What size Generator do I need?

You’ve decided that you need a generator for your home to ensure the safety and comfort of your loved ones during power outages, but are unable to figure out what size generator do I need? 

A standby / whole house generator is an important investment for your home and it’s crucial to select the right size. If your generator is too small, it may not be able to handle the load and could fail during an emergency. Or it may not be able to handle large motor loads and may damage itself or the motor. 

You can choose a portable generator if you do not want to power all the loads and want to save on the initial costs. 

We can help you determine the size of generator you need with an easy-to-use generator sizing guide, that takes into account your home’s running and starting loads, and the wiring configuration.

How large a generator do I need?

All home electrical loads can be categorized into resistive and reactive loads. Resistive loads include light bulbs, stove elements, etc, and consist of all the household appliances that generate heat or light with the exception of fluorescent lamps. To calculate how much power these loads consume, we need to merely add up their individual wattages.

Switching on reactive loads results in a surge of current that the generator has to supply. The reactive loads include motors, fluorescent lamps, welding machines, battery chargers, uninterruptible power supplies, air conditioners, and any transformer. The magnitude of the surge current depends on the constructional details, exact wattage, and load apportioning of motors. The surge wattage rating of the generator comes into play in these scenarios.

Consumer-grade generators can provide surge currents 1.2 to 1.5 times their continuous rated current, while some of the best construction-grade generators can deliver 2.0 to 2.5 times the normal value. So you should consider the starting wattage for the reactive loads in addition to the running wattage.

Any generator adjusts its output to match the power needs of the connected loads. If it is unable to match the power demand, as in the case of starting watts, the voltage drops resulting in flickering & dim lights and the slow turning of motors. The engine may itself stall. Persistent overloads can burn the motor and generator windings, causing damage and failure. 

Induction motors act like short-circuited transformers during the start-up resulting in very high startup currents. The maximum current flows when the rotor speed is zero at the startup. So, this current is also called the Locked Rotor Current (LRA). The current reduces as the motor picks up speed and is significantly lower at 70% full speed.

The continuous current that flows when the motor is subjected to its rated load is known as the full load current (FLA). LRA is usually 3 to 8 times the FLA and requires the higher starting watts from the generator. Once the motor reaches significant speed, reduced current results in lower values of running wattage.

Typical Loads and their Indicative Running and Starting Watts

The table above has common home electric appliances’ energy consumption listed along with their starting wattage and running wattage for your guidance.

Equipment TypeRunning WattsStarting Watts
Resistive Loads
75 W Incandescent Lamp750
1500 W Space Heater15000
Electric Water heater (40 Gal)4500 – 55000
4 Cup Coffee maker6000
Electric Range (6″ element)15000
Electric Range (8″ element)21000
Reactive Loads
Microwave Oven7001000
Refrigerator7003200
Freezer7003200
Clothes Washer12002400
Central AC
20000 BTU25003300
24000 BTU38005000
32000 BTU50006500
40000 BTU60007800
Window AC 10000 BTU15002200
Garage Door Opener
1/4 HP5501100
1/2 HP7301400
Sump Pump
1/8 HP8001300
1/2 HP11002150
Well Pump
1/8 HP8001300
1/2 HP11002150
Well Pump Submersible
3/4 HP9005400
1 HP12007200
Drill
1/8 HP5001000
1/2 HP9001800
Circular Saw, 8 Inch15003000
Disc Grinder, 7 Inch20004000
Air Compressor
3/4 HP9005400
1-1/2 HP9006000
Capacitor Start180010800

Many generator sizing guides, generator manufacturers, and retailers provide wattage charts similar to the ones above or load calculators. While generator wattage calculator methods can provide you with an approximate size of the generator, the method has the following drawbacks:

  • These guides assume average numbers that may vary significantly from the actual wattage of your particular electric equipment depending on its make, year of manufacture, and size.
  • The sequence of switching reactive loads impacts the generator rating. If you start one reactive load at a time, you will require less starting wattage, but if you are using a transfer switch or switch multiple home appliances simultaneously, you need to factor in higher starting watts, while selecting the generator.
  • Load imbalance, discussed below, is not factored in these guides.

