Generator Wattage Calculator: How to Size a Generator for Your Home

Why Getting the Size Right Matters

Buying a generator that is too small means it trips under load or cannot start your appliances. Buying one that is too large means you wasted money on capacity you will never use and are running an oversized engine at low efficiency. Sizing a generator correctly is one of the most important decisions you will make — and it is not complicated once you understand how wattage works.

This guide walks you through the exact process of calculating the generator size you need, with a worksheet you can fill in for your specific home.

Understanding Running Watts vs Starting Watts

Before you calculate anything, you need to understand the difference between two types of wattage:

• Running watts (also called rated watts): The continuous power an appliance draws while it is operating normally. This is what you see listed on most appliance labels.
• Starting watts (also called surge watts): The brief spike of power any motor-driven appliance needs to start its compressor or motor — typically 2 to 3 times the running wattage, lasting only 1 to 2 seconds.

Your generator must handle both. Its continuous (rated) output must cover the total running watts of everything running simultaneously. Its surge (peak) output must handle the highest starting surge of any single appliance, added to the running load of everything else already on.

Step 1: List Every Appliance You Plan to Power

Start by deciding which appliances you must have running during an outage. Be realistic — you do not need to power everything in the house, just the essentials. Common priority appliances for most homeowners include:

• Refrigerator
• Freezer (if separate)
• Sump pump
• Furnace or central heating (blower motor)
• Window or central air conditioner
• Well pump (if applicable)
• Lighting (LED circuits)
• Phone chargers and small electronics
• CPAP or medical equipment

Step 2: Find the Running and Starting Watts for Each Appliance

Use this reference table as a starting point. For your specific appliances, check the yellow EnergyGuide label, the appliance data plate, or the manufacturer’s spec sheet online.

• Refrigerator (full-size): 150–400W running / 800–1,200W starting
• Chest freezer: 100–200W running / 400–600W starting
• Sump pump (1/2 hp): 800W running / 1,300W starting
• Sump pump (1 hp): 1,050W running / 2,150W starting
• Furnace blower motor: 600–875W running / 1,000–2,350W starting
• Well pump (1/2 hp): 750W running / 1,500W starting
• Window AC (10,000 BTU): 1,200W running / 1,800–3,600W starting
• Central AC (2.5 ton): 3,500W running / 5,000–7,000W starting
• LED lighting (10 bulbs): 100W running / 100W starting (no surge)
• Phone/laptop chargers: 50–100W running / 50–100W starting
• CPAP machine: 30–60W running / 30–60W starting
• Television (50″): 100W running / 100W starting

Step 3: Calculate Your Total Running Load

Add up the running watts of every appliance you plan to run simultaneously. This is your total running load — the generator’s continuous output rating must exceed this number.

Example household running load:

• Refrigerator: 200W
• Sump pump: 800W
• Furnace blower: 700W
• LED lighting: 150W
• Phone chargers: 100W
• Total running load: 1,950W

Step 4: Identify Your Highest Starting Surge

The most important surge to plan for is whichever motor-driven appliance has the highest starting wattage. In the example above, the sump pump starts at 1,300W and the furnace blower starts at approximately 1,750W. The furnace is the highest.

Your generator’s peak surge rating must handle: Total running load of everything else + starting surge of the highest-draw appliance

In the example: 1,250W (everything else running) + 1,750W (furnace surge) = 3,000W peak required

Step 5: Apply the 80% Rule

Never size a generator exactly to your calculated load. Generators are most efficient and longest-lasting when run at 50–80% of their rated capacity. Running at 100% of rated output continuously shortens engine life and increases fuel consumption.

Divide your total running load by 0.80 to get your minimum recommended generator size:

1,950W ÷ 0.80 = 2,437W minimum continuous rating

For the example home, a 3,500W generator covers the running load with a comfortable margin, handles the furnace startup surge, and runs at a healthy 55% load under normal conditions.

Generator Size Quick Reference by Home Type

• Apartment or small home — essentials only: 2,000–3,500W
• Suburban home — no AC, essential circuits: 3,500–5,000W
• Suburban home — with window AC or sump pump: 5,000–7,500W
• Larger home — central AC, well pump, full circuit coverage: 10,000–15,000W (standby)
• True whole-house — all circuits simultaneously: 20,000W+ (standby)

Should You Size Up or Down When Between Sizes?

Always size up. A generator running at 60% load is more reliable, more fuel-efficient, and longer-lasting than one straining at 95%. The price difference between a 5,000W and 6,500W generator is typically $100–$200 — a small premium for meaningful headroom.

Bottom Line

To size your generator correctly: list your essential appliances, add up their running watts, identify the highest starting surge, apply the 80% rule, and round up to the next available size. For most suburban homes powering essential circuits without central AC, a 5,000–7,500W generator covers everything comfortably. Add central AC and you need to step up to 10,000W or consider a standby unit.