How to Size a Generator: Running Watts vs Starting Watts

Why This Article?

Every week, generator distributors hear the same two complaints from customers:

• "I bought a 3kW generator, but it can't even start my air compressor. Is the machine under-powered?"

• "I added up my appliances and they only total 1,500W, so why does the 2kW generator overload and shut down?"

In almost every case, the problem is not a defective generator—it is a sizing mistake. The buyer added up the wrong number.

This guide gives you a repeatable 3-step method to size any generator correctly, so you can close sales faster and stop "under-powered" returns before they happen.

The One Concept That Solves 90% of Sizing Mistakes

Every appliance has two different power numbers, and most buyers only look at one of them.

1. Running Watts 

The steady power an appliance needs to keep working normally, printed on the label as "W" or "Watt".

• A 600W water pump needs 600W to run.

• A 60W LED bulb needs 60W to run.

2. Starting Watts / Surge Watts 

The short, powerful burst an appliance needs for the first 1–2 seconds to get moving from a dead stop. This only applies to devices with motors, compressors, or pumps.

• That same 600W water pump may demand 1,800W–2,400W for the first second (3x–4x its running watts).

• A 1,200W air compressor may spike to 3,600W–6,000W on start-up.

   Appliance label says: 600W
   ─────────────────────────────────────────────
   First 1 second (starting):  ~1,800W – 2,400W  ← the surge
   After 2 seconds (running):  600W
   ─────────────────────────────────────────────
   If your generator only covers 600W → it stalls or trips!

The golden rule: a generator must be sized for the surge, not just the running total. A generator that cannot deliver the starting surge will either trip its breaker or stall the engine the moment the motor kicks in.

The 3-Step Sizing Method

Step 1 — Add up the running watts of everything you want to run at the same time

Write down every appliance that will be switched on together. Sum their running watts.

   Example: a small retail kiosk during a power cut
   ─────────────────────────────────────────────
   LED shop lights       2 x 20W  =   40W
   Cash register / POS              120W
   Small refrigerator              180W
   Phone & laptop charging         100W
   ─────────────────────────────────────────────
   Total running watts            = 440W

Step 2 — Find the single biggest motor and add its starting surge

Look at your list. Which single device has the largest electric motor or compressor? Take only the biggest one (motors rarely all start at exactly the same instant) and add its extra surge on top of the running total.

   Biggest motor: the refrigerator (running 180W, starting ~600W)
   Extra surge to add = 600W - 180W = 420W

   Required = Running total 440W + Extra surge 420W = 860W

Step 3 — Add a 20% safety margin

Generators run best when they are not pushed to 100% of their rating. Adding about 20% headroom protects the engine, extends its life, and leaves room for that one extra appliance you forgot to count.

   Required 860W  x  1.20  =  ~1,030W
   ─────────────────────────────────────────────
   Recommended generator:  at least 1.0kW–1.2kW

Put the three steps into one formula:

Generator Size=(Sum of running watts+extra surge of the biggest motor)×1.2Generator Size=(Sum of running watts+extra surge of the biggest motor)×1.2

Typical Appliance Power Reference Table

Use this as a starting point when a customer does not have their appliance labels handy. Values are typical industry figures—always confirm with the actual nameplate before finalizing an order.

   ┌──────────────────────────┬───────────────────┬──────────────────────┐
   │ Appliance                │ Running Watts     │ Starting Surge       │
   ├──────────────────────────┼───────────────────┼──────────────────────┤
   │ LED bulb                 │ 10–20W            │ –                    │
   │ Laptop / phone charger   │ 50–100W           │ –                    │
   │ WiFi router              │ 10–20W            │ –                    │
   │ LED TV                   │ 50–150W           │ –                    │
   │ Microwave                │ 800–1,500W        │ –                    │
   │ Electric kettle          │ 1,000–2,000W      │ –                    │
   │ Electric heater          │ 1,000–2,000W      │ –                    │
   │ Refrigerator / freezer   │ 150–800W          │ 2x–3x running        │
   │ Water pump / sump pump   │ 400–800W          │ 3x running           │
   │ Air compressor           │ 1,000–2,000W      │ 3x–5x running        │
   │ Circular saw             │ 1,200–1,800W      │ 2x–3x running        │
   │ Angle grinder / drill    │ 500–1,500W        │ 2x–3x running        │
   │ Washing machine          │ 500–1,500W        │ 2x–3x running        │
   │ Window / split AC        │ 1,000–2,000W      │ 2x–3x running        │
   └──────────────────────────┴───────────────────┴──────────────────────┘

Dealer tip: when the customer lists "lights + TV + fridge + one power tool", they almost always forget the fridge and tool surges. Walk through the table with them and the right size reveals itself.

Three Extra Factors That Change the Number

1. Altitude 

Internal combustion engines produce less power as the air gets thinner. As a rule of thumb, an engine loses roughly 8–10% of its output for every 1,000m of altitude. A generator that comfortably runs a site at sea level may need to be one size larger at 2,000m elevation.

2. High Temperature

Sustained high ambient temperature also reduces engine and alternator capacity. For customers in hot climates (the Middle East, North Africa, Southeast Asia), recommend one size up from the calculated number.

3. Motor Start Sequence 

If several large motors must start in a strict sequence (e.g. a workshop with a compressor and a welder), sizing for the single biggest surge is enough only when the customer is disciplined about starting them one at a time. If they will start them together, sum the surges.

Matching the Right Huahe Series to the Application

Huahe's portable generator line spans roughly 1kW to 7kW+ for petrol models and extends into larger diesel sets, so one sizing method covers every customer from a camping retailer to a construction depot.

   ┌──────────────────────────┬──────────────────┬─────────────────────────────┐
   │ Application              │ Typical Need     │ Suggested Series            │
   ├──────────────────────────┼──────────────────┼─────────────────────────────┤
   │ Camping / sensitive gear │ 1.0kW–2.5kW      │ Inverter series (clean sine)│
   │ Home backup (fridge+TV)  │ 2.0kW–3.0kW      │ Portable petrol series      │
   │ Small shop / food stall  │ 3.0kW–5.0kW      │ Portable petrol series      │
   │ Construction site tools  │ 5.0kW–7.0kW+     │ Heavy-duty / diesel series  │
   │ Continuous industrial    │ 7.0kW+           │ Diesel sets                 │
   └──────────────────────────┴──────────────────┴─────────────────────────────┘

Note: the "Inverter series" matters for customers powering laptops, drones, smart fridges, or medical devices—see the companion article on THD for why clean sine-wave output protects sensitive electronics.

Summary Checklist for Distributors

1. Two numbers per appliance ➡️ Always ask for running watts AND starting surge. Most complaints come from counting only the running watts.

2. Only add ONE surge ➡️ Size for the single biggest motor's surge, plus the running total, unless the customer insists on starting everything at once.

3. Add 20% headroom ➡️ A generator at 100% load wears out faster and stalls on the day a new appliance is added.

4. Factor in the environment ➡️ High altitude or hot climate? Recommend one size up.

5. Match clean power to the load ➡️ Sensitive electronics need an inverter (low THD) model, not just "enough watts".

Need help building a per-SKU sizing table or a customer-facing calculator for your market? Contact the Huahe support team—we can provide factory wattage data for each model to complete your sales sheet.