Size really does matter - at least when it comes to matching portable or towable generators to the tools on the job, and the extension cords used to connect them.

"Contractors may not be optimally sizing their generators for their applications," states Todd Howe, generator product marketing manager at Ingersoll Rand. "They tend to either overload the generator or underload it, and waste money by purchasing a larger unit than they need."

David Brown, vice president of MQ Power, agrees, noting, "Misapplication is the most common mistake we see contractors making. Proper selection considers the application, duty cycle and load. All three factors are important to proper generator selection."

According to Tim Sheehan, industrial sales manager of the power equipment division of Yamaha Motor Corp., "Contractors generally under-size the generator because they want to purchase the smallest wattage generator for the least cost possible."

"The problem is that contractors don't always consider all the equipment that could possibly be running at once, or they don't build flexibility into their generator selection," explains Derwin Pepper, product manager, Generac Power Systems. "Sometimes conditions on the actual jobsite require different equipment than expected. That really becomes a problem when larger tools demand more power."

Problems can occur
Potential repercussions can occur from using a generator that's not properly and adequately rated to the tool's amperage and load requirements. Possible scenarios include:

• You overload the generator. Its circuit breakers pop and you waste time resetting them.
• You cause damage to the generator. "Every time the breakers trip because of overload, it weakens the generator, as well as the tools. This could lead to downtime and lost income," Sheehan points out.
• Your workers have to alternate tool use. In the worst-case scenario, that could mean one crew is sitting idle, while those with tools the generator can power are working. In any case, it slows work flow and productivity.
• You could damage tools and decrease their life. "When you subject a motor to voltages below the name-plate rating, some motor characteristics change dramatically," Brown explains. "When amps go above the name-plate rating, heat builds up in the tool's motor; this damages the motor and shortens its life. It's most efficient when you operate motors at voltages very close to the name-plate ratings. Stay away from the 'outer limits.' "
• You could experience "wet stacking" on an under-utilized diesel towable generator, especially if the low load occurs over an extended time period. Wet stacking is excessive oil bypass in the system as the generator pushes oil into the exhaust system.

"The extra carbon causes engines to smoke and you get a performance loss, neither of which is desirable," Howe notes. "The solution is to put a heavy load on the generator, and in a couple of hours the engine will clean itself up." However, it's best to properly apply the generator to avoid this problem altogether.

Calculate demand
You have to do a little homework, including some math, to match a generator to your current job and potential new ones.

Begin by identifying the power draw (wattage requirements) for all the tools and appliances you want to power. Even though it's unlikely they will all be used at once, it's still a possibility.

"You can find the information on each tool's identification plate or in the owner's manual," Sheehan says. "If the power requirement is given in amps, multiply the amps times volts to get the watts."

Once you have individual tool wattage requirements, it's time to add these numbers together. "You add up all the wattages and compare your total with the generator's output specifications," Howe explains. "As long as you're in a 75% to 95% load factor window, you have a generator sized in the optimum range."

Pepper recommends an 80% figure. "If you add up all potential loads and use a generator that will at least provide 80% of the total, you should be fine," he says.