HOME GENERATORS SIZING GUIDE AND ADVICE



WHAT GENERATOR DO I NEED?



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 many devices is variable, there are three basic generator sizing requirements: continuous (running) watts, peak (startup) power and surge current capabilities.





RATINGS TABLE OF STANDBY MODELS FOR HOME USE

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 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 load 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. Obviously, this info 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 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 refrigerators, a/c, and other appliances are continuously cycling on and off. It is highly unlikely that all of them would operate at the same time. The U.S. National Electrical Code (NEC®) recognizes this fact and allows electrical system designers to use a diversity factor (0.67 or so), anticipating that not everything is running at full load all the time. 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. For the same reason, measuring the rotation time of your electricity meter's revolving disk as some suggest, will not will give you sufficient information.
WHAT GENSET DO I NEED: RULES OF THUMB
  • 2-3 essential loads (fridge, small furnace, window A/C)- 4,000 W 120V ($400);
  • Most items in a small home-
    8,000 W portable 120/240V ($1000);
  • Whole house with a central A/C - 15-20 kW standby ($3,700-$5,000 + $2,000-$6,000 installation).

DETERMINING RUNNING WATTS.

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. 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 (see the diagram), read the value of current on each of two "hot" wires separately. Then multiply the greater of these two values by 240V. 120/240V split phase 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. If the loads were balanced (in our example 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.

Note that single phase residential and commercial generator sets are normally rated for the loads with power factor PF=1.0. This means that your total load's volt-amps can't exceed the genset's rating. In other words, you have to count volt-amps rather than watts. For an explanation of the difference between these quantities see this guide on power factor calculation. For a reference, average or real power is P=V×I×PF, where PF is always less than or equal to 1. 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. If you don't feel like doing all these measurements and calculations and want a simple guide, see the rules of thumb above.

HOW TO SIZE A STANDBY HOME GENERATOR TO START an A/C OR OTHER MOTORS.

Motor driven appliances (such as refrigerators and air conditioners) require larger amounts of current for initial start-up than when they are running. This is because induction motors initially act like a short-circuited transformer. The maximum start up current is referred to as "Locked Rotor Amps" (LRA). The current of a motor will drop significantly when it accelerates to about 75% full speed. The LRA of a typical motor is 3 to 8 times its continuous operating current (called full load amps, or FLA). Note, this does not equate to 3 to 8 times real (active) power because the power factor of a starting motor is low (<0.5). That's why when selecting a home system for starting requirements, you must consider primarily the current surge capability. By the way, fridges may have lesser ratio between LRA and FLA because of the resistive heaters that are periodically connected to defrost the freezer.

To find your appliance starting amps, look for LRA on its motor nameplate. If LRA is not indicated on the label, you can likewise measure it although it may not be easy. Just don't try to do it unless you have proper electrical training. You will need to reach a single wire in the cable that feeds your device. If it is a cord and plug device, you may take an extension cord, remove a few inches of the external sleeve and pull black lead out of the bundle. Use line-worker's rubber gloves, set amp meter to Peak (surge) reading and enclose the wire within the clamp-on device. Then have someone turn on the device and take the surge current reading. If you have several motor driven loads, find the one with the greatest difference between surge and running currents. Add that difference to the total running current of all appliances assuming multiple devices rarely start up at exactly the same time. This will give you net surge current requirement of your backup system at nominal line voltage. Most tutorials will tell to pick a generator with surge current matching LRA of your motor. Well, with such an advice you may wind up with much larger device than you really need. The case is that nameplate LRA is given for full voltage starting. When motors start, they draw a current surge causing a voltage dip of the generator. When voltage drops, motor current is reduced proportionally. Most appliances can start with 30% voltage sag, that is at currents 30% lower than their nominal LRA. As the result, starting volt-amps can be 0.7×0.7=0.49 of nominal.

For reference, here are typical nominal starting requirements of single-phase 240VAC central air conditioners depending on their size.

A/C Cooling
Capacity		 Nominal
BTU/hour		 LR Amps
at 240VAC		 Starting kVA
@30% voltage dip
1 ton12,000333.9
2 ton24,000677.9
3 ton36,00010011.8
4 ton48,00011713.8
5 ton60,00014517.1
Genset manufacturers often specify their models' surge wattage capability, but unfortunately, they rarely state LRA capability. The chart below shows surge current capability of Generac's standby generators at 30% voltage dip based on their sizing guide.

Generator's Rated Power (kW)7101316171820
Surge Current Capability at 240VAC 1 Ph
(LR Amps @ 30% Voltage Dip)		466395117125133145
If you are buying an automatic system without an "intelligent" load control, be aware that after detecting a service interruption it may try to start all your motors simultaneously. With such a system you'll need a genset with the capacity to provide the necessary total starting current for all your loads. Otherwise, the motors may trip the genset's circuit breaker or can overheat and even burn out. Alternatively, you may set your backup system to manual mode. Then in emergency you will first turn on the central a/c and then all other loads sequentially.

If generator's surge current capability is less then LRA of your air conditioner, it may require some form of assisted starting. You may need to install a "hard start" kit, which is quite cheap. It is basically a large AC capacitor in series with a solid-state relay. It has to be wired with "piggy-back terminals" parallel to the existing "run capacitor" (these terminals may be marked RUN). The solid state material rapidly increases in resistance as it is heated when an electric current passes through it. As the result, it is disconnecting the start capacitor from the circuit soon after power is applied to the motor. The solid state material then remains hot from the "trickle current" that continues to flow through it as long as voltage is applied to the motor. Note that when the power is disconnected from the motor, the material begins to cool down, which takes one to two minutes. If AC voltage is re-applied during the cooling off period, the hard start capacitor may be ineffective because it is still disconnected.

For an average home with up to 5-ton a/c, you likely need at least a 15-17kW system with proper surge capability. In this class among top brands my pick is GUARDIAN® 17kW/16kW model, which has one of the lowest cost per kW. For most small homes such a system is usually adequate. I understand that regardless on where you bought a Generac® system, you can order installation and service from their local dealer or hire your own contractor.



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.