Buy a generator that is too small and it will trip its breaker the moment your refrigerator compressor kicks on. Buy one that is too big and you will overpay by hundreds of dollars, burn extra fuel every hour it runs, and haul around weight you never needed. The difference between those two outcomes is about twenty minutes of math, and most people skip it.
This guide walks through the exact process electricians use to size a generator, step by step: list your loads, find the real wattage numbers, account for motor surges, and add sensible headroom. By the end you will have a specific wattage target instead of a guess, along with three worked examples you can adapt to your own home.
Quick Answer: How to Calculate Generator Size
- Step 1: List every appliance you actually need during an outage, not everything you own.
- Step 2: Find the running watts for each item from its label, manual, or a plug-in watt meter.
- Step 3: Identify starting (surge) watts for motor-driven appliances, typically 2 to 3 times running watts.
- Step 4: Add up the running watts of everything that will run at the same time.
- Step 5: Add the single largest surge on top of that total (not every surge, just the biggest one).
- Step 6: Add roughly 20% headroom so the generator is not running flat out.
- Step 7: Match the result to the nearest generator class: 2,000W, 3,500W, 5,500W, 7,500W, 9,500W, or a standby unit.
As a shorthand: (sum of simultaneous running watts + largest single surge) x 1.2 = your target generator size. Now let’s do it properly.
Step 1: List the Appliances You Actually Need
Start with a sheet of paper and two columns: “must run” and “nice to have.” During a typical outage, most households genuinely need the refrigerator, some lights, phone charging, internet equipment, and one or two comfort or safety items such as a furnace fan, sump pump, or a window air conditioner.
Be honest here, because every item you add pushes you into a bigger, louder, thirstier generator. The electric dryer can wait. The dishwasher can wait. If you are sizing a portable generator, the goal is essentials plus a little comfort, not replicating normal life. (If you do want normal life, skip ahead to the standby generator section.)
Step 2: Find the Running Watts for Each Item
Running watts (also called rated or continuous watts) are what an appliance draws once it is up and going. You can find them three ways, in order of accuracy:
- A plug-in watt meter (best): A $15 to $25 meter between the appliance and the wall shows real consumption, which is often well below the label. A “700W” refrigerator may actually cycle at around 150 watts.
- The nameplate label: Look for a sticker on the back or bottom listing watts, or volts and amps (multiply them: 120V x 5A = 600W). Labels show maximum draw, so treat them as conservative.
- The EnergyGuide tag or manual: The yellow EnergyGuide label lists annual kWh. Divide by 8,760 hours for a rough average draw, useful for refrigerators and freezers that cycle on and off.
Write the running watts next to each item on your list.
Step 3: Identify Starting (Surge) Watts
Here is where most sizing mistakes happen. Anything with a motor or compressor draws a brief spike of power at startup, typically 2 to 3 times its running watts, lasting a second or two. The usual suspects:
- Refrigerators and freezers: around 150-200W running, 1,000-1,200W starting
- Sump pumps: around 800W running, 1,300-2,000W starting
- Well pumps: around 1,000-1,500W running, 2,000-4,000W starting
- Air conditioners: a window unit around 900W running can spike past 2,500W; central AC spikes far higher
- Furnace blower fans: around 400-700W running, up to 1,500W starting
Pure resistance loads (light bulbs, space heaters, toasters, coffee makers, electric griddles) have no meaningful surge: their starting watts equal their running watts. Electronics like TVs, laptops, and routers also start gently.
Step 4: Add Up Simultaneous Running Watts
Total the running watts of everything that will be on at the same time. This is not the sum of your whole list; it is the sum of a realistic worst moment. Your refrigerator, freezer, sump pump, furnace fan, lights, and router might all run at once on a stormy night. Your microwave and coffee maker probably will not, and you can manage them by simply not running both together.
This is the cheapest sizing tool you have: load management. Choosing to stagger heavy appliances can drop you a full generator class, which saves real money.
Step 5: Add the Single Largest Surge
Take your simultaneous running total and add the largest single starting surge from your list, minus that appliance’s running watts if you already counted it. Why only one? Because startup spikes last a second or two, and the odds of your well pump, sump pump, and refrigerator all starting in the same instant are extremely low. Generators are also built to tolerate brief surges above their rated output.
