 watts, where this page gives 2,520 W, because this page adds the whole 400 W of starting watts to the running watts. Cummins works an RV list: an air conditioner at 1,800 W running and 2,800 W starting, a fridge at 400 W running and 1,200 W starting, a coffee maker at 600 W, a 40-inch TV at 70 W, lights at 60 W and two cell chargers at 20 W. It adds the starting watts of its largest starting load, the air conditioner, to the running watts of all the other loads: “We calculate 3950 starting watts. We add all the running power values and calculate 2950 running watts.” On the same list this page gives a peak of 5,750 W, or 6,900 W with the 20% margin: it adds the 2,800 W to all 2,950 W of running watts, the air conditioner's own 1,800 W included, so its peak reads 1,800 W above Cummins's 3,950 W. Generac's sizing paper works a 1,200 W submersible pump at 5 times its running watts and asks for a surge rating of at least 6,000 W for the start; this page adds the pump's running watts too, for a peak of 7,200 W.

Can I run my central air conditioner on a portable generator?

It depends on the AC unit size and the generator. Champion's wattage chart prints a 3-ton central AC at 3,000 to 3,500 running watts and 5,000 to 6,000 starting watts. For 3,500 running watts and 6,000 starting watts alone, this page gives a peak of 9,500 W, or 11,400 W with the 20% margin. Window AC units are much more manageable: Champion's chart prints a 10,000 BTU window unit at 900 to 1,200 running watts and 1,800 to 2,400 starting watts. For whole-house backup including central AC, enter every load the house runs at once and compare both figures with the generator's spec sheet.

Do I need a different generator size for a construction site vs home backup?

The sizing method is the same, but the load profiles differ. Construction sites have frequent, heavy motor surges from power tools (table saws, compressors, concrete mixers) that kick on and off throughout the day. Home backup tends to have steadier loads with occasional surges from the fridge or sump pump. Job site generators also need to be more rugged and are typically rated for continuous duty, while home backup generators may only run during outages. Calculate your specific loads either way: the generic "5kW for a job site" rule breaks down quickly once you add a second compressor or a welder.

Generator Sizing Calculator - Surge Watts

9 min read

Find out what size generator you actually need before you buy one that is too small or waste money on one that is too large. The generator sizing calculator adds your total running watts to the highest starting watts of any one appliance and gives the two figures a generator's spec sheet lists: running watts and peak watts, plus the peak with a safety margin. If you already own a generator and want to plan battery backup alongside it, our inverter battery sizing calculator shows how much battery capacity covers the loads a generator cannot.

Portable generator with engine, outlet panel, fuel cap, and wheeled frame.

What Happens When You Undersize a Generator

An undersized generator does not just struggle: it fails. When the load exceeds the rated capacity, the voltage and frequency drop. Sensitive electronics (computers, medical equipment, modern refrigerators with inverter compressors) can be damaged or destroyed by low-voltage output. The generator itself overheats, and sustained overloading shortens its lifespan dramatically or trips the circuit breaker, cutting power to everything.

An oversized generator wastes fuel and money. A 10kW generator running a 3kW load burns more fuel per useful watt than a properly sized 5kW unit. Generac's sizing paper says that for diesel gensets, when less than 30% of the rated capacity is applied, the generator has been oversized, which can lead to “wet stacking”: unburned fuel escapes into the exhaust side of the engine, engine performance decreases and the exhaust and emissions components can be damaged. The same paper puts the optimum load at 70-85% of the generator's rated capacity.

The biggest sizing mistake people make is forgetting about motor starting surges. Champion's wattage chart prints a 1 HP well pump at 1,000 to 2,000 running watts and 2,000 to 4,000 starting watts. If your generator cannot deliver that momentary spike, the pump will not start, even if the generator handles the running load fine. That is why this calculator separates running watts from starting watts.

Generator sizing table: fridge 1,200 W starting, sump pump 800 W running and 2,000 W starting, window AC 1,200 W running. — Starting watts run well above running watts for motor-driven appliances: Generac's rule of thumb works a 1,200 W motor at 5× (6,000 W starting). The table's sump pump draws 2,000W on startup but only 800W while running.

