How Do I Know What Size Portable Generator I Need?
75How Many Watts Do You Need?
Choosing the right size camping portable generator or portable generator for home emergency use is determined by how many total watts of power you will need for the electrical devices you want to operate. The basic formula is amps x volts = watts. For example, a 100 watt light bulb running from a 120 volt generator uses one amp. If you wanted to run ten 100 watt light bulbs you would need: 10 x 120 = 1,200 watts.
Build in a Safety Margin
The math is fudged a bit to build in a safety margin. 100/120 = .8333 amps but you want to round that up to 1 amp. The breaker switches on portable generators, and in your house too, are rated in amps. You could run into a situation where the generator is technically rated to handle the required wattage but the breaker switch is not able to handle the amps. The generator shuts itself off when the breaker switch is tripped.
Steady-State and Surge Wattage
There are two wattage ratings that are important when choosing a portable gas generator: steady-state wattage and surge wattage. As in our example, a 100 watt incandescent light bulb requires, as you would expect, 100 watts. Incandescent light bulbs have only a steady-state wattage requirement. Our 100 watt bulb requires that wattage both when you turn it on and while it is running.
A fan motor, on the other hand, might require 150 watts to get it started and 75 watts while it is running. That extra wattage to start the motor is called the surge wattage and is required of anything that contains an electric motor, and several other electrical appliances as well.
One other factor not considered here is that if you need 'clean" AC power, like you get out of the wall plug, for sensitive electronic equipment then you may need a portable inverter generator.
Here are the usual wattages of some of the devices found in a typical household:
The portable generator you select must be able to safely handle the combined surge or start-up wattage of all the electrical appliances that you want to run at the same time.
On your next camping trip you want to power three 100 watt bulbs, a fan, and a color TV. Your total surge wattage from the above table = 690 watts. But wait. Let's do the amp calculation. 690/120 = 5.75 amps, that we round up to 6 amps. Recalculate wattage based on amps. 6 x 120 = 720 watts. A 720 - 750 watt generator will handle your power requirements in this example.
What happens when you add another fan and a microwave with a 1,000 watts surge?
Total surge requirements jump to 1,840 watts. Do the amp calculation. 1840/120 = 15.33 amps. Round up to 16 amps and do the math. 16 x 120 = 1,920 watts. All 2,000 watt portable camping generators will do the trick.
There may be additional factors that reduce the power of your generator:
Normal operating altitude
With any generator your power will reduce by 3.5% for every 1000 feet above sea level.
Average ambient temperature
Again with any generator your power will reduce 1% for every 10 degrees over 85 degrees.
Extension Cords
An extension cord must have adequate wire size (AWG or American Wire Gauge) for safety, and to prevent loss of power and overheating. The smaller the gauge number of the wire, the greater the capacity of the cable, i.e. 16 gauge has more capacity than 18 gauge. When using more than one extension cord to make up the total length, be sure each individual extension cord contains at least the minimum wire size.
You will still have to check for the amp rating on the breakers. If your generator has two outlets and the breakers are rated at 15 amps on each outlet then you will have to split the load and use both outlets so you don't overload the one.
All electrical appliances with a UL rating will show steady-state and surge wattage requirements. If you want to be sure you can get a line tester that shows you watts/amps in a digital readout for any plug-in electrical appliance.