Adding every surge together is the most common way people end up buying twice the generator they need.
Step 6: Add Roughly 20% Headroom
Multiply your total by 1.2. This buffer matters for three reasons: generators run most efficiently and last longest at around 50 to 80% load rather than maxed out, wattage labels are estimates, and you will inevitably want to plug in one more thing. Headroom also compensates for output loss at altitude and in extreme heat, where generators produce a few percent less than rated.
Step 7: Match Your Number to a Generator Class
- Under 2,000W: Small inverter generators. Phones, lights, fridge, TV, laptops.
- 3,000-4,500W: Mid-size inverters. Adds a sump pump or furnace fan, microwave in bursts.
- 5,000-6,500W: Full essentials including a well pump or window AC.
- 7,500-9,500W: Near-whole-home for average houses, usually with a transfer switch.
- 10,000W+ portable or 14-26kW standby: Whole-home coverage including central air.
Appliance Wattage Reference Table
Use these typical figures as starting points, then verify your own appliances. Real numbers vary by model, age, and size.
| Appliance | Running watts (approx.) | Starting watts (approx.) |
|---|---|---|
| Refrigerator | 150-200 | 1,000-1,200 |
| Chest freezer | 100-150 | 800-1,000 |
| Sump pump (1/3 HP) | 800 | 1,300-2,000 |
| Well pump (1/2 HP, 240V) | 1,000 | 2,000-3,000 |
| Furnace blower fan | 400-700 | 1,000-1,500 |
| Window AC (8,000 BTU) | 650-900 | 1,800-2,600 |
| Central AC (3-ton, 240V) | 3,000-3,800 | 7,000-9,000 |
| Space heater | 1,500 | 1,500 |
| Microwave (1,000W cooking) | 1,000-1,500 | 1,000-1,500 |
| Coffee maker | 800-1,200 | 800-1,200 |
| Toaster | 850-1,200 | 850-1,200 |
| Electric range (one burner) | 1,500-2,500 | 1,500-2,500 |
| Dishwasher | 1,200-1,500 | 1,500 |
| Washing machine | 500-800 | 1,200-2,000 |
| Electric dryer (240V) | 5,000-5,400 | 6,700 |
| Electric water heater (240V) | 4,000-4,500 | 4,000-4,500 |
| TV (LED, 50-inch) | 60-120 | 60-120 |
| Laptop | 50-100 | 50-100 |
| Wi-Fi router and modem | 10-25 | 10-25 |
| Phone charger | 5-20 | 5-20 |
| LED lights (per bulb) | 8-12 | 8-12 |
| Garage door opener | 400-550 | 1,000-1,400 |
| CPAP machine (no humidifier) | 30-60 | 30-60 |
| Portable fan | 50-100 | 100-200 |
Three Worked Examples
Example 1: The Minimal Kit (lands around 1,800W)
Goal: keep food safe and stay connected.
- Refrigerator: 200W running
- LED lights (5 bulbs): 50W
- Wi-Fi router and modem: 20W
- Phone and laptop charging: 80W
- Small TV: 80W
Simultaneous running total: around 430W. Largest surge: the refrigerator, adding roughly 1,000W on startup. Peak requirement: about 1,430W. With 20% headroom: roughly 1,720W. A 1,800 to 2,200W inverter generator covers this comfortably, runs quietly, and sips fuel.
Example 2: Comfortable Essentials (lands around 5,500W)
Goal: a stormy week in a house with a basement and gas furnace.
- Refrigerator: 200W
- Chest freezer: 150W
- Sump pump: 800W
- Furnace blower fan: 600W
- Lights, router, TV, charging: 300W
- Microwave (short bursts): 1,000W
Simultaneous running total: around 3,050W. Largest surge: the sump pump, adding roughly 1,200W beyond its running draw. Peak requirement: about 4,250W. With 20% headroom: roughly 5,100W. A 5,500W class generator is the right fit, ideally feeding a transfer switch so the furnace and sump pump connect safely.
Example 3: Near-Whole-Home (lands around 9,500W)
Goal: rural home with a well, running almost everything except central AC and the electric dryer.