Generator Sizes for Common Scenarios

Scenario	Running Load (example)	Highest Starting Watts	Peak	Peak + 20% Margin
Camping / Tailgating	1,000-2,000 W	1,000 W	2,000-3,000 W	2,400-3,600 W
RV / Travel Trailer	2,500-4,000 W	2,800 W (13,500 BTU RV air conditioner)	5,300-6,800 W	6,360-8,160 W
Home Backup (Essentials)	3,000-5,000 W	2,500 W	5,500-7,500 W	6,600-9,000 W
Home Backup (Full House)	8,000-15,000 W	5,000 W (3-ton central AC)	13,000-20,000 W	15,600-24,000 W
Job Site (Power Tools)	3,000-7,000 W	7,500 W (2 HP air compressor)	10,500-14,500 W	12,600-17,400 W
Food Truck / Market Stall	4,000-8,000 W	3,000 W	7,000-11,000 W	8,400-13,200 W

These rows are examples worked through this calculator with a 20% margin on the peak; enter your own loads above rather than relying on generic ranges. The appliances named in the starting column are printed on Champion's wattage chart: a 13,500 BTU RV air conditioner at 2,800 to 3,500 starting watts, a 3-ton central AC at 5,000 to 6,000, and a 2 HP air compressor at 7,500. A 7-1/4 inch circular saw alone is printed at 2,400 to 3,600 starting watts. If you are not sure what your appliances draw, our amps draw calculator converts watts to amps.

How to Size a Generator in 4 Steps

List every load that will run simultaneously. Write down each appliance with its running wattage (check the nameplate label or owner's manual). Include everything that will be on at the same time, not just the big items. LED lights, phone chargers, and Wi-Fi routers all add up. Our electrical load calculator does a different job: it gives a dwelling's demand-factored service load by the optional method that municipal worksheets print, not a list of running watts.
Find the highest starting watts. Find the one appliance with the highest starting (surge) wattage. Motor loads such as AC compressors, well pumps, refrigerator compressors, and power tools draw several times their running watts for a moment as they start: Generac's sizing paper works a 1,200 W motor at 5× in its rule of thumb, 6,000 W of starting wattage. If the label does not list starting watts, look the appliance up on its maker's wattage chart.
Add them to get the peak. Your running watts are the first figure: the generator's running (continuous) rating must exceed them. Add the highest starting watts to the running watts to get the peak: step 4 adds your margin to it, and this page holds the generator's peak (surge) rating against that margined peak. This page divides by no power factor: Generac's paper prints that kW and kVA are the same for single-phase gensets because the power factor is 1.0. Use the kVA to amps calculator to dig into the relationship between apparent and real power.
Add a safety margin to the peak. Champion's tailgating guide applies a 20% safety margin to the peak: a 2,000 W peak becomes 2,400 W. 20% is the default in this calculator, and the margin is your input. Champion then holds that margined figure against the continuous rating: “Target a generator rated for at least 2,400 continuous running watts to handle unexpected load spikes without interrupting the tailgate.” This page holds the generator's peak (surge) rating against the margined peak and its running (continuous) rating against your running watts; a generator whose running rating also covers the margined peak meets Champion's target as well. Champion also warns that running a generator at its absolute limit causes excessive heat and shortens its lifespan.

Worked Examples

Hurricane Season Home Backup in Florida

Context

A homeowner in Tampa wants a portable generator for hurricane-season outages. Essential loads: refrigerator (200W running), window AC unit (1,200W running, 2,400W surge), Wi-Fi router (30W), LED lights (100W), phone and laptop charging (80W), and a sump pump (800W running, 2,000W surge). Not all motor loads start at the same time.

