Do you want to buy a home generator, but don’t know what size generator you need to run your home? Generator sizing can be confusing if you don’t know where to start. And buying the wrong size, either too big or too small, can be an expensive mistake to make. But don’t worry! In this post, we’ll show you how to easily calculate the number of watts you need to keep your home running during a grid outage.

Generating sizing guide summary

Here’s a summary guide for what size generator you’ll need at home:

• If you just want to run your most critical household equipment like lighting circuits and a fridge, choose a generator about 2,000 to 3,000 watts (2-3 kW).
• For emergency situations like a power outage of 24 hours plus, to run a few critical home appliances choose a generator that has a wattage around 5,000 watts (5 kW).
• If you need a home generator for extended outages, choose one around 7,500 watts (7.5 kW).
• For heavy-duty applications like running multiple energy hungry appliances at once (toaster, kettle, fridge, freezer), look for at least 10,000 watts (10kW).

If you want to know how to work this out for your own power needs, keep reading! We’ll show you in 6 easy steps.

Why work it out for yourself?

Because if you buy bigger than you need, you’ll be paying too much for already expensive energy.

If you buy something too small, you’ll be tripping your power supply and cursing your purchase.

So let’s get started!

Generator sizing in 6 easy steps

To get exactly the right sized generator for your household needs, follow these 6 easy steps:

Step 1 – Make a list of each appliance you want to run at the same time
Step 2 – Categorise each appliance as ‘critical’ or ‘other’
Step 3 – for each ‘critical’ appliance write down the ‘starting watts’ and ‘running watts’.
Step 4 – convert all other units of measure into watts (see how below).
Step 5 – Calculate your peak power requirements

Step 6 – Convert watts to kVA, so you can choose a generator size (peak output and rated output) for your power requirements.

We explain ‘starting watts’, ‘running watts, ‘peak output’ and ‘rated output’ as we take you through each step below. We’ve also created a free, downloadable Generator Size Calculator that does all the calculations for you.

Before we begin, let’s talk about why emergency backup power is trending in Australia. We’ll also share some rule of thumb generator sizes for 4 different scenarios.

Why emergency back up power?

In mid 2021, severe storms in the Dandenong ranges left hundreds of families without power for an extended period. Some families lost grid power for weeks.

While you many think that could never be you, the science shows that extreme weather events like this are escalating in Australia.

Backup power, provided by a generator, is one option to keep the fridge running, the lights on, and the internet ticking over. In regional and rural Australia, outages are doubly disruptive. No power equals no pumps to run water.

For Australians, energy is an essential service because when the power goes out, life is thrown into turmoil. A home generator can power your whole house if need be, and keep things on track at home when disaster hits.

Generator sizing guide

Before we show you how to properly size your home generator based on your exact energy requirements, here is a rule of thumb generator sizing guide. This is a rough guide to help you understand how much power you might need for 4 different life scenarios:

Work-from-home backup power

If you work from home and want uninterrupted supply or backup power to your computer and the internet, a small portable generator or camping generator might be a low budget option.

• A Macbook Pro laptop = 100 watts
• An inkjet printer = 50 watts
• Iphone = 20 watts

A small camping generator might be enough to provide standby power and run these electronics.

Critical household equipment

If you just want to run your most critical household equipment like lighting circuits and a fridge, choose a generator about 2,000 to 3,000 watts (2-3 kW). Smaller generators like a portable generator or inverter generator may suffice.

Emergency situations

For emergency situations, to run multiple home appliances choose a generator that has a wattage around 5,000 watts (5 kW).

Extended blackouts

If you need a home generator for extended outages, choose one around 7,500 watts (7.5 kW). You won’t power an air conditioner or multiple motors with this size, but you will power most other devices connected to your home.

Heavy duty energy needs

The kilowatts you need can stack up easily for multiple appliances. For heavy-duty applications like running multiple energy hungry appliances at once (toaster, kettle, fridge, freezer), look for at least 10,000 watts (10kW) but you may need closer to 20,000 watts (20kW) to run air conditioners. Portable generators don’t usually come this large! Larger generators, like standby generators, are best.

Why home generator sizing matters to your hip pocket

Generator sizing can be completed in just 6 steps and is the single most important thing you can do to get the right generator for your home. The right size generator to meet your power requirement is important for two main reasons:

1. generators are expensive – buy something too big you’re overpaying for your power needs – more than you pay for your utility bill!
2. energy demand and supply must be in balance – a too small generator will trip off and could cause asset damage or other problems.

What you’ll need to start your generator sizing calculations

The first step in working out your home generator sizing is to calculate your power requirement. Here’s what you’ll need:

• A list of the electronics you want to power during an outage
• The wattage rating of each appliance – either on the equipment itself, or in the manual
• An idea of which items on your list you’ll run at the same time.

