A "genset" refers to a backup generator that uses an internal combustion engine. It comprises a generator and a motor. It can come in various shapes, sizes and fuel-supply choices. Small ultra-portable gensets are unsuitable for home-emergency or battery-charging applications. In general, a light generator like this is too small for any home backup use. The DuroStar Generator is about the smallest practical generator for household use. Alternatively, a 6,000W unit is usable, although its performance, battery charging time and fuel economy may not be very encouraging.

Generator Rating

Generators are rated like inverters: in watts (W) or volt-amps (VA). VA caters to inductive loads (like welding units) which differ from resistive loads (like heating units and lights). Even though a generator is powerful enough to operate the required load, a higher-capacity unit may be preferred to overcome the inductance effect or the power factor. Surges in power demand can cause small generators to hiccup, especially when there is a huge inductive load. Cheap gensets tend to use weak engines, cheap generators and poor support electronics which work for light household applications but not for big motor-driven appliances. Components like air compressors, furnaces and well pumps demand extra power from the genset when they start up.

Motor loads can have a start-up power requirement of 3-4 times higher than their normal operational needs.

It's the same with battery charging; a small, weak genset cannot sustain the battery charger which consumes power at the alternating current's waveform peak. Units under 5,000W are unable to provide the required power even if the battery charging power is very small.

A generator using a gas engine is known as a reciprocating device. AC voltages are produced by rotating generators. From the far left of the sine wave, voltage begins to rise slowly from zero to its peak at 170V, then the polarity begins its reversal. You may wonder about the 170V genset output voltage which is very different to standard 120V. Since the AC sine wave voltage fluctuates with time, it is difficult to determine the right point for voltage measurement on the wave. Use the "root mean square" mathematical formula to compute the voltage in "the area under the curve" - simply speaking, 120V is the "average" voltage of the sine wave.

Battery charging can only occur with a high peak output voltage from the generator. The inverter's battery-charging unit has a transformer capable of stepping up or down applied voltages depending on operation mode selection. For example, a 12V battery inverter is capable of stepping up the voltage to 120V AC for connected loads. The inverter reverses the process to charge the battery by stepping down the 120V AC from the generator to a sufficient DC voltage level that will "push" current in to charge the battery. When the generators step down, it causes a period of no charge as the voltage is less than the battery bank's voltage.

Please see part two of this article for further explanations.