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How to Size a Generator? Portable, Backup & Standby for Home & Commercial Applications

How to Calculate the Right Size Generator for Home and Commercial Applications?

A generator is a suitable alternative to provide electric power where and when you need it in case of power failure and emergency power outage, blackouts, construction sites, camping, outdoor and RV (Recreational Vehicles) applications etc.

A generator can be hooked up to the main service panel via ATS (automatic transfer switch). You may do so by obtaining a building permit with the help of a qualified and licensed electrician. Multiple generators are available for small, medium and large potions the the type of generator (such as portable, backup, standby for home or commercial) depends on the load requirements and applications.

In the following post, we will show how to find the suitable size of generator for home applications for different scenarios like whole or selected circuits and types of loads etc.

How to Size a Generator? Portable, Backup & Standby for Home & Commercial Applications

How Much Size Portable Generator Do I Need for Home Applications?

Generally, a portable generator is preferred for small and selected load points in a house. No matter if you go for selected circuits or whole loads connected to the main panel, you will have to find the wattage rating of all appliances and add them to get the estimated value of the generator in kW.

Keep in mind that most household appliances like general lighting circuits are resistive loads and you may add the exact value of these kinds of loads. In case of high inductive loads (such as compressors, electric ranges, air-conditioners etc.), the starting current and wattage are bigger than the running amperes and wattage. No worries, we will do it in the next following solved example.

Keep in mind that alternators and generators are always rated in kVA (kilo-volt-amperes) but in our examples, we will express the rating of the generator in kW (kilowatt) as we use it on load side where most of the home appliances are rated in watts.

To know the wattage rating of a device, simply refer to the nameplate data printed on it. If not available, you may multiply the voltage to the amperage to get the wattage value. For example, the wattage rating of a 120V, 0.8A fan is 96W i.e. (120 volts x 0.8 Amps = 96 Watts).

In short, you may use the following formula to calculate the wattage rating of an appliance.

P = V x I

Where:

In simple words, 

Power in Watts = Voltage in Volts x Current in Amps

Watt = Volt x Amps

VA or W is the basic unit of electric power. For higher values, we use k (kilo) e.g. 1000W = 1kW. You may simply find the W from the A and vice versa using the Watts to Amps Calculator & Amps to Watts Calculator respectively.

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Sizing a Portable Generator

Now, let’s see a solved example about sizing a portable generator according to our needs.

Example:

What is the suitable size of portable generator for the following household appliances?

Small Load

  • 4 No of Fans each of 80W = 320W
  • 4 No of LED Light each of 25W = 100W
  • 2 No of LCD TV each of 120W = 240W
  • 1 No of Laptop having 110W = 110W
  • 2 No of Phone Charger each of 25W = 50W
  • Dishwasher: 120V x 8A = 1200 VA = 0.96 kW
  • Garbage disposal = 120V x 6A = 960VA = 0.72 kW
  • Other general small appliance load = 1.5 kW

Total Small Load = 320W + 100W + 240W + 110W + 50W + 960W + 720W + 1.5kW

Total Small Load = 4 kW

Large Load

  • Air-conditioner: 240V x 15A = 3.6 kW
  • Electric range: 240V x 25A = 6 kW
  • Electric heater: 240V x 30A = 7.2 kW
  • Clothes dryer: 240V x 10A = 2.4 kW
  • 1HP ( 746W) Air compressor (L-Type motor) = 120V x 5A x 6* = 3.6 kW
  • Circular saw (G-type universal motor) = 120V x 5A x  3* = 1.8 kW

You may have noticed that we have multiplied x6 and x3 with the 1HP L-type motor and G-type universal motor. This is due to this kind of motor taking very high initial starting currents and running smoothly with normal load current when achieving the running speed.

In addition, We must take the demand factor into account as we know that all the appliances won’t run at once continuously. For example, a heater and air-conditioner won’t be operated at the same time. In this case, we will count the highest rated appliance (electric heater in our case as its wattage rating (7.2kW) is more than the air-conditioner which is 6kW). (NEC® Article 220.82(C)). And the allowable demand factor for 7.2 kW electric heater is 5.76 kW (NEC Table 220.55). Electric range is excluded as it runs only for a short time period.

Demand Factor for Small Load

  • Electric range: = 3.6 kW
  • Electric heater: = 5.76 kW now ( after demand factor 7.2 kW)
  • Clothes dryer: = 2.4 kW
  • 1HP ( 746W) Air compressor (L-Type motor) = 120V x 5A x 6* = 3.6 kW
  • Circular saw (G-type universal motor) = 120V x 5A x  3* = 1.8 kW

Total Large Load After demand factor = 3.6 kW + 5.76 kW + 2.4 kW + 3.6 kW + 1.8 kW = 17.16 kW.

Similarly, no one uses all the electrical connected load at once like fans, lighting points, hair-dryers, washing machines, electric ranges etc. According to (NEC Table 220.42), the first 3kVA or kW should be rated at 100% while remaining load can be rated at a demand factor of 35%.