Calculation

Total running watts = 200 + 1,200 + 30 + 100 + 80 + 800 = 2,410W

Highest single starting watts = 2,400W (window AC)

Peak = 2,410 + 2,400 = 4,810W

With 25% margin on the peak: 4,810 × 1.25 = 6,012.5W, rounded up to 6,013W

Interpretation

The generator needs a running (continuous) rating above 2,410W and a peak (surge) rating of at least 6,013W: the 4,810W peak with the 25% margin on it. A generator whose running rating covers 2,410W but whose peak rating is below 4,810W would trip when the AC compressor starts; the margin up to 6,013W covers unexpected load spikes on top of that peak.

Takeaway

Motor surges are the deciding factor for most home backup systems. Pair the generator with a UPS battery backup to keep critical loads running during the seconds it takes for the generator to start.

Remote Construction Site with Power Tools

Context

A contractor runs a job site with no grid power. Loads: 15A circular saw (1,800W running, 3,600W surge), air compressor (1,500W running, 4,500W surge), LED work lights (500W), battery charger station (300W), and a radio (50W). Only one motor tool runs at a time, but the compressor cycles automatically.

Calculation

Total running watts = 1,800 + 1,500 + 500 + 300 + 50 = 4,150W

Highest single starting watts = 4,500W (air compressor)

Peak = 4,150 + 4,500 = 8,650W

With 25% margin on the peak: 8,650 × 1.25 = 10,812.5W, rounded up to 10,813W

Interpretation

The air compressor's 4,500W starting load sets the peak at 8,650W, or 10,813W with the 25% margin. In practice, the circular saw and compressor may not run simultaneously since one person operates the saw. If you can guarantee they never overlap, take the saw out of the running load (4,150W becomes 2,350W) and let the compressor keep the highest-starting slot at 4,500W, which brings the peak down to 6,850W, or 8,563W with the margin. But on a busy site with multiple workers, plan for worst-case overlap.

Takeaway

Job sites benefit from slightly oversized generators because load profiles are unpredictable. Before purchasing, tally every tool's running and surge wattage using our load calculation guide to build a complete picture. It is easy to forget items like battery chargers and compressor duty cycles that add up quickly.

Frequently Asked Questions

Glossary

Running Watts (Rated Watts)

The continuous power a generator can deliver indefinitely under normal operating conditions. This is the number you size against for your total simultaneous load. A generator rated at 5,000 running watts can sustain that output all day. Exceeding this number causes overheating and trips the overload breaker.

Starting Watts (Surge Watts)

The brief peak power a generator can deliver for a short time to start electric motors. Motor compressors in air conditioners, refrigerators, and pumps draw several times their running watts for a moment as they start. A generator's peak (surge) rating is higher than its running rating: compare each with the two figures this page gives.

Power Factor

The ratio of real power (watts) to apparent power (volt-amperes). Generac's sizing paper prints that kW and kVA are the same for single-phase gensets because the power factor is 1.0, so this page adds watts and divides by no power factor. Three-phase sets are a different case: Generac rates them in kVA at a 0.8 lagging power factor.

Once you know your generator size, find out how long it will run on a tank of propane with our propane generator runtime calculator.

A generator is only as good as the sizing behind it. Err on the side of slightly larger rather than smaller. The fuel cost difference between a 5kW and 7kW unit running at moderate load is minimal, but the headroom gives you room to add loads later without buying a second generator. If fuel consumption is your primary concern, run your load figure through our propane generator runtime guide to estimate daily fuel costs for the size you are considering.

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Last updated: October 26, 2026

Written and maintained by Dan Dadovic, Commercial Director at Ezoic Inc. & PhD Candidate in Information Sciences. On the energy side, Dan has hands-on experience with residential solar panel installation, DIY battery bank construction, off-grid power systems, wind power, and EV ownership/driving, all from building, maintaining, and operating his own systems.

Disclaimer: Calculator results are estimates based on theoretical formulas. Actual performance varies with temperature, battery age, load patterns, and equipment condition. For critical electrical work, consult a licensed electrician.

Methodology reviewed by Doc. dr. sc. Danijel Jerković-Štil, Assistant Professor, FERIT Osijek.