Sizing electric generators

The right "size" electric generators provide just enough wattage to operate all your essential equipment. Too few watts and you can overwork the electric generator, potentially damaging it and anything plugged into it. Too many and you'll waste money on the initial purchase and on the fuel to run it.

How to measure wattage
There are two main ways to assess the size electric generator you need - and one way to avoid. First, you can determine the power you're using directly. Using an ammeter, a device that measures electrical current, a qualified electrician can determine exactly how much power each piece of equipment needs. Total up the watts the electrician measures for each item to be powered by the generator, and you'll have the baseline power you'll need. Some generator dealers will visit your location and do this as part of the sales process.

The second way is to determine how many watts your equipment will draw on your own. Most pieces of equipment will list their power requirements either in the instruction manual or on the nameplate found on most electronic devices. The number you want to get to is watts, and as you may remember from high school physics, watts = volts x amps. Many devices will list amps at a given voltage, so just do the multiplication to see how many watts they draw.

The way not to decide what size you need is to estimate. Dealers can often come up with a good guess as to what size electric generators your business might need - but they should work with you to total up all the devices you want to power before finalizing the decision. Rules of thumb based on the size of business are never a good substitute for actual investigation of the devices your generator will be powering.

Running versus startup power
One important characteristic to know about electric motors is that they take much more to start than they do to run. Any electronic appliance with significant moving parts - fans, pumps, compressors on refrigerators or air conditioners - can require up to three times as much electricity to start up than it does to run continuously.

Because of this, you need to increase your wattage requirements. However, multiple devices rarely start up at exactly the same time: even a refrigerator that's plugged in only uses the compressor occasionally. So you only need to add enough power for the biggest startup requirement. Figure out what equipment has the largest difference between running load and startup load. Add that number to the running load of all your equipment added together to get your wattage requirement.

What size electric generators to buy
Once you've factored in your startup requirements, you'll have an idea how many watts you need from your generator. To avoid overworking your generator, you should plan on purchasing a generator with a rated capacity of around 20% more than your exact requirements. This will give you room to add a few small devices, in addition to helping extend the lifespan of the generator.