Demand Factor for Small Load

Total Small Load = 4 kW

  • The first 3 kW at 100% = 3 kW
  • Remaining 1 kW (4 kW – 3 kW) at 35% = 350 W

Total Small Load After demand factor = 3 kW + 350W = 3.35 kW.

Now the total load in watts (small load + large load) = 17.16 kW + 3.35 kW = 20.51kW

20% Future Expansion

Finally, add a 20 to 25% future expansion load to the calculated value. This way, you may smoothly connect an additional load in the future to the generator. Moreover, this extra large capacity will easily handle the voltage spikes and transients etc. In addition, it will expand the life of the generator as running a generator at 100% rated load continuously will shorten the useful life of the generator.

In short, by adding an additional 20% wattage capacity to the rated generator:

  • You can add additional load in the future without affecting the efficiency of the generator.
  • It will easily handle the transient voltage and sudden load spikes during the operation.
  • It will reduce the generator from unwanted noise.
  • It will expand the life expectancy of the generator as running the generator at 100% rated load simultaneously will reduce the lifespan of the generator.

By doing so, the 20% extra wattage of the total calculated value of 20.51kW

20.51kW + 20% = 4.1 kW

Now the the total required size of generator would be:

20.51kW + 4.1 kW

Required size of the generator: 24.61 kW.

The next available and suitable size of generator is 25 kW.

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Sizing a Commercial Generator for Standby & Backup Power

A portable generator also known as emergency generator set as the name suggests, provide power when and where needed in case of emergency blackout. On the other hand, backup and standby generator are always connected to the main panel via ATS switches and automatic electronics circuits where they automatically restore the electric power within seconds when main power is not available from the electric power service providers due to multiple reasons.

Commercial generators are used for large scale applications and places where the business operates 24 hours a day like resorts, restaurants and retailer places, gas filling stations, bank and financial institutes, security and hospitals, manufacturing plants etc.

Keep in mind that large scale commercial backup generator require a proper designing and installation plan and compliance with National Electrical Code (NEC) requirements (700, 701, 702 and 708). As the applications and operations of same devices are different for different users and costumers depends on system requirements, a commercial backup power generator may be sized using the following methods.

Measurement of Full Load kW Capacity

This is quite a simple calculation based method. Just pick an ampere meter (or clamp-on ammeter) and measure the full load current in amperes of each leg (electrical service entering to the main panel) during the peak usage. Just add all the three values to get the total amperage for the service facility.  

If the wiring system is three phase, just divide the total measured amperes on 3. Otherwise, divide the measured amperes by 2 for single phase. Now, multiply the resulting amperes by the supply voltage (e.g. 120V, 230V or 240V etc.). This way, you will get the wattage rating required by the facility. Now, divide by 1000 to get the size of the generator in kW rating. Finally, add 20% additional wattage capacity to the calculated value. This is the estimated and needed size of the generator required for the desired facility. 

Estimate the required generator capacity by taking full-load current measurements during peak usage at the service panel. Use a clamp-on ammeter on each leg of the electrical service and add the measurements together. This provides the total amps used by the facility.

Example:

What size commercial generator do I need in the following scenario.

Service Panel, Single Phase, 240V

  • Amp in Leg 1 = 175A
  • Amp in Leg 2 = 165A

Solution:

Just add the two values and divide by 2.

175A + 165A = 340A

340A ÷ 2 = 170A.

As the service voltage is single phase i.e. 240V (US-NEC), just multiply the average value of measured current.

170A x 240V = 40,800 W

Just divide by 1000 to get the kW rating

40,800 W ÷ 1000 = 40.8 kW.

Now add the 20% additional capacity for future load.

40.8 kW + 20% = 8.16 kW.

Now add the calculated wattage rating plus future expansion load of 8.16 kW.

40.8 kW + 8.16 kW = 48.96 kW

The next available right size commercial generator is 50 kW. Follow the NEC articles (700, 701, 702 and 708) while calculating the full load kW for sizing a commercial generator.

Full Load Capacity From Utility 

This is the easiest way to find the size of commercial generators especially for businesses operating 24 hours a day. To do so, just evaluate and analyze the utility bill from energy providers. You may find the peak demand factor and power usage over monthly and yearly bases. Just pick the highest peak demand value in kW and add 20% as additional capacity for future load. This is the estimated value of kW rating of the required generator.

Full Load kW Capacity of Extensive Motor

In this method, select the largest size of motor among others which are frequently ON and OFF for different applications. You may multiply the amperage of this large motor to its rated voltage to determine the wattage rating.

Now do the same e.g. find the wattage rating of small motors and non-motors loads and add them all. Finally, add a 20% future expansion capacity to the calculated value as shown in the first example. Divide the calculated value by 1000 to get the kW rating of the generator. This is how they size a commercial generator using the full load capacity of an extensive motor.

Measurement of Square Foot

This method is commonly used by retail applications for sizing a commercial generator.

In this method, they add 10 watts per square foot for retail applications while 5 watts per square foot for other general and commercial applications. For example, to determine the size of the generator:

  • Retail Applications = 75 kW + 10 W per ft2
  • Other general applications = 75 kW + 5 W per ft2

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