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In a perfect world there would be temporary power where and when you needed it, but in reality, many contractors must rely on portable generators some or all of the time. As a framing contractor, I've had to use them myself, so I understand how important it is to get a dependable machine that can provide the required power. In this article I will explain how to size a portable generator and what to look for in terms of features.

To make it easier for you to find an appropriate generator, I have included a list of gasoline models (the most common type) with built-in GFCI protection (to ensure OSHA compliance) and 3,500 to 7,000 watts of rated output (see specification chart on page 3). The manufacturers and equipment-rental folks I interviewed said the typical construction crew needs between 4,000 and 6,000 watts of rated power. I broadened that range, because it seems overly restrictive and I've often seen units smaller and larger than that on job sites.

Output

Two of the more important specifications for a generator are rated and maximum output. Rated output is easy to understand: It's the number of watts the machine can produce for an indefinite period of time. As one manufacturer put it, the generator can produce that amount of power as long as there is fuel in the tank and oil in the crankcase.

Maximum output is more complicated, because with no industry-wide standard for what, exactly, this term refers to, manufacturers are free to measure it however they like. Most treat it as the highest number of watts the generator can produce, even if it can only produce that wattage for a fraction of a second. The length of time that the generator can sustain this output is up to the manufacturer; it could be anywhere from less than 1 second to 10 seconds or more – long enough to provide the surge of power it takes to start a tool.

A few manufacturers carry an intermediate number that falls between rated and surge. For example, GMC does not test its machines for surge; for this company, maximum output is the highest output the generator can sustain for two hours. Honda carries three numbers: rated, maximum, and "iAVR." Maximum is what the machine can sustain for at least 30 minutes, and iAVR is akin to what others call surge or starting output.

When I refer to output in this story, I will use the terms maximum, surge, and starting interchangeably, because that's what most manufacturers do.

Sizing a Generator

The trickiest part of buying a generator is deciding how much capacity you need. If you undersize the unit, it will strain to meet demand – and both it and your tools will work harder and wear out more quickly. In extreme cases, the panel breaker will trip and the generator will stop delivering electricity.

An oversize generator will work just fine – but it costs more, consumes more fuel, and is harder to haul around.

Types of loads. The first step in sizing a generator is determining which tools you will be using together and how much power they require. Resistive loads like lights and electric heaters need the amount indicated on their labels: A 100-watt bulb uses 100 watts and a 1,000-watt heater uses 1,000 watts. It's not so simple with power tools. They use more power at startup than they do when running, and you'll need to get both numbers.

The starting and running wattages of common tools can be found in the generator manufacturers' sizing guides. If the guide does not contain data for a tool you plan to use, you'll have to get it from the tool's manufacturer or make an estimate based on the amp rating on the tool's nameplate. Since generators are rated in watts, you'll need to convert the amp rating to watts. The conversion formula is volts x amps = watts, so if a tool draws 15 amps of current, it will require 1,800 watts (120 volts x 15 amps) of power.

Most handheld and benchtop tools have universal motors, which use about twice as much power at startup as they do when running. If you can't find the running amps for a tool that has a universal motor, it's safe to use the nameplate amp rating instead. Bear in mind that this will overstate the running amps, because when a tool is running but not under load it will draw somewhat less current than the nameplate amount.

If the machine has an induction motor, you will need to find the amp rating and motor code for that device. The amp rating will be on the nameplate; if the motor code is not there too, you'll have to get it from the manufacturer. A contractor-style table saw or brick saw might have a Code G motor, meaning that it starts with no load and load is gradually applied once it comes to full speed. This type of motor will draw about 3-1/2 times its rated power at startup. Tools that start under load – such as air compressors – typically have Code L or M motors. These motors draw about six times their rated power at startup.

Load management. Generator manufacturers assume that you'll be smart enough not to start all of your tools at once, and most recommend buying a machine capable of providing the number of watts it takes to start your largest tool while your other tools are running, plus 10 percent (as a safety factor). This means carrying starting watts for your largest tool and running watts for the rest.

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In most cases, the biggest tool will be a table saw, miter saw, demo hammer, or compressor. A compressor may require so much power at startup that it's impractical to run it off a generator. For this reason, contractors who regularly run big compressors will buy gas-powered models so that their generators need to power only smaller tools.

The adjoining chart shows how to size a generator for a framing crew that is using a circular saw, a 1/2-inch drill, and a miter saw:

You'll need 5,830 watts for the brief period of time that your miter saw is starting while your drill and circular saw are running. Most of the time you'll need less – 4,180 watts when all three tools are up and running. Based on these assumptions, you should buy a generator with a rated output of about 4,180 watts and a maximum (surge) output of about 5,830 watts.

These are conservative numbers that reflect the fact that manufacturers want you to have enough capacity.

I would personally have no problem omitting the recommended 10 percent safety factor – and even downsizing a bit more – because it's very unlikely that every other tool will be running during that fraction of a second when the biggest tool starts.