What Is The Difference Between Running And Starting Watts (Load Sizing)
Running watts are the power a generator can supply continuously. Starting watts are extra power it can provide for a short time to start certain motors. To size a generator, add the running watts of loads you’ll use together, then check that its surge rating can handle a motor starting while the other planned loads are already running.
Choosing the right size helps you keep essential equipment running without paying for capacity you do not need. The key is to plan for two different demands: the steady power appliances use while operating and the brief surge some motors need at startup. I use those as separate checks, because passing one does not guarantee passing the other.
Running Watts vs. Starting Watts: What’s the Difference?
Running watts, also called rated watts, are the generator’s continuous output limit. Starting watts, also called surge watts, are a temporary limit for short bursts, often needed when a motor begins to turn. The generator manual or rating label defines how long that temporary output is allowed.
A refrigerator, pump, air conditioner, or power tool with a motor may draw more power as it starts than it uses once running. Many simple heating loads, such as resistive heaters, draw a steadier load, but always check the equipment label or manual rather than guessing.
Do not treat surge watts as extra power you can use all day. A generator might have enough surge capacity to start a motor but not enough running capacity to keep that motor and the other connected appliances operating together. If the steady load is too high, reduce it.
How to Calculate the Generator Size You Need
For a reliable estimate, list the loads you want to run together, find each load’s running watts, and add them. Then identify motor startup demands and check whether the generator’s surge rating covers the largest startup while accounting for loads already running.
Step 1: Find each load’s power
Use the appliance nameplate or manufacturer’s specifications when available. If a single-phase appliance lists volts and amps but not watts, calculate:
Watts = volts × amps
For example, at 120 volts, a 10-amp load uses 1,200 watts. At 240 volts, 10 amps equals 2,400 watts. Use the stated wattage when the equipment provides it.
If the generator or appliance lists volt-amperes (VA) as well as watts, check both limits. VA measures electrical demand in a different way from watts. Because power factor affects the relationship, a load’s VA demand can be higher than its watt demand.
Step 2: Add the loads that will run together
Include only the equipment you plan to operate at the same time. This is your combined running load. Then compare that total with the generator’s running-watt rating.
Step 3: Check startup demand
Use the appliance’s starting-watt, locked-rotor, or startup information if the manufacturer provides it. Do not apply one multiplier to every motor: startup demand varies by equipment.
When a motor starts, account for its startup demand plus the running loads that remain on. If more than one motor might start at once, check that scenario too, or plan to start them one at a time.
Generator Setup, Load Calculation & Safety Prep
Before connecting equipment, check the generator and appliance labels, the manuals, cords, outlets, and fuel. Confirm the generator can supply the loads’ required voltage, phase, and frequency, and that its receptacles and any transfer switch match the intended connection.
The goal is not just to find a large enough watt number. Every connection must suit the equipment. A generator with the right watt capacity may still lack the correct outlet or electrical supply for a particular load.
A simple planning table helps separate steady demand from startup demand. The example below uses illustrative numbers only, not specifications for particular products.
| Planned load | Running watts | Startup information | Load-sizing note |
|---|---|---|---|
| Lights | 200 W | No motor startup listed | Add to the running total |
| Refrigerator | 700 W | 1,400 W, hypothetical | Check startup with lights already on |
| Pump | 800 W | 2,000 W, hypothetical | Check if it may start while other loads run |
With the lights on and refrigerator running, the steady load is 900 W. If the pump then starts, the pump’s hypothetical startup demand must be considered along with the loads already operating. The example shows the method, not a guaranteed generator size: use actual manufacturer data and confirm how the generator defines its surge rating.
Before operation, place a portable generator outdoors on a stable, dry surface, with exhaust directed away from people and openings. Keep it at least 20 ft (6.1 m) from occupied buildings unless the manufacturer or local rules require more distance. Never run it indoors, in a garage, or in another enclosed or partly enclosed space. Use a working carbon monoxide alarm in nearby occupied spaces.
Starting Loads and Field Operation
Start the generator according to its manual, with major loads disconnected. After it stabilizes as directed, connect equipment progressively. Starting large motor loads one at a time can help avoid overlapping surges, but it does not replace checking the generator’s ratings.
