Generator Sizing Guide

Buying a generator is typically a significant investment for a home or business owner. It’s also something most consumers don’t do very often, so there’s a lot of confusion around which fuel type to use (diesel, natural gas, propane), which brand is best (Caterpillar, Cummins, MTU, Generac, Kohler, HIPOWER), and most especially, what size generator is appropriate.

To answer that last question, your first and best option is always to consult a certified electrician to assess your specific needs. However, if you opt to do it yourself, you can still make an informed generator purchase or rental by following a few basic guidelines. We’re laying those out in this guide to help you know how to determine what size generator you need.

Generator Classes: Residential Vs. Industrial

You should already have an idea of what class of generator you need. If you’re a homeowner looking for backup or standby power, you need either a small, portable generator or a stationary standby generator. These sizes run from 2kW-2000 watts or less for a recreational unit, up to about 50kW for a whole-house standby generator. These generators typically use a single-phase current, which are sufficient to power smaller equipment that don’t require constant, high-voltage power.

Industrial generators are available in a range of sizes, from around 20kW to well over 3MW. Larger business and industrial applications obviously require more capacity and thus often utilize three-phase motors for higher power. Office buildings, manufacturing facilities, data centers, and building complexes such as shopping malls, educational institutions, and living centers all require larger-capacity generators. This is the case whether it’s regarding primary power supply or emergency back-up power generation.

How To Size A Generator

The basic generator sizing formula goes like this:

Form a list of all items that will be powered.Determine the starting wattage (energy needed to turn it on) and running wattage (energy needed to operate it) for each piece of equipment. These figures are typically inscribed somewhere on the equipment itself, as well as recorded in the owner’s manual.Calculate your total power requirement by adding up these kW or KVA figures.

For equipment rated in amperes, you can convert amps to watts with the following formula:

For resistive loads (most common type): Wattage = amperes x voltsFor reactive loads: Wattage = amperes x volts x load factor

Load factor is the ratio of your electric energy use in kilowatt-hours to your peak demand in kilowatts. You can calculate it by consulting your utility bill for data and using it in the following formula:

Total kWh for the previous month / (your peak demand for the period x 30 days x 24 hours)

Even if you lose the manual, you should be able to find wattage details online. However, if you can’t determine a piece of equipment’s power draw figures, you can use a ballpark number. For example, these are the starting and running figures for some common tools and electronics:

Once you know the estimated wattage needed, the generator size is easier to choose. Whatever number you come up with, choose a generator with a capacity that’s 10-20% larger than your requirement. This will give you some leeway when and if you update your equipment and end up needing more power as a result. It also helps you manage “de-rating,” or under performance of the generator compared to its manufacturer-claimed abilities due to adverse operating conditions, such as extreme temperatures or high altitudes.

Buying A Single Generator Vs. Paralleling

If your power needs are large enough, you may wonder whether it’s better to go with a single, huge diesel generator, or split the burden between two or more smaller generators. For example, instead of a 1200kW generator, you could install three 400kW generators. This is called paralleling, and it can be a great option in the right circumstances. Here’s why:

#1. Greater flexibility. It’s very likely the reason you’re buying a generator at all is the additional reliability, a hedge against an interrupted power supply causing a slowdown or halt to your business’s operations. With multiple generators, you don’t have to worry if one has to be shut down for maintenance. You simply shift the burden to the others and keep right on working.

With just one generator, if it has to stop, so do you.

#2. Can be more cost-effective. Obviously the economics will vary widely depending on brands, new vs. used generators, prices in your geographic location, etc. But all things being equal, once you breach a certain generator size, it becomes more economical to parallel than use a single unit. Specifically, once you go over the sizes of gas or diesel engines mass-produced for use in cars, as well, those engines are harder to come by and are thus more expensive. And the engine comprises more than half the cost of a generator.

The threshold for diesel engines is 600kW and for gas engines it’s 150kW.

#3. Can be more practical. Depending on your physical space, it may be easier to fit two or three parallel generators into a room than one oversized unit. Among large commercial generators, smaller capacities can mean the difference in 25 or more square feet per unit.

#4. Easier on the generators. Load sharing among two or more generators can extend the life of each generator and avoid the risk of overburdening a single unit. For continuous usage, loads up to 80% generator capacity are ideal.

Generator Sizing And Fuel Consumption

In the same way a 3500-level truck uses more fuel than a 1500, larger generators consume fuel faster than smaller units. This is important to remember when “running the numbers” on what a new or used generator may cost. Not only do you have to refuel larger generators more often, it may be necessary to store extra fuel on-site, precipitating the need for additional storage tanks.  

For this reason, when it comes to residential generators, Consumer Reports recommends getting the smallest portable or home standby generator that meets your needs to limit the amount of fuel you have to store to run it.

For industrial-strength generators, consult our approximate diesel fuel consumption chart to estimate how much fuel you’ll burn in a particular generator size operated at either a quarter-, half-, three-quarter, or full load.

Why It’s Crucial To Choose The Right Generator Size

It may be tempting to skimp on the size of your generator, especially when doing so might save you a couple thousand dollars. However, selecting a generator that’s too small for your applications is a recipe for disaster. Not only can you damage the appliances and devices pulling power from the generator, you can harm the generator itself.

Here are some more benefits of erring on the side of a generator that’s too powerful, as opposed to too weak: