The single most important thing to determine when choosing a backup power system for your home is how much watts you will need in an emergency. Since the electric consumption of many devices is variable, there are three basic generator sizing requirements: continuous (running) watts, peak (startup) power and surge current capabilities. 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. However, 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. There are guides that tell you to find the rated power of your devices from their nameplates and then to add up all the wattages. The problem with such method is you will wind up with a significantly oversized unit: the nameplate rating usually is just the worst-case number required by UL. In almost all cases, 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 load diversity of 50%. Another important thing to note is the above calculation methods do not take into account possible imbalance of the loads on two 120V lines of your home wiring. The case is, most homes in U.S have 120/240V service which has two separate 120V buses derived from line transformer (see diagram below). A genset likewise has two outputs. Each of them can provide not more than half of the total rated power. For this reason, measuring the rotation time of your electricity meter's revolving disk as some suggest, will not will give you sufficient information because the result reflects only the lump consumption.


In my view, the best way to determine how big home generator you need is to actually measure your required power. First, decide what lights and equipment you will need to operate simultaneously during an emergency. Measuring current with ammeter Then have your electrician measure your electric consumption when all these loads are running. If you have a proper electrical training and know how to safely work with electricity, you can do it by yourself. All you will need is a hand-held clamp-on amp meter with "Peak" function to capture inrush currents. To do the measurement wear line-worker's rubber gloves, remove front panel of the main disconnect box and enclose the line cable within the clamp-on device. If you have a typical for US split phase 120/240 volt service, read the value of current on each of two "hot" wires separately. Then multiply the greater of these two values by 240V. For example, if your measurements on the two lines are I1=30A and I2=20A, then you need a genset rated for 30A×240V=7,200 volt-amps. This number takes into account imbalance of your loads. 120/240V split phase If the loads were balanced (in our example they would be 25A each), you would need 25×240=6,000 VA. Make several measurements with 10-minute intervals and take the largest reading. Note that for a 2-wire single-phase service, such as 220-230VAC in Europe, or old 120VAC in US, you need to measure the current in just one AC line and multiply it by the working voltage. Finally, add 10 to 20 percent safety margin for system derating and to prevent false tripping of your generator circuit breaker. This will be your required continuous volt-amps rating. Again, do not attempt to do these measurements by yourself unless you have proper electrical qualifications: you are dealing with lethal voltage levels. If you don't feel like doing all these measurements and calculations and want a quick simple guide, see the rules of thumb above.

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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 and volt-amps are always greater than watts. This means that you have to count your total volt-amps, which can't exceed the genset's rating. For an explanation of the difference between these quantities see this guide to power factor calculation. For a reference, real power is P=V×I×PF. Also note that if for a cost reason you have selected a genset that can't supply your entire house, in an emergency you can rotate usage among essential appliances as you need to. For example, run a refrigerator for an hour, then run an a/c, etc.

For an average single-family home with up to 5-ton a/c, you likely need a 15-17kW system. In this class among top brands my pick is GUARDIAN® 17kW/16kW model, which has one of the lowest costs per kW. For most small homes such a system is usually adequate. In general, if you need to operate a central a/c or another large motor-driven load, you need to know its start up current and select a genset with proper surge capability. For starting requirements see our guide to sizing generator for air conditioner or a motor. Generator buyers leads

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.