Of course, there are plenty generator buying guides on the web. However, in my view, most of them are either useless or even misleading. For example, many power calculators give you the wattage charts for various categories of appliances. I see two problems with this method. Firstly, within each category actual wattage of different models often varies within 2 to 1 range (see for example our quick study of refrigerator power consumption). Obviously, these charts may be good only for ball park estimates, but not for real sizing. Secondly, this method does not take into account possible

- For 2-3 essential loads (fridge, small furnace, window A/C)- 4,000 watt 120V ($400);
- To run most items in a small home- 8,000 watt portable 120/240V ($1000);
- For whole house with a central A/C - 8-20 kW standby ($2,000-5,000 plus $1,500-8,000 installation).

If you are sizing a portable generator to run a few cord and plug appliances, you may find the rated power of your devices from their nameplates and then add up all the wattages.

RATINGS TABLE OF STANDBY MODELS FOR HOME USE

A little problem with such method is you may wind up with an over-sized unit: the nameplate rating usually is just the worst-case number required by UL. Sometimes it is well above the actual operating level, often by up to 50%. In addition to this, compressors of fridges and a/c are continuously cycling on and off. It is highly unlikely that everything would operate at full load at the same time. The U.S. National Electrical Code (NEC®) 2014 recognizes this fact and in certain cases allows electrical system designers to use adjustment factors from 50 to 80% under assumption of For a complete step-by-step sizing procedure for different setups see my Home Generator Guide.

Note that single phase residential and commercial generator sets are normally rated for the loads with power factor PF=1.0. In reality, PF is always less than 1.0 and volt-amps (VA) are always greater than watts. This means that your total volt-amps should not exceed the genset's rated wattage. For an explanation of the difference between watt and VA see this guide to power factor calculation. For a reference, real power is

Many genset's manufacturers provide "intelligent" control to aide in running air conditioning systems. For example, Briggs and Stratton offers automatic transfer switches with optional AC Power Control module (ACCM) that lets you prioritize your loads. It monitors the currents in selected circuits and if necessary, temporarily turns off lower priority lines to allow the air conditioner to start. Generac Guardian® automatic systems likewise offer load management. Intelligent control costs more, but you may save by choosing a smaller genset.