How to Size a Generator System

Critical Power Supplies has pleasure in providing you with the following sizing guide for sizing your generator solution.

Estimating the generator size so that it can manage all your power generation needs is one of the most critical stages of a critical power project. Whether you are interested in prime or standby power, if your new generator can’t meet your specific requirements today then it should not be purchased without first knowing that it can adequately meet your requirements. However, estimating exactly the size of generator required can be difficult and involves a number of factors and considerations.

Making a choice amongst single phase, three phase, kW, KVA, welder, standby or motor starting generators can be daunting. However, to prevent confusion, Critical Power Supplies has developed this guide to help you get a better understanding of how the sizing process works and some key things to keep in mind. This is not a substitute for a site survey but can assist you in the first few “in-house” stages of your Critical Power Project.

Generator Rating Definitions

Standby Rating based on Applicable for supplying emergency power for the duration of normal power interruption. No sustained overload capability is available for this rating. (Equivalent to Fuel Stop Power in accordance with ISO3046, AS2789, DIN6271 and BS5514). Nominally rated.

Typical application – emergency power plant in hospitals, offices, factories etc. Not connected to grid.

Prime (Unlimited Running Time) Rating based on: Applicable for supplying power in lieu of commercially purchased power. Prime power is the maximum power available at a variable load for an unlimited number of hours. A 10% overload capability is available for limited time. (Equivalent to Prime Power in accordance with ISO8528 and Overload Power in accordance with ISO3046, AS2789, DIN6271, and BS5514). This rating is not applicable to all generator set models.

Typical application – where the generator is the sole source of power for say a remote mining or construction site, fairground, festival etc.

Base Load (Continuous) Rating based on: Applicable for supplying power continuously to a constant load up to the full output rating for unlimited hours. No sustained overload capability is available for this rating. Consult authorized distributor for rating. (Equivalent to Continuous Power in accordance with ISO8528, ISO3046, AS2789, DIN6271, and BS5514). This rating is not applicable to all generator set models

Typical application – a generator running a continuous unvarying load, or paralleled with the mains and continuously feeding power at the maximum permissible level 8760 hours per year. This also applies to sets used for peak shaving /grid support even though this may only occur for say 200 hour per year.

As an example if in a particular set the Standby Rating were 1000 kW, then a Prime Power rating might be 850 kW, and the Continuous Rating 800 kW. However these ratings vary according to manufacturer and should be taken from the manufacturer’s data sheet.

Often a set might be given all three ratings stamped on the data plate, but sometimes it may have only a standby rating, or only a prime rating.

Generator Size Variations

With the latest advancements in the field of electrical engineering, generators are now available in a wide range of sizes. Generators with power supply capacities of 2kW to 50kW are readily available in the personal and home use markets, while industrial generators are anywhere from 15kW to over 3 Megawatts. Handy and portable gensets are available for homes, and small offices, but larger businesses, data centers, buildings, plants, and industrial applications need to use the much larger sized industrial generators to meet their higher power requirements.

Generator Sizing – How Much Power?

Many organisations and people believe smaller generators can be used for standby electric power because they are not running all the time. This is not only a myth but can actually be very detrimental. Unfortunately, generator under sizing is one of the most common mistakes committed by buyers. Not only does it involve the risks of damaging your new asset (the generator), but it can also damage other assets connected to it, create hazardous situations, and even limit overall productivity of the unit and/or the business relying on it. If nothing else, the key thing to remember here is that more is always better than less. As one aspect of over sizing a generator will simply mean the fuel tank will last longer then expected.

How to Determine the Right Size Engine or Generator

While there is no substitute for having a certified electrician perform an inspection and calculate everything for you, the guidelines below do offer some great starting points and should at least get you started in the right direction:

Know Your Requirements
Simply buying the best or cheapest generator available “today” new or second hand without any other consideration is clearly not the best approach. It is always better to delve deep into your power generation requirements before making a choice. You can do this in the following ways:

– Make a list of the items that need to be powered by the generator
– Make a note of the starting and running wattage of the respective items
– Calculate the total power requirements in KVA or KW

How to Find the Starting and Running Wattage
Getting the right starting and running wattage of the devices you intend to power is crucial for calculating the accurate power requirements. Normally, you will find these in the identification plate or the owner’s manual in the buyer’s kit of each respective device, tool, appliance, or other electrical equipment. If you have any questions or need a site survey why not contact us

Ampere – Watt Conversion
You may often find power requirements of tools stated in amperes. In order to convert the power requirement of a tool from ampere to watts, follow these calculations, or you can also use our handy conversion tool on our power calculator web page.

For resistive load: Wattage = amperes x volts For reactive load: Wattage = (amperes x volts) x load factor

Different ways of Calculating
Depending upon the type and number of devices, and the way the generator is scheduled to be used, there are a few different ways of calculating power requirements:

– Single motor running
– Multiple motors running simultaneously
– No electric motors.

Correct Generator Installation.

To ensure correct functioning, reliability and low maintenance costs generators must be installed correctly. To this end manufacturers provide detailed installation guidelines covering such topics as:

– Sizing and selection
– Electrical factors
– Cooling
– Ventilation
– Fuel storage
– Noise
– Exhaust
– Starting systems

Advantages of choosing the right size generator and installing it / using it inline with manufacturers guides.

– No unexpected system failures
– No shutdowns due to capacity overload
– Increased field life of the generator
– Guaranteed performance
– Smoother hassle-free maintenance
– Increased system life span
– Assured personal safety
– Significantly reduced chance of asset damage