THE HOME GENERATOR SIZING GUIDE

HOW MUCH BACKUP POWER DO I NEED?

POWER REQUIREMENTS

. The single most important thing to determine when choosing a backup power system for your home or business is how much wattage 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

FIND ELECTRICAL POWER GENERATORS BY SPEC

There are plenty generator buying guides and load calculators on the web. However, in my view, most of them are either useless or even misleading. For example, many sizing calculators give you the wattage charts for various categories of appliances. However, since within each category actual wattage of different models often varies within 2:1 range, this info may be good only for ball park estimates, but not for real sizing of a generator. Other reference guides tell you to find the rated power of your devices from their nameplates and then to add up the watts of all your devices. The problem with such method is you will wind up with an 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 most appliances (such as refrigerators, a/c, etc.) 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 all devices are running at full load all of the time. Another important thing is the above approaches do not take into account possible imbalance of the loads on two 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.

A RULE OF THUMB:

2-3 essential loads (fridge, small furnace, window A/C)- 4,000 W ($400);
Most items in a small home with no central a/c- 8,000 W ($800);
Whole house with a central A/C - 15,000 W ($3,600 + $1,000 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 power outages. 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 measure it by yourself. All you will need is a hand-held clamp-on amp meter or a power analyzer 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 wire within the clamp-on device. If you have a typical for US split single-phase 120/240 volt service, read the value of current on each of two "hot" wires (which are usually black) separately. Then multiply the greater of two values by 240V. For example, if your measurements on the two lines are I1=30A and I2=20A, then you need a genset with 30A×240V=7200 volt-amps. This number takes into account imbalance of your loads. 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 wire and multiply it by the working voltage. Finally, add 10 to 20 percent safety margin to the obtained wattage value 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 a 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 not only your total load's wattage, but also total volt-amps can't exceed the genset's rating even if active (average) power consumption of your devices is less. For an explanation of the difference between watts and VA see watts, VA, and power factor calculation. Also note that if for a cost reason you bought a genset that can't supply your entire home, in an emergency you can rotate usage among essential appliances as you need to. For example, run the refrigerator for an hour, then run the freezer, the water pump, etc. If you don't feel like doing all these measurements and calculations, see the rule of thumb above.

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

Motor driven appliances (such as refrigerator, furnace fan, air conditioner) 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 5 to 8 times its continuous operating current (called full load amps, or FLA). Note, this does not equate to 5 to 8 times active power. The power factor of a starting motor is low (<0.5). Depending on the load type, the starting real power could be below the generator rated wattage, while the motor starting current may be well above its rated current. That's why when rating home system for starting requirements, you must consider not just starting kW, but also the current surge capability. Also note that most fridges have lesser ratio between LRA and total operating current because of the resistive heaters that are periodically connected to defrost the freezer.

To find your refrigerator or other appliance starting amps, look for LRA on its motor nameplate. If LRA is not indicated on the nameplate, you can actually measure it. Just don't try to do it unless you have proper electrical training. Wear line-worker's rubber gloves, set amp meter to Peak (surge) reading and enclose the wire that feeds your load 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 device with the greatest difference between surge and running current (FLA). Add that difference to the total running current of all appliances assuming multiple devices rarely start up at exactly the same time. If you are looking for a genset that supplements your solar system, you also need to take into account a recommended charging current of the storage battery bank. 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 advice you may wind up with 30% larger device than you really need. The problem 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. Similarly, starting volt-amps can be 0.7×0.7=0.49 of nominal.

For reference, here are typical nominal starting current and AC power 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 ton	12,000	33	3.9
2 ton	24,000	67	7.9
3 ton	36,000	100	11.8
4 ton	48,000	117	13.8
5 ton	60,000	145	17.1

Due to low power factor, surge watts may not be an issue. High surge power often shown in various buying guides is actually volt-amps rather than real power. The main issue is usually with the starting current. For reference, average or real power is volt-amps times power factor: P=V×I×PF, where PF is always less than 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 single phase standby generators at 30% voltage dip based on their Sizing Guide.

Generator's Rated Power (kW)	7	10	13	16	17	18	20
Surge Current Capability at 240VAC 1 Ph (LR Amps @ 30% Voltage Dip)	46	63	95	117	125	133	145

If you are buying an automatic system, after detecting a power outage it may try to start several motors simultaneously. So, you'll need a genset with the capacity to provide the necessary total starting current for all these loads. Without sufficient starting current, motors can overheat, burn out, or trip the genset's circuit breaker. Alternatively, you may set your backup system to manual mode, and in emergency 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 or some other major load, your load's motor will require some form of assisted starting. In this case 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 and START). 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 solid state material begins to cool down, which takes one to two minutes. If 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 a 5-ton central air conditioner, you likely need at least a 15-17kW system with proper surge capability. In this class among top brands a popular air-cooled GUARDIAN® 17kW/16kW emergency generator has one of the lowest cost per kW. For up to 5-ton a/c, such a backup power system is probably 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 load currents in these circuits and if necessary, temporarily turns off lower priority lines to allow the air conditioner to start. Generac® Guardian® residential automatic systems likewise have what they call a "priority load center".

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