Picture a blackout at home. The fridge is warming up, the Wi-Fi is down, the water pump has stopped, and you’re wondering whether the generator in the shed can actually handle the load.
That’s when choosing the right generator size becomes important.
So, what size generator do you need to power your house? It comes down to one key question: What do you need to keep running?
A home that only needs to power the fridge, freezer, lights and a few essentials will have very different requirements from one running air conditioning, water pumps and other high-demand appliances.
The right generator size depends on your household’s actual power needs, not simply the number of bedrooms or the size of your home.
You’ll need to consider how much power your appliances use, which ones run at the same time, and whether they require extra power when starting.
This guide walks you through generator sizing step by step, so you can estimate the capacity your home actually needs.
Why Does Generator Sizing Matter?
Choosing the right generator size means having enough power for your household without paying for more capacity than you need.
A generator that’s too small may struggle when several appliances are running or when equipment such as a fridge, air conditioner or water pump starts up.
This can overload the generator, cause it to trip or leave you without enough power for essential appliances.
Go too large, and you may be paying for more generator capacity than your household actually needs.
The goal isn’t simply to buy the biggest generator you can. It’s to find one that comfortably handles your expected load, including those short bursts of extra power some appliances need when they start.
What Affects the Generator Size You Need?
There are three main things to consider: what you need to power, how much electricity those appliances use, and what may be running at the same time.
Starting loads from appliances such as fridges can affect the generator capacity you need.
What Appliances Do You Actually Need to Run?
Start with your must-haves. During a blackout, that might include your:
- Fridge and freezer
- Lights
- Wi-Fi
- Water or pressure pump
- Television
- Selected power points
Then consider the nice-to-haves, such as air conditioning, a microwave, a washing machine or a dishwasher.
Separating essential and optional loads can help you avoid sizing a generator to run every electrical appliance in the house at once.
Looking at a different type of backup power? Our guide to choosing the right solar generator explains what to look for when comparing solar generator options.
How Much Power Do Your Appliances Use?
Electrical appliances have a power rating, usually shown in watts (W) or kilowatts (kW).
As a simple conversion:
1kW =1,000W
The higher the combined wattage of the appliances you want to run, the more generator capacity you'll need.
For the most accurate figure, check the appliance’s data plate, owner’s manual or manufacturer specifications rather than relying only on a generic wattage estimate.
What’s the Difference Between Running Watts and Starting Watts?
This is one of the most important parts of generator sizing.
Running watts are the power an appliance uses during normal operation.
Starting watts are the higher amount of power an appliance may briefly require when switching on.
Equipment with motors or compressors, including fridges, freezers, air conditioners and water pumps, can have a much higher start-up demand than their normal running load.
That means a generator might comfortably handle your normal running load but struggle when a pump or compressor starts.
What Will Be Running at the Same Time?
You don’t necessarily need enough generator capacity to power every appliance in your home simultaneously. Think about how you’ll actually use your generator.
For example, you may decide not to run another high-demand appliance while the air conditioner is running. Managing larger loads this way can reduce the generator capacity you need.
How Do You Calculate What Size Generator You Need?
A simple way to estimate your required generator size is to calculate the combined running load of the appliances you’ll use at the same time and then account for any additional start-up demand.
Four steps to estimate the generator size you need.
Step 1: List What You Need to Power
Write down the appliances and equipment you want to use during a blackout or while running on generator power.
Start with essentials such as your fridge, freezer, lights and water pump. Then add other appliances you want to use, such as a microwave, television or air conditioner.
Step 2: Find the Running and Starting Wattage
Check the data plate, owner's manual or manufacturer's specifications for each appliance. Record the running watts and, where applicable, the starting watts.
Pay particular attention to appliances with motors or compressors because their start-up demand can be considerably higher than their normal running load.
Step 3: Calculate Your Maximum Power Requirement
Add together the running wattage of the appliances likely to operate at the same time.
Then account for the additional starting power of the appliance with the highest start-up demand.
A useful basic formula is:
Total running watts + highest additional starting watts = estimated maximum power requirement
For example, if your appliances use 3,000W while running and the largest start-up demand requires another 1,500W above its normal running load:
3,000W + 1,500W = 4,500W (4.5kW)
Your generator would therefore need to handle at least 4,500W (4.5kW) at that point.
If several high-starting-load appliances could switch on together, this also needs to be considered.
Step 4: Allow Some Extra Capacity
Avoid choosing a generator that only meets your calculated requirement. Some extra capacity gives you room for changes in load and helps prevent the generator from operating at its calculated limit.