Whole house Generator Sizing

In North America, most electric appliances sold have a 120V rating. Hence, 3-wire 120/240V system is followed within the households for distributing electrical power, wherein two separate 120V lines, L1 and L2, are wired to feed all your appliances. Most 120V devices like refrigerators, stoves, room air conditioners, etc., are cord and plug devices and connected to one of the above lines.

Depending on the location of these devices and the power outlets, which in most houses is quite random, it is very common to have an unbalanced load distribution among the two lines. Most portable generators have two 120V outputs with a common neutral for the two. When hooked up, each of these outputs or buses can only supply half of the generator’s rated wattage. So in a 5000 W generator, each bus can supply only 2500 W.

So, to select a whole house generator, it is not sufficient to just know the total power requirements. In fact, you must know the system configuration, wiring, and the possible load imbalance to figure out what size generator you need correctly.

If all the electrical devices of your entire home need 5.8 kW, you will be inclined to pick the generator of a 6 KW rating. But if any one of the lines draws 4 kW power, you need a generator with a total power output of 8 kW. In Europe and elsewhere, with the two-wire systems, this is not an issue.

To select the correct size generator for the entire power demand of your house, you should measure the actual currents on the two 120 V lines as detailed below.

Sizing procedure

  • Turn on all the electrical loads and lighting circuits that you want to operate during a power outage on your standby generator.
  • Ask your electrician to measure the continuous electric currents on both lines L1 and L2 with a clamp-on multimeter.
  • Choose the largest of the two currents and multiply it by 240 to get the correct wattage rating of your home standby generators.

If the measurements give L1 = 16A and L2 = 24A, you need your generator to supply at least 24A from each of its outlets. It must be rated for at least 24 x 240 = 5760 watts to do so. If you still want to go for a smaller generator, you will have to deploy an electrician to swap some circuits to reduce the imbalance.

Sizing a Generator for Partial Loads.

To provide backup only to certain selected critical household equipment and lighting circuits during power outages, circuits for all these appliances will have to be transferred to an additional sub-panel connected to two 120 V outputs of the standby generators. Follow the sizing procedure as above, considering only the loads on the 120 V lines going to the sub-panel. Take the larger current of the two and multiply it by 240 to get the required wattage.

Sizing a Generator for Central Air Conditioning Units

Large motor-driven equipment like a central air conditioner has much higher Locked Rotor Currents (LRA) requiring considerable surge capacities from the generators. LRA values are usually mentioned on the motor’s nameplates, else you may have to contact the manufacturer or assume a conservative value.

Most documentation will suggest selecting the surge rating of your generator to match the nominal LRA of the air conditioning unit. You will select a much higher size generator by this method. The nominal nameplate LRA of any machine is given for full voltage starting. In actual conditions, the surge current required by your air conditioner’s motor will cause a dip in generator voltage, which proportionally reduces the motor current.

Most residential appliances are designed to start with even 25 – 30% voltage dips, resulting in 25 to 30% reduced LRA from its nameplate value. Commercial equipment can work with 15% voltage drops. Most generator manufacturers specify only the surge wattages of their machines and remain silent on the surge currents.

This restricts you from simply comparing LRA with the surge current capability of the generator to complete the selection. Instead, you must work out the requirement of starting watts of your airconditioning unit and check it against the surge wattage of the generator. You can see the example of a 3-ton air conditioner below to understand the procedure.

At 30% voltage drop, the required starting watts for 240 V appliance will be (0.7 x 240) x (0.7 x LRA), which is equal to 117.6 X LRA.

For a 3 Ton Air conditioner:

Nominal BTU per Hour = 12000 x 3 = 36000 BTU per Hour, where BTU is British Thermal Unit.

Voltage = 240 V,

EER = Energy Efficiency Ratio = 10,

where EER is the ratio of cooling capacity in BTU and the power input to the appliance. The higher the EER value, the more efficient is your air conditioning unit. Here a value of 10 is taken for ease of calculation.

The KW requirement of the unit will be (BTU per Hr) / EER = 36000 / 10 = 3600 Watts

Assuming a power factor of 0.8, the running kVA required at 240V will be 3600 / 0.8 = 4500 VA = 4.5 kVA.

Full Load Amps (FLA) = 4500 / 240 = 18.75 Amps

LRA at 240 V for a 3-ton unit is typically in the range of 100 A. So LRA is 5.33 times the FLA value for the unit. But the LRA at 30 % dip shall be 0.7 x nameplate LRA = 100 x 0.7 = 70 A.

