Wattage Basics: Running vs Starting
Every generator has two watt ratings: running (or rated) watts and starting (or surge/peak) watts. Running watts is the continuous power the generator can deliver hour after hour — this is the number that matters for sustained operation. Starting watts is the momentary burst the generator can handle for a few seconds to kick-start motorized appliances like refrigerators, air conditioners, and pumps. A generator rated at "7,500 running / 9,500 starting" can continuously supply 7,500 watts while handling brief startup surges up to 9,500 watts.
Understanding the difference between these two numbers is the foundation of proper generator sizing. Your total running load must stay below the running watt rating at all times. Your single largest startup surge — which happens when a compressor or motor kicks on — must be within the starting watt rating. If either number is exceeded, the generator will trip its overload breaker, shut down, or potentially suffer damage.
Running wattage is printed on the nameplate or specification label of virtually every appliance. Starting wattage is trickier — it's often not listed on the appliance itself. As a general rule, motorized appliances (those with compressors or electric motors) have starting wattages 2–3 times their running draw. Resistive loads like heaters, light bulbs, and toasters have no startup surge — their starting and running wattages are identical.
How to Calculate Your Total Load
The sizing formula is straightforward, but the process requires being thorough about what you actually plan to run. Here's the step-by-step method that works for any scenario — emergency backup, camping, construction site, or outdoor event:
Step 1: List everything you'll run simultaneously. Be specific. "The kitchen" isn't a load calculation — "refrigerator, microwave, 6 LED bulbs, and a coffee maker" is. Don't include things you'll cycle on and off; only count what's plugged in and running at the same time.
Step 2: Look up running watts for each item. Check the appliance nameplate (usually on the back or bottom), the owner's manual, or the manufacturer's spec sheet. Nameplates sometimes list amps instead of watts. To convert: Watts = Volts × Amps. For standard US household circuits, that's 120V × Amps = Watts.
Step 3: Add up all running watts. This gives you your total continuous load — the running watt rating your generator must exceed.
Step 4: Identify the highest single starting wattage. Find the one appliance on your list with the largest startup surge. This is almost always the air conditioner, well pump, or refrigerator. Only one appliance starts at a time (you'll stagger startups), so you only need to account for the single largest surge.
Step 5: Total running watts + highest starting wattage = minimum generator size. Then add 15–20% as a safety buffer. This is your generator's minimum starting watt requirement.
Common Appliance Wattage Chart
| Appliance | Running Watts | Starting Watts | Type |
|---|---|---|---|
| Refrigerator | 100–400 | 1,200–2,100 | Motor |
| Chest/Upright Freezer | 50–100 | 500–1,000 | Motor |
| Sump Pump (1/3 HP) | 800 | 1,300 | Motor |
| Well Pump (1/2 HP) | 1,000 | 2,100 | Motor |
| Window AC (5,000 BTU) | 450–550 | 1,400–1,650 | Motor |
| Window AC (10,000 BTU) | 1,000–1,200 | 3,000–3,600 | Motor |
| Central AC (2 ton) | 2,500–3,000 | 5,000–7,000 | Motor |
| Central AC (3 ton) | 3,000–3,500 | 7,000–10,500 | Motor |
| Gas Furnace Blower | 300–800 | 600–1,300 | Motor |
| Portable Space Heater | 1,500 | 1,500 | Resistive |
| Microwave (1,000W) | 1,000 | 1,500 | Motor |
| Toaster | 800–1,500 | 800–1,500 | Resistive |
| Coffee Maker | 600–1,200 | 600–1,200 | Resistive |
| LED Bulb (10W equiv) | 10 | 10 | Resistive |
| Incandescent Bulb (60W) | 60 | 60 | Resistive |
| Laptop | 50–100 | 50–100 | Electronic |
| Desktop Computer + Monitor | 200–500 | 200–500 | Electronic |
| WiFi Router | 10–20 | 10–20 | Electronic |
| Phone Charger | 5–25 | 5–25 | Electronic |
| TV (50" LED) | 80–120 | 80–120 | Electronic |
| Circular Saw | 1,400 | 2,300 | Motor |
| Drill (1/2") | 600 | 900 | Motor |
| Air Compressor (1 HP) | 1,500 | 4,500 | Motor |
| Electric Water Heater | 4,500 | 4,500 | Resistive |
| Garage Door Opener | 550 | 1,100 | Motor |
| Security System | 5–15 | 5–15 | Electronic |
| CPAP Machine | 30–60 | 30–60 | Electronic |
Sizing Examples by Use Case
Example 1: Essential Home Backup
Refrigerator (200W running / 1,200W starting) + freezer (80W / 800W) + 10 LED lights (100W) + WiFi router (15W) + phone charger (15W) + gas furnace blower (500W / 1,000W) = 910W running. Highest single startup: refrigerator at 1,200W. Total: 910 + 1,200 = 2,110W minimum. With 20% buffer: 2,530W starting watts needed. A 3,000W inverter generator handles this with room to spare.
Example 2: Home Backup With Window AC
Same essential load above (910W running) + window AC 10,000 BTU (1,200W running / 3,600W starting) = 2,110W running. Highest startup: window AC at 3,600W. Total: 2,110 + 3,600 = 5,710W. With buffer: ~6,850W starting watts needed. A 5,500W running / 7,000W+ starting generator fits — look at dual-fuel models in the 7,500W class.
Example 3: Camping Setup
Portable fridge (50W) + LED lights (15W) + phone chargers (20W) + laptop (80W) + CPAP (50W) = 215W running. No significant startup surge. A 500–1,000Wh portable power station runs this load for a full night easily.
Example 4: Job Site
Circular saw (1,400W / 2,300W) + drill (600W / 900W) + work lights (200W) + radio (50W) + chargers (100W) = 2,350W running. Highest startup: saw at 2,300W. Total: 2,350 + 2,300 = 4,650W. With buffer: ~5,580W needed. A 5,000W+ generator or a beefy inverter generator covers this.
Sizing Mistakes to Avoid
Counting Everything You Own
You don't need to power your entire house. Generator sizing is about what you'll run simultaneously during an outage, not what your house uses on a normal Tuesday. Most families find their true essential load is surprisingly modest — 1,000–3,000 running watts covers the basics for most homes.
Ignoring Motor Startup Surges
This is the number one sizing mistake. A generator that handles your running load perfectly may trip and shut down the instant your refrigerator compressor kicks on because the startup surge exceeds the generator's peak capacity. Always size for the startup spike, not just steady-state running.
Running at Full Capacity
Operating a generator at 100% of its rated capacity is hard on the engine, reduces fuel efficiency, and leaves zero headroom for unexpected loads. The ideal operating range is 25–75% of rated capacity. This extends engine life, improves fuel economy, and gives you margin for startup surges and load fluctuations. If your calculation puts you at 90% of a generator's capacity, size up to the next tier.
Forgetting About Altitude
Generators lose approximately 3–4% of their rated power for every 1,000 feet of elevation above sea level. If you're sizing a generator for a mountain cabin at 7,000 feet, you could lose 20% or more of the manufacturer's sea-level rating. Factor altitude derating into your calculation if you're above 3,000 feet.
Building in the Right Safety Buffer
A 15–20% buffer above your calculated minimum is the standard recommendation. This accounts for appliance wattage variations (a "150W" refrigerator might actually draw 180W on a hot day), wiring losses, aged equipment drawing more power than spec, and the occasional forgotten load you didn't include in your calculation. For critical applications (medical equipment, sump pump protection), consider a 25–30% buffer. The cost difference between generator sizes is usually modest — a 30% larger generator rarely costs 30% more.