- Refrigerator: 200W
- Chest freezer: 150W
- Well pump (1/2 HP): 1,000W
- Sump pump: 800W
- Furnace blower fan: 600W
- Window AC in the bedroom: 900W
- Lights throughout: 300W
- Microwave: 1,000W
- TV, router, electronics: 300W
- Washing machine (occasional): 600W
Simultaneous running total: around 5,850W. Largest surge: the well pump, adding roughly 2,000W beyond running draw. Peak requirement: about 7,850W. With 20% headroom: roughly 9,400W. A 9,500W class portable with a 240V outlet and a properly installed transfer switch handles this. Note the well pump detail: it is a 240V load, so the generator must have 240V output, not just a big wattage number.
Common Sizing Mistakes to Avoid
- Ignoring starting watts entirely. The classic result: a 2,000W generator that runs everything fine until the fridge compressor kicks in and trips the overload. Always account for the largest surge.
- Adding all the surges together. The opposite error. Summing every appliance’s starting watts produces a monster number and a generator twice as large, loud, and expensive as needed. Count only the single largest surge.
- Forgetting 240V loads. Well pumps, central AC, electric dryers, and electric water heaters run on 240V. They need a generator with a 240V outlet and a transfer switch, and they are usually the loads that push you from a mid-size portable to a large unit or standby. Decide early whether you truly need them during an outage.
- Sizing from nameplate maximums. Labels show worst-case draw. Measured consumption, especially for refrigerators, is often half the label. A watt meter pays for itself on the first appliance.
- Buying with zero headroom. A generator running at 100% load runs hot, drinks fuel, produces dirty power under strain, and wears out early. Aim for your calculated peak to sit around 80% of the generator’s rated output.
Portable vs. Standby: The Sizing Rules Change
Everything above describes sizing a portable generator, where you pick which loads matter and manage them manually. Standby generators work differently: they connect to your electrical panel through an automatic transfer switch and are sized with a formal load calculation based on your whole panel, the same NEC-style math an electrician uses for a service upgrade.
Practical differences worth knowing:
- Standby sizing starts bigger. Typical homes land between 14kW and 26kW because the system must handle whatever the household turns on, including central AC, without anyone managing loads.
- Load-shedding modules shrink the requirement. A smart management module can lock out the dryer while the AC runs, letting a 14kW unit protect a home that would otherwise need 22kW. Ask the installer about this before accepting a bigger quote.
- Portables reward discipline; standbys reward planning. With a portable, you can undersize slightly and stagger loads. With a standby, undersizing means nuisance shutdowns you cannot easily manage from the couch, so the load calculation needs to be right the first time.
Frequently Asked Questions
What size generator do I need to run a whole house?
For a typical U.S. home, running everything including central air conditioning takes roughly 14kW to 26kW, which is standby generator territory. If you exclude central AC and 240V heavy hitters like the electric dryer and water heater, a 7,500 to 9,500W portable with a transfer switch covers most homes’ realistic needs.
Will a 5,000-watt generator run my refrigerator and air conditioner together?
A refrigerator (around 200W running) plus a window AC (around 900W running) totals about 1,100W running, with the largest surge around 2,600W from the AC. That is roughly 3,700W peak, so yes, a 5,000W generator handles both with room to spare. Central AC is a different story: its 7,000W+ starting surge alone exceeds a 5,000W unit.
Is it bad to run a generator at full capacity?
Continuously, yes. Sustained operation at or near 100% load shortens engine life, increases fuel burn per useful watt, and can cause voltage sag that stresses electronics. Size your generator so typical demand sits around 50 to 80% of its rating; brief surges above that are what the surge rating exists for.
How do I find the wattage of an appliance with no label?
Use a plug-in watt meter for 120V devices; it is the most accurate method regardless of labels. For hardwired or 240V equipment like a well pump, check the breaker and motor plate (volts times amps gives watts) or look up the horsepower: each horsepower translates to roughly 750 running watts plus 2 to 3 times that at startup.
Wattage figures in this guide are typical estimates and vary by model and condition. Transfer switches and standby generators must be installed by a licensed electrician, and portable generators must always run outdoors, well away from windows and doors, due to carbon monoxide risk.