Information & Advice

How To Calculate Your Generator Size / Wattage Needs

The very first question that people usually ask when considering purchasing a generator is, “What size generator do I need?” In this article we’re going to answer that question by explaining the power terminology for generators, how to calculate the wattage of an appliance, and from those two things, how to calculate exactly what sized generator will best fit your needs.

Starting and Running Watts

Make sure you know both the starting and running watts of the generators you evaluate.

Most generators will state their power output in watts, but to confuse matters they will usually have two numbers – one that refers to starting watts, and another for running watts. What does this mean?

When you first turn on an appliance, there is usually a surge of electricity that is drawn which is needed to get the appliance to start. Once it has started, it will usually then run on less electricity than was needed to start it. One mistake that some people making when sizing their generator, is ignoring the start wattage and looking only at the running watts of each appliance.

If your appliances have a high start (or surge) watts, and your generator cannot accommodate that amount, you will not be able to start it.

Power Load

If you have read our buying guide, you’ll know that we recommend running your generator at only 50% load for the majority of the time. What this means, is that if you have a 3000w generator, you are only drawing 1500 running watts from the appliances that are plugged in.

This is better for the health of the generator, it will increase its run time, and it also means that the surge watts are less of a problem because there is usually power to spare.

Calculating Your Power Needs

So now that you understand the power output of a generator, you need to ask yourself what you are intending to plug into it, and then figure out how much electricity each of those things actually use. So where do you find this information?

Many items will list the running watts

Most electrical items will have some kind of information label with some essential information such as the voltage requirements and the wattage. However, most tend to only list the running watts and are not specific about starting watts. But if you follow our rule of thumb above, to only load your generator to 50% of its capacity, any surge when starting up appliances will hopefully be accommodated.

For large appliances such as fridges, freezers etc, you may well have to pull it away from the wall and look around the back of the unit to find the information.

Watts, Amps & Volts

Some appliances will not list watts but volts and amps instead. This is a very simple conversion:

Watts = Amps * Volts

Typically in the United States most appliances are running at 120 volts. In many other countries such as the UK it is usually 240 volts.

You may also find that some appliances list a range of volts rather than a fixed number. It may say 220v – 240v for example. Here you’ll use the large number as it is best of over-estimate. However, sometimes you’ll have a massive range such as 100v – 240v. In this case it means that the appliance will work in any country regardless of the voltage. In this case, use the typical voltage for your country. So if you are in the US, take that as 120 volts.

If the label shows a large range in voltage, use the one most common in your country.

Variable Wattages

Many appliances will vary the wattage based on how they are used.

One thing to watch out for is those kinds of appliances whose power requirements depend on the kind of usage. For example, if you are using an air conditioner or a heater, they may draw a lot more electricity if you turn them right up and they are having to do a lot of work. Other items like a mobile phone charger should be fairly consistent.

It can work the other way too. Electrical labels will always list the maximum wattage of an appliance but they may not run at that wattage all the time. For example a fridge is only drawing maximum power when the compressor is on, which is not all the time.

Estimations vs Accurate Calculations

We will be putting together some articles with the typical wattages of many appliances that people typically want a generator for. If you are not entirely sure what you want to run, or what your budget is and you just want to get a ballpark then this is a good place to start.

However if you want to be totally accurate then you can purchase an energy usage monitor. This is a small device that you use to read the exact electricity drawn from a specific appliance. They are fairly cheap (around $20 – $30) so if you are planning on buying a high end generator, it would be worth this small investment up front to make sure you don’t end up buying a generator way bigger than your needs and waste a lot of money.

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