Generator Size Calculator: How to Find the Right Generator Size
Generator Size Calculator
Add the appliances you want to run at the same time. The calculator totals running and starting watts and recommends a generator size.
Add your own appliance
Only amps on the label? Multiply amps × volts. Example: 6 A × 120 V = 720 W.
Engines lose roughly 3.5% of output per 1,000 ft of elevation. The calculator sizes up to make up for fuel and altitude losses.
| Appliance | Qty | Running W | Starting W | |
|---|---|---|---|---|
| Total | 0 | 0 |
Edit the watts to match your appliance labels for an exact result.
Wattages are typical averages for planning. Check appliance labels and the generator maker’s specs, and use a transfer switch installed by a licensed electrician.
Find the Right Generator Size in Under a Minute
Buying the wrong generator is an expensive mistake. Too small, and it trips its breaker the moment your refrigerator or well pump kicks on. Too big, and you pay more upfront, burn more fuel, and haul around weight you never use.
Our free generator size calculator removes the guesswork. Add the appliances you want to power, and it totals your running watts and starting watts, then recommends a generator size with a safe buffer built in.
How to use the calculator:
- Select each appliance or tool you plan to run at the same time.
- Adjust the quantity, or enter a custom wattage from the appliance label.
- Read your total running watts, peak starting watts, and recommended generator size.
- Export or download your results to compare generator models.
[Insert Generator Size Calculator tool here]
Below, we explain how the math works, so you can double-check any result and buy with confidence.
Running Watts vs. Starting Watts: The Key to Generator Sizing
Every generator has two ratings, and you need to understand both before you buy.
Running watts (also called rated or continuous watts) are the power an appliance draws while it operates normally. A refrigerator might run steadily at about 700 watts.
Starting watts (also called surge or peak watts) are the extra burst of power a motor needs for the first few seconds after it switches on. That same refrigerator may need around 2,200 watts for a moment to get its compressor turning.
Appliances with motors or compressors create these surges: refrigerators, freezers, air conditioners, well pumps, sump pumps, furnace blowers, and power tools. Resistive loads, such as light bulbs, toasters, and space heaters, draw roughly the same power from start to finish.
Why it matters: your generator’s running wattage must cover everything running at once, and its starting wattage must cover the biggest single surge on top of that. Ignore surge, and your generator stalls the moment a compressor kicks on.
How to Calculate What Size Generator You Need (Step by Step)
The calculator does this automatically, but here is the exact method it uses.
- List what you must power at the same time. Focus on essentials during an outage: refrigerator, sump pump, lights, furnace fan, phone chargers, internet router.
- Find each item’s running watts. Check the data label on the appliance or the owner’s manual. If it lists only volts and amps, multiply them: watts = volts × amps.
- Add up all the running watts. This is your total continuous load.
- Find the single highest starting wattage. Only one motor usually starts at a time, so you add just the largest surge, not every surge.
- Add the extra surge to your running total. Extra surge = that appliance’s starting watts minus its running watts.
- Add a 10–20% safety buffer. Generators run cooler, quieter, and longer when they are not pushed to 100%.
Generator size = (Total running watts + (Largest starting watts − that appliance’s running watts)) × 1.2
Tip: if your label shows amps only (for example, 6 amps at 120 volts), the running load is 6 × 120 = 720 watts.
Appliance Wattage Chart: Running and Starting Watts
Use these typical figures when an appliance label is missing. Actual wattage varies by model, age, and efficiency, so always trust the label first.
| Appliance / Tool | Running Watts (W) | Starting Watts (W) |
|---|---|---|
| Refrigerator / freezer | 700 | 2,200 |
| Chest freezer | 500 | 1,500 |
| Sump pump (1/2 HP) | 1,050 | 2,150 |
| Well pump (1 HP) | 1,000 | 2,000 |
| Furnace fan (1/2 HP) | 800 | 2,350 |
| Window AC (10,000 BTU) | 1,200 | 3,600 |
| Central AC (3 ton) | 3,800 | 11,400 |
| Space heater | 1,500 | 1,500 |
| Microwave (1,000 W) | 1,000 | 1,000 |
| Electric stove (1 burner) | 1,500 | 1,500 |
| Coffee maker | 1,000 | 1,000 |
| Electric water heater | 4,000 | 4,000 |
| Washing machine | 1,150 | 2,250 |
| Electric clothes dryer | 5,400 | 6,750 |
| TV (LED, 55″) | 150 | 150 |
| Laptop | 60 | 60 |
| Wi-Fi router + modem | 25 | 25 |
| LED light bulb | 10 | 10 |
| Phone charger | 10 | 10 |
| Circular saw | 1,400 | 2,300 |
| Air compressor (1 HP) | 1,600 | 4,500 |
| RV air conditioner (13,500 BTU) | 1,300 | 2,800 |
Figures are approximate industry averages for planning only.
Generator Sizing Example: Powering Home Essentials in an Outage
Say you want to keep these running during a storm power cut:
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator | 700 | 2,200 |
| Sump pump (1/2 HP) | 1,050 | 2,150 |
| Furnace fan (1/2 HP) | 800 | 2,350 |
| 10 LED bulbs | 100 | 100 |
| TV | 150 | 150 |
| Router + modem | 25 | 25 |
| 2 phone chargers | 20 | 20 |
| Total | 2,845 | — |
- Total running watts = 2,845 W.
- Largest extra surge = furnace fan: 2,350 − 800 = 1,550 W.
