How Much Power Do You Need in a Generator?

The amount of electricity that a device needs to operate is measured in watts or kilowatts (1,000 watts). Most devices display their power needs on the plate or label where the model number and serial number are located. It might be stated as a single number, like 2900; or it may have two numbers, like 3500/2900. The two numbers means that the device needs more power to start up, in this case 3,500; but less power to continuously operate, the 2,900.

Generator specification sheets show the power of the generator. This is the maximum capacity of the generator in kilowatts. That number needs to be at least as high as the total needs of all of the devices you need to power at one time.

Residential Generators

In residential applications, it is important to follow the Canadian Electrical Code (CEC) in order to correctly size a generator for the desired electrical loads. This can vary from a whole home backup system, to a smaller system that only powers a few essential devices. While sizing for a smaller isolated system is quite simple, a whole home system will require the load calculation guidelines set forth by local governing safety authorities. The guideline starts with a base load demand calculated by the square footage of the dwelling followed by calculating the electrical needs of items such as stoves, air conditioners, and dryers, etc. From there, any other type of heating or motor loads must be added and calculated at the full rating of the device.

Startup vs Operational

The start up requirements vs the operational requirements are determined by how the generator will be used. For example, if you are using the generator as the prime power source for an industrial operation at a remote site, your total startup needs might be 400 kilowatts (kW), and your operational needs 310kW. If all of the equipment has to be started at the same time, then your generator needs to be at least 400kW. But, if you can start up each piece of equipment in a series, and the most power you need for any one piece of equipment is 200kW, then your total needs will be somewhere between 310kW and 400kW. In the example below, Equipment 1 (E1) needs 200kW to start up, so it’s started first. E1’s needs then drop to 150kW. To keep E1 going while we start E2, we need a total of 270kW, then it drops to 250kW. To keep E1 and E2 going while we start E3, we need 330kW. So, our total needs are 330kW and not 400kW.

  Start Operate Start Next Maintain
E 1 200 150   150
E 2 120 100 270 250
E 3 80 60 330 310
Total 400 310    


Let’s look at the opposite end of the scale, a portable generator for your tools. If you’re only using one tool, then you only need enough power to start the tool, as the power maintenance requirements are always lower. 

For a backup generator for your home, business, or industrial facility, you need to add up the kilowatts needed for each item you need to power at the same time. So, for example, at a restaurant, if a power outage occurs, you’ll need backup power for the refrigerators, freezers, the heating, cooling, and exhaust systems, the payment system, and at least some emergency lighting. You’ll also want all of those items to startup as soon as possible to avoid interruptions. If you add up all of those energy needs, you’ll end up with the number of kilowatts you’ll need at one time. Let’s say that number is 85kW. Then you’d choose a generator that has a capacity of at least 85kW.

Now, let’s talk about voltage. Voltage is the amount of force with which the electrical energy leaves the generator to travel through the electrical system. The amount of energy decreases as it travels. The more force it starts with, the less energy is lost along the way. In practical terms, your voltage needs will differ depending on the distance and overall amount of space you need to cover. A home, for example, can do with a lower voltage than a large factory. The higher voltage for the factory will ensure that the item farthest away from the generator will get the power it needs, and the overall distribution will be more efficient. 

If power requirements are expressed in amps instead of kilowatts, you can use this formula to convert between them: Watts = Volts x Amps.

You don’t need to worry about doing all of these calculations yourself. We’re here to help, and will be happy to review your needs and make a recommendation. Just give us a call at 1-604-746-0606 or contact us today.