Connect loads only as the manual instructs. Use suitable cords and outlets, and follow any transfer-switch instructions for home circuits. Do not connect equipment in a way that conflicts with the generator or transfer-switch manual.
If a breaker trips or the generator struggles, disconnect loads and reassess the total. Do not repeatedly reset a breaker while the overload remains. Reduce the load, then restart or reconnect only as the manual allows.
Load Monitoring, Maintenance & Inspection
While equipment runs, keep track of what is connected and listen for changes in the generator’s operation. Stay within both the continuous watt rating and any applicable VA limit. A successful motor start does not prove the steady load is safe.
A common sizing mistake is to focus only on startup watts. A generator may start a motor on surge power, then become overloaded when the motor keeps running alongside other appliances. Sustained overload can trip protection or overheat the generator and connected equipment. Reduce loads promptly instead of repeatedly resetting a breaker.
Check the generator, cords, and connections for damage before use, and follow the maker’s inspection and maintenance schedule. If a label is missing or a load’s startup demand is unclear, consult the equipment manufacturer rather than relying on a generic estimate.
Shutdown, Storage & Seasonal Care
When the job is done, disconnect loads and follow the generator manual’s shutdown steps. If the manual directs you to let the unit run unloaded first, do so. Allow it to cool before refueling or storage, and complete any inspection or cleaning the maintenance schedule calls for.
For the next use, review your load list again. What you connect together determines the running demand, while the order in which motor loads start can affect the surge demand. Keep the generator’s instructions with your notes so the ratings and safe operating steps are easy to check.
Example Load-Sizing Scenarios
These scenarios show how to apply the calculation. The appliance figures are hypothetical; actual starting demand depends on the specific equipment, so check its nameplate or manufacturer data.
| Scenario | Running-load check | Startup check |
|---|---|---|
| Lights and refrigerator | Add both running-watt values | Check whether the generator can start the refrigerator with lights on |
| Pump and lights | Add pump and light running watts | Check the pump’s startup demand with lights already running |
| Two motor loads | Add both running-watt values if both will operate | Check whether they could start together; otherwise, plan to start them separately |
| One motor load, started alone | Include other loads that stay on | Compare the motor’s startup demand plus those running loads with the surge rating |
The practical rule is to size for the highest load combination you actually expect, not every appliance you own. If you cannot control when motors start, account for the possible overlap or choose equipment whose ratings cover it.
FAQ: Running Watts, Starting Watts, and Generator Sizing
What are running watts on a generator?
Running watts are the continuous electrical output the generator is rated to provide. Keep the combined running load within this limit.
What are starting watts?
Starting watts are a temporary output limit for brief startup demands, such as those from some motors. The manufacturer defines the allowed duration.
Are starting watts and surge watts the same?
They often refer to the generator’s temporary output rating. Check the manufacturer’s wording and instructions to confirm how the rating applies.
Do I add running watts or starting watts to size a generator?
Add the running watts of the loads you plan to operate together. Then check that the surge rating covers a motor starting while the other planned loads are running.
Can a generator start a motor but still be too small?
Yes. It may have enough surge capacity to start the motor but not enough running capacity for the motor and other loads afterward.
How do I find an appliance’s starting watts?
Check its nameplate, manual, or manufacturer’s startup or locked-rotor data. Do not assume every motor needs the same multiple of its running watts.
Can I use volts and amps to estimate watts?
For a single-phase load, multiply volts by amps. For example, 120 V × 10 A = 1,200 W; 240 V × 10 A = 2,400 W. Use nameplate watts when listed.
What if the generator lists VA as well as watts?
Stay within both the watt and VA ratings. A load’s VA demand can be higher than its watt demand because of power factor.
Can I start two motor appliances at the same time?
Only if the generator’s surge rating can cover that startup combination plus loads already running. Otherwise, start motors one at a time if the manuals allow it.
Where can I run a portable generator?
Run it outdoors, at least 20 ft (6.1 m) from occupied buildings unless greater clearance is required, with exhaust directed away from people and openings. Never run it in a building, garage, or partly enclosed space.
(This article was written by one of our staff writers, Evan Bradley. Visit our Meet the Team page.)
