When there is a necessity to ensure a continuous energy supply because of a mains failure at our home or at our work place, or to reduce our spending on energy during peaks, then follow our handy guide to choosing the right power generator.

This is not a substitute for a certified electrician, which we always suggest talking to before buying, but it should provide you with enough information to get a solid understanding of some of the key things that are involved.

 What do you need in order to keep working in case of a sudden or programmed grid failure?

Firstly we need to know the starting power and normal running power of all of the items that need to be powered by the generator. All of your items should tell you the nominal running power, however they might not tell you the start-up power required. This depends on the type of mains connection. If it is star or delta, then the start-up power required should be calculated at 3 times the normal running power. If it’s a DOL rating then the start-up power will be equal to 6 times the normal running power of the item. For example in the case of Elevators/Lifts, they may contain a a soft starter of variable frequency drive (VFD) and the starting current can be estimated at 2.5 times the rated current.

In summary as long as we now the kilowatts (kW) of the items we need to keep running in the event of a mains failure, we can then calculate the overall start up power; the final figure being dependent on the type of mains system in use – star, delta or DOL.



Which areas of your installation must have basic lighting?

You need to calculate the total amount of light sources, for that you need to add the consumption of each light expressed in watts (W) then dividing the total by 1.000 to obtain the kilowatts of power consumption from your lighting needs.

By adding the figures from the previous question to the figures obtained for lighting, you will know the approximate power of the generator you need.

Who is in charge of calculating the power needed for the genset?

The decision of the size and power of the genset and its electrical configuration are the responsibility of the electrical, mechanical or engineering department, depending on the kind of project. This department will define:

  • Power needed to cover present and future necessities.
  • The operational requirements to be incorporated in the equipment: manual or automatic start, redundancy with other units or with the mains, soundproofing, auxiliary fuel tanks, heating system etc
  • Legal regulations and local laws (electric, noise, pollution).
  • Selection of the right place to install the genset or gensets (outdoors or indoors, elevation above sea level, dusty environments etc)

Example: How to estimate the power of a generator?

To end this article, we will estimate the power of the genset needed for a 12 floor apartment building, with the following power necessities:

  • 1 elevator with a 15 kW engine: starting with a VFD, 380V.
  • 2 water pumps: 10 HP per unit, star-delta starting (S-T), 380V.
  • 1 Boiler pump: 5 HP, DOL starting, 380V
  • 1 pressurized fire ladder: 15 HP, DOL starting, 380V.
  • Corridors lighting: 5,5 kW (55  100W units), 220V.
  • Entrance hall lighting: 1,5 kW (15 100W units), 220V.
  • Garage door: 0,5 kW, DOL starting, 220V.

Next we design a load table and define the starting power for each item. For this example, we will assume a scenario with the biggest potential demand, one with the elevator and the fire ladder included, although in reality both cannot work at the same time.

Load table
Equipment Rated power (kW) Voltage (V) Equivalent power (kW)  380 V  Start Starting factor Starting power   (kW)
Elevator 15 380 15 VF 2,5 37,5
Water pump 7,5 380 7,5 E-T 3 22,5
Water pump 7,5 380 7,5 E-T 3 22,5
Boiler pump 3,7 380 3,7 DOL 6 22,2
Fire ladder 11,2 380 11,2 E-T 3 33,9
Corridor lighting 5,5 220 1,9 1 1,9
Entrance hall lighting 1,5 220 0,5 1 0,5
Garage door 0,5 220 0,5 DOL 6 3,0
Total with elevator TOTAL 36,6
Total with ladder TOTAL 32,8
Total starting power with elevator (kW) 110,1
Total starting power with ladder (kW) 106,2


We must remember that these figures are an estimate, the definitive power of the genset won’t be bigger than this, but it could be lower. For more exact calculations we must consider other factors as the type of load (lineal or non-lineal), usage factors, simultaneity of start ups, etc.

Worst case scenario is with the elevator working, 110.1 kW, this power is the maximum transient power that could be demanded eventually to the genset, which has a power factor of 0.8 (standard for AC alternators). Now we can calculate the apparent power: kVA=kW/0.8, so the apparent power in our example will be 110.1/0.8= 137.6 kVA, maximum transient power demanded from the genset, based on all the equipment start at the same time. However, the power needed by this equipment when running normally will be 36.6/0.8=45.8 kVA.




Simultaneous start or sequential start?

To define the capacity of the genset we need to consider other factors, like simultaneity and usage factors, applied by the electrical contractors. In our example, we can apply a 0.8 factor to the maximum power calculated, 137.6×0.8=110 kVA, which will be the stand-by power or emergency power that the genset needs to provide. However, if a sequenced start could be programmed, the most critical scenario will be when the elevator starts with all the other equipment running, with this power demand, (36.6 kW – 15 kW) + 37.5 = 59.1 kW. In this scenario, the apparent power will be 59.1/0.8=73.9 kVA. You will notice the difference between both scenarios, 110 kVA vs 73.9 kVA, equivalent to 33% less with the sequential start.

With this information, you are now able to do a preliminary evaluation of your needs of back-up power. To summarize, to choose the right genset you must consider:

  1. Define which equipment/items will be connected to the genset.
  2. Select the power of the genset based on the equipment that will be connected.