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Generator Selection Guide

Sep. 07, 2021

Buying a generator is usually a major investment for a home or business owner. It is also something that most consumers don't do very often, so there is a lot of confusion about which fuel type to use (diesel, natural gas, propane), which brand is best and, most particularly, what size generator is appropriate.


To answer this last question, your first and best option is always to consult a certified electrician to assess your specific needs. However, if you choose to do it yourself, you can still buy or rent a generator wisely by following some basic guidelines. We have listed these in this guide to help you understand how to determine the size of generator you need.

 50kw Diesel Generator Set

 50kw Diesel Generator Set

Why choosing the right size generator is critical?

Saving on the size of your generator can be tempting, especially as doing so could save you thousands of dollars. However, choosing a generator that is too small for your application can lead to disaster. Not only will you damage the appliances and equipment that are powered from the generator, you will also damage the generator itself.


How to size a generator?

The basic generator sizing formula is as follows.


Form a list of all items that will be powered. Determine the starting wattage (the amount of energy required to turn it on) and the operating wattage (the amount of energy required to run it) for each unit. These figures are usually engraved somewhere on the device itself and recorded in the user manual. Calculate your total power requirement by adding up these kW or KVA figures.

64kw Diesel Generator Set

 64kw Diesel Generator Set

For equipment measured in amps, you can convert amps to watts using the following formula.


For resistive loads (the most common type): Wattage = Amps x Volts For reactive loads: Wattage = Amps x Volts x Load Factor


The load factor is the ratio of electrical energy usage in kilowatt hours to peak demand in kilowatts. You can calculate it by looking up the data in your utility bill and using it in the following formula.


Total KWh for the previous month / (your peak demand for the period x 30 days x 24 hours)


Once you know the estimated wattage required, the generator size is easier to choose. Whatever figure you come up with, choose a generator with a capacity 10-20% larger than your requirements. This will give you some leeway if you upgrade your equipment and end up needing more power. It can also help you manage "derating" or under-performance of your generator due to adverse operating conditions such as extreme temperatures or high altitude, compared to the manufacturer's claimed performance.

Generator selection and fuel consumption

In the same way that a 3500 class truck uses more fuel than a 1500 class truck, a larger generator consumes fuel faster than a smaller unit. It is important to remember this when "calculating the cost of a new or used generator". Not only will you need to refuel larger generators more frequently, but you may also need to store additional fuel on site, resulting in the need for additional storage tanks.  


For this reason, when it comes to residential generators, Consumer Reports recommends buying the smallest portable or home standby generator that will meet your needs to limit the amount of fuel you have to store to run it.

Let us help you find the best generator. Welcome to contact us today or visit our website to learn more about generator set.

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