Generator Size Calculator

The Generator Size Calculator tells you exactly what wattage generator you need to power your appliances during an outage, on a job site, or on the road. Instead of guessing, you pick your appliances from a built-in library of running and starting (surge) wattages — or enter your own — and the tool sizes the generator the correct way: it adds up every appliance's continuous running watts, then adds only the single largest startup surge, because motors and compressors never all switch on at the same instant. A safety margin and a recommended generator class complete the picture.

How to Size a Generator

Correctly sizing a generator comes down to four steps, all of which this calculator automates:

1. Add every running watt. Total the continuous (rated) watts of everything you want to run at the same time.
2. Add one surge — the biggest. Find the appliance with the largest extra startup demand and add just that surge on top. You never add every surge together.
3. Apply a safety margin. Add roughly 20–25% headroom so the generator is not run flat-out.
4. Match a generator class. Choose a generator whose starting (peak) rating meets or beats the number from step 3.

Running Watts vs Starting (Surge) Watts

Running watts (also called rated or continuous watts) is the steady power an appliance uses while it operates. Starting watts (also called surge or peak watts) is the brief burst of extra power that anything with an electric motor or compressor — a refrigerator, air conditioner, well pump, or power tool — demands for a fraction of a second as it spins up. That surge can be two to three times the running figure. A generator must supply enough starting watts to cover the biggest surge, then enough running watts to keep everything going.

Why You Only Add One Surge

This is the mistake most generator buyers make. If you added the starting watts of every motor together, you would size a generator two or three times larger — and far more expensive — than you actually need. In reality your appliances' motors start at slightly different moments, so the worst case is: everything running plus the single largest motor starting. That is precisely how this calculator computes your peak, and it flags exactly which appliance sets that surge.

Typical Appliance Wattages

These are representative averages; your specific models may differ, so check the nameplate label where possible. Resistive loads — heaters, kettles, toasters, incandescent and LED lights — have equal running and starting watts because they contain no motor.

What Size Generator for a House?

For most homes, a 5,000–7,500 watt portable generator covers the essentials: refrigerator, lights, a well or sump pump, phone and internet, and a window air conditioner. Add central air conditioning, an electric range, or electric heat and you are usually looking at a 10,000 watt or larger unit, or a permanently installed standby generator wired through a transfer switch. Small inverter generators of 2,000–3,500 watts are ideal for RVs, camping, and running a few critical items quietly.

How to Use This Calculator

1. Select your appliances: Tap each appliance you want to power. Use the quantity steppers if you have more than one of something.
2. Add custom loads: Anything not in the library can be added with its own running and starting watts.
3. Pick a safety margin: 20% is the sensible default; choose more for future expansion.
4. Click Calculate: You get a recommended generator class, an animated surge chart, a capacity comparison, a generator-size ladder, and a full per-appliance breakdown with the math.

Frequently Asked Questions

What size generator do I need?

Add the running watts of everything you want to power at once, then add the single largest starting surge among those appliances, because motors do not all start at the same instant. Add roughly 20–25% as a safety margin, and choose a generator whose starting (peak) rating meets or exceeds that number. This calculator does all of that automatically.

What is the difference between running watts and starting watts?

Running watts is the steady power an appliance draws while operating. Starting watts is the brief spike of extra power that motors and compressors need for a fraction of a second when they switch on — often two to three times their running watts.

Do I add up the starting watts of every appliance?

No. Appliances almost never start at the same instant, so you only add the single largest startup surge on top of the combined running watts. Adding every surge together would badly oversize the generator.

What safety margin should I use?

About 20–25% is common. Running a generator continuously at 100% of its rating shortens its life, wastes fuel, and leaves no room for extra loads. The buffer also covers the fact that published wattages are averages your models may exceed.

Will a generator run my whole house?

It depends on your total load. A 5,000–7,500 watt portable generator runs the essentials of most homes. Powering central AC, electric heat, or an electric range simultaneously usually needs 10,000 watts or more, or a standby generator.

Can a generator be too big?

Yes. An oversized generator costs more, burns more fuel, and runs inefficiently when lightly loaded. Aim for a generator whose starting rating comfortably covers your peak surge with a sensible margin — not one far larger than you will ever use.

Additional Resources

• Electric Generator - Wikipedia
• Inrush Current (Startup Surge) - Wikipedia