Also check the generator's rated or continuous output, not just its maximum or peak output.
Maximum output is generally available for short periods, while rated output indicates how much power the generator is designed to supply continuously.
The appropriate amount of headroom depends on the generator and the equipment you're powering, so check the manufacturer's specifications before making your final choice.
What Does a Generator Size Calculation Look Like in Practice?
Say you want to keep a fridge, freezer, lights, Wi-Fi and a water pump running during a blackout.
For this example, we'll use the following hypothetical appliance ratings:
Starting loads can increase the generator capacity you need.
The combined running load is 1,520W.
The water pump has the largest starting requirement. Because its 1,000W running load is already included in the total, you only need to account for the additional 2,000W needed during start-up:
1,520 W + 2,000 W = 3,520W
So the generator needs to handle at least 3.52kW when the pump starts.
If, for example, you allowed around 20% extra capacity:
3.52 kW × 1.2 = 4.22kW
You could then compare generators by their rated and maximum output to narrow down suitable options.
💡Note: These figures are an example only. Appliance power consumption and start-up requirements vary considerably, particularly for fridges, air conditioners and pumps.
Check the specifications of your actual equipment before choosing a generator.
How Much Power Do Common Household Appliances Use?
The figures below can be useful for an initial estimate, but household appliances vary widely in power use.
Check each appliance’s data plate or manufacturer specifications when sizing your generator.
For a more accurate calculation, check the data plate, owner's manual or manufacturer's specifications for each appliance you plan to run.
If you're sizing a generator to power most or all of your home, or you have large loads such as air conditioning or pumps, consider having a qualified professional assess your electrical load.
This can help ensure the generator is correctly sized for your actual requirements.
What Do kW and kVA Mean When Choosing a Generator?
When comparing generators, you'll often see output listed in kilowatts (kW) or kilovolt-amperes (kVA).
Understanding the difference helps you compare your calculated power needs with the generator's actual capacity.
Kilowatts (kW) measure real power—the power available to do useful work. One kilowatt equals 1,000 watts, so a household load of 4,000W equals 4kW.
Kilovolt-amperes (kVA) measure apparent power and take power factor into account. Because of this, a 5kVA generator doesn't necessarily provide 5kW of usable power.
You can estimate kW output using:
kW = kVA × power factor
For example, if a 5kVA generator has a power factor of 0.8:
5kVA × 0.8 = 4kW
So a 5kVA rating doesn’t automatically mean you have 5kW of usable power.
Power factor varies between generators and loads. Check the manufacturer's specifications for the generator's rated kW output and power factor rather than assuming a particular conversion.
Frequently Asked Questions
The right generator size depends on the household's running and starting power needs.
Is a 5kVA Generator Enough to Run a House?
A 5kVA generator may be enough for household essentials such as a fridge, freezer, lights and Wi-Fi, depending on their combined load and the generator’s rated kW output.
You also need to check whether it can handle the starting requirements of appliances such as pumps, fridges and air conditioners.
What Size Generator Do I Need for a Three-Bedroom House?
Don't size a generator by bedroom count. A three-bedroom home with basic electrical loads may need far less power than one with air conditioning, electric hot water and pumps.
Instead, calculate the appliances and systems you actually need to run.
What Size Generator Do I Need to Run an Air Conditioner?
Check the air conditioner's running and starting power requirements and add them to any other loads that will operate at the same time.
Larger split systems and ducted air conditioning will generally require more generator capacity than smaller units. The manufacturer’s specifications are the best place to start.
What Is the Difference Between Rated and Maximum Generator Output?
Rated or continuous output is the power a generator is designed to supply continuously. Maximum or peak output is generally available for short periods.
When sizing your generator, pay close attention to rated output while also checking that the generator can cope with your appliances’ starting demands.
What Size Generator Do I Need to Power My Whole House?
There is no standard generator size for every home. Calculate the maximum load of the appliances and systems you expect to run simultaneously, including relevant start-up demands.
For whole-home backup or more complex installations, it’s worth having a qualified professional assess your electrical load and installation requirements.
So, What Size Generator Should You Choose?
The right backup setup depends on which household loads you need to keep running.
The right generator isn’t necessarily the biggest one. It’s one that can comfortably handle the appliances you actually need, including their start-up demands, with appropriate capacity to spare.
Start by listing your essential loads, check their actual wattages and work through the calculation above.
Then compare your requirement against the generator's rated output, startup capability and manufacturer information.
If the numbers still feel a bit murky, no worries.
Eco Off Grid can help you compare generator options for household backup, rural properties and off-grid setups without pushing you towards more capacity than you need.