Before you begin – starting and running wattage

It’s important to understand ‘starting wattage’ and ‘running wattage’ before you begin.

One ‘watt’ is a unit of power that is equal to one joule of energy per second. It’s a way to measure how much power your appliances need so we need to know the watts for each appliance.

When you’re looking up the wattage of equipment, you might see two numbers: starting wattage and running wattage.

The starting watts are the amount of power that’s required to start the appliance, while the running watts are the amount of electricity that’s required to keep it going (running load).

Some items, like an air conditioner, use way more power to start up than to run. For example, a coffee maker might need 600 watts to start and 100 watts to run.

Write down both figures wherever you have them.

How to determine starting and running wattage

To determine the starting watts, look at the appliance’s nameplate or owner’s manual. The starting wattage will be listed under “startup watts” or “peak watts.” If it’s not there, try searching online.

Look in the same places to find the running wattage. It should be listed under “run watts.”

Calculating your power requirements

Step 1 – Make a list of each appliance you want powered by your generator.

Step 2 – Categorise each appliance as ‘critical’ (must have) or ‘other’ (want).

Lights, fridge and internet might be critical. Toaster might be ‘other’.

Home generators increase in price as they do size. Their fuel consumption and cost does too. Step 2 will help determine the cost of meeting your basic electricity needs, versus the cost of running discretionary items.

Step 3 – write down the running and starting watts of each listed appliance.

If the unit measure you find here is not in watts, write down the unit measure provided. We will convert these numbers to watts next.

Converting into watts

If the information you have is not provided in watts, then it will likely be in Amps, HP (horse power), or kW (kilowatts). Some electric tools, like an air compressor or motors may be measured in kVa.

Step 4 – convert other units of measure into watts.

We need to normalise all measurements of power on your list, so that we can add everything together in the next step to get to your ‘total power requirements’.

Here are the formulas to convert other measurements into watts:

Step 5 – calculate your peak power requirements

Select the highest starting wattage for all devices you will run at the same time in your ‘critical’ category.

Add to this number to the ‘total running wattage’ of all ‘critical list’ appliances.

Here is the formula:

Highest starting watts + total running watts (for appliances that don’t have a given starting watts) = your peak demand for power.

So why should you add the highest starting watts number only?

To save you money.

Using your highest starting wattage number only just assumes you’ll only need to start up one appliance at a time. Or in other words, you won’t start them up together. You can avoid overloading your generator and get by with a smaller generator (which will save you money) if you start up your appliances in a staggered manner.

Generator sizing – choosing what’s right for you

What is generator ‘peak power’ and ‘rated output’?

When you’re looking at generators online, you may come across two different unit measures in the power information – ‘peak power’ and ‘rated output’.

Both are important to generator sizing.

‘Rated output’ is the amount of electricity a generator can produce over time, typically measured in either kilovolt amperes (kVa) or watts (W).

Generator ‘peak power’ (also called maximum power), on the other hand, is the maximum amount of electricity a generator can produce in a short burst.

Peak power is used to start equipment that requires a large amount of electricity to start. But it is only required for a short period of time, so it’s different from a generator’s overall output capacity.

Instead, ‘rated output’ is a more accurate indication of how much electricity a generator can provide over an extended period of time.

Step 6 – choose the right sized generator based on your peak power requirements and running load.

Here are 2 rules of thumb to size the generator for your power needs:

1. Your peak power requirements (highest starting watts + total running watts) must be less than your generator ‘peak power’.
2. Your ‘total running watts’ must be less than the generator ‘rated output’.

When you’re shopping for a generator, you might find the ‘peak power’ and ‘rated output’ listed as ‘kVA’ instead of watts.

But don’t worry, our generator sizing calculator will do all the conversions for you. And it’s free!

Add some extra watts to your totals to give you some flexibility with how you operate the generator.

Generator sizing is just that simple!

So now you know what peak power and rated output you need to run essential appliances and other equipment.

Time to go shopping! And remember, whichever you chose make sure you have a certified electrician do the installation!

Conclusion

To work out your generator sizing calculations, follow this simple process step by step:

• Step 1 – Make a list of each appliance you want to run
• Step 2 – Categorise each appliance as ‘critical’ or ‘other’
• Step 3 – for each appliance write down the ‘starting watts’ and ‘running watts’.
• Step 4 – convert all other units of measure into watts (see how below).
• Step 5 – calculate your peak power requirements
• Step 6 – Convert watts to kVA, so you can choose a generator size for your power requirements using ‘peak load’ and ‘running load’.

That’s it! 6 easy steps that demystify generators and help you calculate exactly what you need!