- Peak load = 2,845 + 1,550 = 4,395 W.
- With a 20% buffer: 4,395 × 1.2 ≈ 5,275 W.
Result: a generator rated around 5,500 starting watts (roughly 4,500 running watts) handles these essentials comfortably.
What Size Generator Do I Need? Quick Guide by Use Case
| Use case | Typical load | Recommended generator size |
|---|---|---|
| Camping, phones, lights, small fan | Under 1,000 W | 1,000–2,000 W inverter |
| Tailgating, TV, small appliances | 1,000–2,000 W | 2,000–3,000 W inverter |
| RV with one 13,500 BTU AC | 2,500–3,500 W peak | 3,500–4,000 W |
| Home essentials (fridge, sump, lights) | 3,000–5,000 W peak | 5,000–7,500 W portable |
| Essentials + well pump + window AC | 6,000–8,000 W peak | 7,500–10,000 W portable |
| Job site power tools | 4,000–7,000 W peak | 6,000–8,000 W portable |
| Whole house with central AC | 15,000–25,000 W | 20–26 kW standby |
| Large home, all-electric | 25,000 W+ | 26–48 kW standby |
These ranges are starting points. Run your own list through the calculator above for an exact figure.
Portable vs. Inverter vs. Standby Generators
Once you know your wattage, pick the type that fits how you will use it.
| Type | Typical size | Best for | Trade-offs |
|---|---|---|---|
| Inverter | 1,000–4,000 W | Camping, RVs, laptops and other sensitive electronics | Clean, quiet power; higher cost per watt |
| Conventional portable | 3,000–12,000 W | Home backup essentials, job sites | Affordable; louder; manual setup and refueling |
| Home standby | 10–48 kW | Whole-house backup | Starts automatically; professional install; highest cost |
Clean power matters. Inverter generators produce stable power with low harmonic distortion, which is safer for computers, TVs, and medical devices. Many larger portables now include inverter technology too.
Fuel type affects real output
- Gasoline: widely available, but stores poorly for long periods.
- Propane: stores indefinitely and burns cleaner, but usually delivers about 10% less wattage than gasoline on the same unit.
- Dual fuel / tri-fuel: flexibility during long outages; check the rated watts for each fuel.
- Natural gas (standby): unlimited runtime from your utility line, with slightly lower output than propane.
- Diesel: efficient and durable for heavy commercial use.
Always size using the wattage for the fuel you will actually use.
Common Generator Sizing Mistakes to Avoid
- Ignoring starting watts. The most common reason generators trip or stall.
- Adding every surge together. Motors rarely start at the same instant; stagger start-ups and add only the largest surge.
- Running at 100% capacity. Constant max load shortens engine life and wastes fuel. Aim for 50–80% of rated running watts.
- Forgetting future needs. A new chest freezer or window AC next summer can push you over the limit.
- Using peak watts as the headline. Some brands advertise peak (starting) watts in large print. Compare running watts between models.
- Overlooking altitude and heat. Engines lose roughly 3–4% of output per 1,000 feet of elevation, and output also drops in high temperatures.
Generator Safety Tips
- Never run a generator indoors or in a garage, basement, or crawlspace, even with doors open. Carbon monoxide can kill within minutes.
- Place it at least 20 feet from your home, with the exhaust pointed away from windows and doors.
- Install battery-powered CO alarms on every level of your home.
- Never backfeed power through a wall outlet. Use a transfer switch or interlock kit installed by a licensed electrician.
- Use heavy-duty, outdoor-rated extension cords sized for the load.
- Let the engine cool before refueling, and store fuel in approved containers away from living spaces.
Frequently Asked Questions
What size generator do I need to run a house?
Most homes need 5,000–7,500 watts to run essentials like the refrigerator, sump pump, furnace fan, and lights. Powering a whole house with central air usually takes a 20–26 kW standby generator.
How many watts does it take to run a refrigerator?
A typical refrigerator uses about 700 running watts and needs around 2,200 starting watts for a few seconds when the compressor starts.
Will a 3,500-watt generator run my house?
It can run a few essentials, such as a refrigerator, lights, a TV, and chargers, but not large loads like central AC, an electric water heater, or an electric dryer.
Can a generator be too big?
Yes. An oversized generator costs more, burns more fuel, and can suffer from “wet stacking” (unburned fuel buildup) when it runs at very light loads for long periods.
What is the difference between rated watts and peak watts?
Rated (running) watts are what a generator can supply continuously. Peak (starting) watts are the short burst it can deliver for a few seconds to start motors.
Do I need an inverter generator for electronics?
Inverter generators are the safest choice for laptops, TVs, and other sensitive electronics because they produce clean, stable power.
How do I convert amps to watts?
Multiply amps by volts. For a standard 120-volt circuit, 10 amps × 120 volts = 1,200 watts.
How accurate is a generator size calculator?
It is accurate when you enter real wattages from your appliance labels. Default values are averages, so check the labels on your largest appliances for the best result.
Conclusion: Size It Right the First Time
The right generator covers your total running watts, handles your biggest starting surge, and leaves a 10–20% cushion. Get those three numbers right and you avoid tripped breakers, wasted fuel, and money spent on capacity you never use.
Scroll back up, add your appliances to the generator size calculator, and download your results before you shop. Then compare models by running watts, fuel type, and noise level to find the one that fits your home, RV, or job site.
Disclaimer: Wattage figures are estimates for planning. Check appliance labels and manufacturer specs, and have a licensed electrician install any transfer switch or standby generator.