Also, the Stating kVA at 30% voltage dip shall be (117.6 x 100) / 1000 = 11.76 kVA.

So the generator must be able to provide 70 A- 18.5 A = 52.5 A as the surge current. Hence, you will require an 8 kW generator to start the 3-ton unit. In this example, we have not considered other loads on the generator. The sizing procedure considering other essential appliances’ power requirements in addition to central AC is detailed below.

Sizing Procedure for Central AC

  • Check the LRA from the nameplate details of the AC unit. Multiply it with 0.7 to calculate LRA at 30% voltage sag.
  • Turn on all other electrical essential appliances and lighting circuits that you plan to run during the electric grid power outages. Measure the continuous currents in L1 and L2 as in previous cases.
  • Take larger of the two currents and add the surge current for central AC as calculated above to arrive at the total surge current. The surge current for central AC is the difference between the LRA at 30% voltage sag and the FLA.
  • You can calculate the total surge watts required from the generator by multiplying the total surge current with 240 x 0.7 = 168 V.

From the above examples, if the L1 = 30 A and L2 = 40 A, the total surge current will be 52.5 A + 40 A  = 92.5 A.

Starting Watts = 168 x 92.5 = 15540 Watts

You may require a 12.5 kW to provide the required starting watts to your loads. The size may vary depending on the make or model you choose. If you want to be more conservative, you may carry out the above analysis with a 20% voltage drop following the same basic methodology.

If several big motors start together, find the load with the largest surge current requirement and add the running watts of tall other appliances. This is with the assumption that all the devices will not start simultaneously, or you may have to run multiple home appliances with a manual and staggered start.

Sizing a Portable Generator

Working out what size portable generator do I need during prolonged power outages involves the same basic three steps as for the whole house generators discussed above:

  • Choosing the essential equipment you want to run during a blackout.
  • Finding the power consumption of these appliances.
  • To understand the imbalance, determine the load distribution of this essential equipment on the two 120 V lines.

As above, any wattage chart or generator calculator found on the net can only provide an approximate idea of how many watts a particular type of device consumes. To exactly work out how large a portable generator you actually need, you will have to assess the exact consumption. This can be done by scanning the nameplate details of your appliances or by actually measuring the consumed power.

For simplicity, assume that you need to feed five appliances consuming 1000 W each. Just going by the methods suggested by most guides, you would think you need a 5 kW portable generator. However, when you distribute these loads between two 120V lines of your generator, one of the output lines would have a 3 kW load connected to it.

As highlighted above the rated power of portable generators is equally divided between the two lines. This means you will need a generator that can supply 3 kW x 2 = 6 kW during a power outage.

Power Needs based on the Application

A generator can be an expensive investment and its usage may vary from an emergency power outage to an outbuilding like a job site, workplace, or recreational comfort for travel or camping.

Some of the common appliances that you may run on your generator sets are listed below. This is by no means a comprehensive list but enough to give you an idea.

Household Needs

You can decide to go with a whole house, a large portable generator for the entire house, or a small size generator for critical appliances. The list includes lights, fridge/freezer, electric stove/oven, window A/C unit, small kitchen appliances, hairdryers, small medical equipment, a water pump or sump pump.

Job or Construction Site

You may power halogen work lights, handheld electric tools like power drills, reciprocating saws, air compressors, or other electrical tools run on a construction site like circular saws.

Workplace

At your workplace, powering sensitive electronics during breakdown becomes a dominant factor in the selection of the generator. Most workplaces have electronic devices like computers with monitors, laser printers, wireless routers, fax machines, etc. Modern electronics need the THD value to be below 3% to function safely and reliably. Most conventional heavy-duty generators can’t comply with these stringent thresholds.

You will need either a standby generator or an inverter generator to safely power sensitive electronics. Inverter generators can be classified into large, mid-sized, and recreational categories.  A large inverter generator can supply around 7500 watts of power, mid-sized around 3500 watts, and a recreational unit around 2000 watts.

Most inverter generators have paralleling capability that allows you to parallel two inverter generators to get more wattage.

Recreation

They are the lightest generators with no installation costs, easy to store and transport. As these are often called, small camping generators mainly power RV appliances and other portable appliances like electric grills, phone chargers, gaming systems, and CD players. 

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