Determining generator size for home and commercial use

A generator helps when there is a power outage, it serves as an emergency power supply backup to homes and commercial places.

This is how to determine the size of generator for your home and commercial needs

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.

We have to take the demand factor into account as all the appliances won’t run at the same time 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. And the allowable demand factor for a 7.2 kW electric heater is 5.76 kW. The electric range is excluded as it runs only for a short time period.

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

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.

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.

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

20% Future Expansion

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 total required size of the generator would be:

20.51kW + 4.1 kW

The required size of the generator: is 24.61 kW.

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

For more information or assistance during normal business hours, call or e-mail TPS at the phone number above. You can also complete and submit our Service Project Inquiry form.