What Size Generator Do You Need for an Event? A Sizing Guide

When there is no outlet near the party, a generator is the answer, and sizing it is straightforward math that people skip. Buy too small and the breaker trips during dinner. Buy too large and you pay for fuel and noise that you do not use. This guide shows the formula, the numbers to gather and a worked example.

The formula

Required watts = (running watts + largest starting surge) × 1.25

• Running watts is what each device draws while operating. It is printed on the label or in the manual.
• Starting surge is the extra draw for a few seconds when motors like refrigerators, fans and pumps start. It can be two to three times the running figure. You only add the largest single surge because motors do not all start at once.
• Headroom keeps the generator below its limit, which helps reliability and fuel use.

If a device lists amps instead of watts, use watts = volts × amps. A device that draws 5 amps on a 120-volt supply uses 600 watts.

The numbers in the figure are illustrative. Use the labels on the actual equipment for your event.

Gather the list

Ask each vendor what they will plug in and what they draw. Do this early, because the caterer's list is the one that surprises people.

Many caterers bring propane or butane for cooking and ask for power only for warming and refrigeration. Confirm instead of assuming. The event power needs checklist lists each category.

Single phase, 120 and 240

Portable generators offer 120-volt outlets, and many also offer a 240-volt outlet. Most event gear needs 120 volts. Each outlet has its own amp limit, so spread the load across outlets and across both circuits if the unit has them. A 20-amp, 120-volt outlet delivers up to 2,400 watts, but keep continuous loads below about 80 percent of that. Never overload one outlet with a multi-plug strip.

Fuel and run time

Fuel use depends on the load, the engine and the fuel type, so check the manufacturer's run-time chart. Half-load run times are the most useful figure. Ask the supplier how long a full tank lasts at your load, and plan a refuel break. Never refuel a hot or running generator. For a four-hour reception, many planners prefer a generator that can run the entire event on one fill.

Dedicated circuits for critical gear

Put the DJ and the sound system on their own outlet or generator if you can. Lights and caterer equipment cause voltage dips that produce hum in audio. A separate small inverter generator for audio costs little compared with a ruined toast.

Where generators go

Placement affects safety, noise and sound quality. See quiet generators and placement. Run cords to the tent in a safe way, covered in extension cord safety.

Chicago notes

Public parks and lakefront venues may restrict generators, and some suburbs limit noise hours. Ask your park district or village hall before you order. For the Forest Preserve District of Cook County, DuPage County or any other forest preserve, check the permit conditions for your site.

Plan the power with the rest of your rentals

Book a free 15-minute seating call and we will help list every power draw, or request a quote.