POWER REQUIREMENTS.

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 most devices varies depending on their mode of operation, there are three basic sizing requirements: continuous (running) watts, peak power and surge (startup) 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. I am going to tell you why. But if you’re in a hurry and and are not interested in technical details, just scroll down to the rules of thumb below.
Most power calculators give you the wattage charts for various categories of appliances. There are two problems with this method. Firstly, in reality, 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 initial ball park estimates, but not for real sizing. Secondly, and even more important, this method does not take into account possible imbalance of the electrical loads on two 120V lines of your home wiring. This is something no other sizing guide would tell you. The thing is, most homes in U.S have 120/240 volt service. It actually appears as two separate 120V buses derived from line transformer (see diagram below). Standby genset and many portable ones likewise have two 120V outputs. Each of them can provide not more than half of the total rated power. For this reason, if you want to connect a generator to your house wiring, the lump power consumption of your appliances will not will give you sufficient information. You need to know how each of two 120V lines are loaded. Two houses which have the same loads may need different generators depending on how the devices and lights are distributed there. That's why for example, measuring the rotation time of your electricity meter's revolving disk as some suggest is not very useful either. The imbalance of the loads is not an issue if you have a two-wire single phase service, like in old homes or in Europe.

SIZING PORTABLE GENERATOR.

If you are looking for a portable generator to run just a few "cord and plug" appliances, you may gather the rated power of your devices from their nameplates or user manuals and then add up all the wattages. 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 in your home would operate at full load at the same time. The U.S. National Electrical Code (NEC®) 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%. Aside from this, this calculation may work. However, for sizing a generator that will be connected to your house wiring you still need to know how the power consumption is split between two 120V busses as I noted above. I'll show you how you can find it. For brief guidelines read below.

If you want more details for various options, my Home Generator Guide provides complete step-by-step sizing procedures for different setups. It may help you avoid costly mistakes.

DETERMINING RUNNING WATTS.

In my view, the best way to determine how big home generator you need is to actually measure your required power on each of two 120V service legs. If you are technically inclined, it's not so difficult as one might think. First, of course, decide what lights and equipment you want to operate simultaneously during an emergency. 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" or "Inrush" function to capture startup currents and line-worker's rubber gloves. To do the measurement wear the gloves, remove front panel of the main disconnect box and enclose the line cable within the clamp-on device. Read the value of current on each of two "hot" wires separately. Make several measurements with 10-minute intervals and take the largest readings. Then multiply the greater of these two values by 240V. For example, if your load measurements on the two lines are I1=30A and I2=20A, then you need a genset rated for at least 30A×240V=7,200 volt-amps. In other words, you need a device with the wattage twice the largest leg loading. This is what the quick calculator on this page does. The result does take into account imbalance of your loads. If the imbalance is substantial, you may want to swap some circuit breakers. Do not attempt to do all of the above by yourself unless you have proper electrical qualifications: you are dealing with lethal voltage levels!
If the loads were balanced (in our example they would be 25A each), you would need 25×240=6,000 VA. 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 only one AC line and multiply it by the working voltage. Finally, you may 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. If you don't feel like doing all these measurements and calculations and want a quick simple guide, see the rules of thumb above.

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 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 always manually rotate usage among essential appliances as you need to. For example, run a refrigerator for an hour, then run an a/c, etc.

RULES OF THUMB.

For an average single-family home with up to 5-ton a/c, you likely need a 15-18kW system. In this class among top brands my pick is GUARDIAN® 18kW model. It 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.

Note that many genset's manufacturers provide "intelligent" control to help 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. Some Generac Guardian® automatic systems likewise offer load management. Intelligent control by itself of course costs more, but overall you may save since it lets you choose a smaller size genset.