What Size Generator Do I Need?

Choosing the right generator size is one of the most important decisions before hiring or buying power equipment.

Too small, and equipment can trip, stall, or shut down unexpectedly.
Too large, and you may pay more in fuel and hire costs than necessary.

The correct generator size depends on your total electrical load, how that load behaves at startup, and whether the generator will be used for backup or continuous power.

This guide explains how to think about generator sizing properly without guesswork and when to use a sizing calculator for an accurate recommendation.

What size generator you need depends on the total running load of all equipment plus the highest starting surge load. Small domestic backup may require only light capacity, while commercial and industrial sites require significantly higher kVA ratings. The safest way to determine the correct size is to calculate running and starting demand before selecting equipment.

What Does “Generator Size” Actually Mean?

When people ask, “What size generator do I need?”, they are usually thinking in watts.

But generators are commonly rated in kVA, not just watts.

That is where confusion often starts.

In simple domestic backup situations, required capacity may be relatively modest if only essential circuits are supported. In commercial or industrial environments, capacity increases quickly due to motor loads, three phase equipment, and simultaneous demand. The scale of application matters as much as the total wattage.

Running Load vs Starting Load

Every electrical device has two important numbers.

Running load is the power required to keep it operating.
Starting load, sometimes called surge load, is the higher power needed when it first switches on.

Motors, compressors, pumps, refrigeration units, and air conditioning systems can draw significantly more power at startup than during normal operation.

If a generator is sized only for running load and not surge load, it can cause:

• Voltage drops
  
• Protection trips
  
• Equipment stalling
  
• Unexpected shutdowns

This is why proper sizing must consider both running and starting load.

Why Generators Are Rated in kVA

kVA measures apparent power, which includes both usable power and reactive power.
Watts measure real usable power.

Most commercial and industrial generators are rated in kVA because it provides a more complete picture of electrical capacity, especially for equipment that includes motors or inductive loads.

Appliance labels usually display watts.
Generators usually display kVA.

That mismatch is one reason why simple guesswork often leads to incorrect sizing.

Generator size is not just about adding up appliance wattage.
It is about understanding how the load behaves in real conditions.

The safest way to determine the correct size is to add together the total running load of all equipment and include the highest starting demand before selecting equipment using a generator size calculator.

The Biggest Mistake People Make When Choosing a Generator

The most common mistake when choosing a generator is guessing.

People often estimate based on what “sounds about right” or choose a size based on availability, price, or what a neighbour used.

In small domestic situations, that guess might work.
In commercial or industrial environments, it usually does not.

When a generator is undersized, problems appear quickly.

• Voltage can drop under load.
  
• Protection systems can trip.
  
• Sensitive equipment can shut down.
  
• Motors may struggle to start.

When a generator is oversized, the problems are less obvious but still costly.

Fuel consumption increases.
Hire costs rise unnecessarily.
The unit may run inefficiently at low load.

Both scenarios lead to avoidable expense.

The issue is rarely the generator itself.
It is the assumption made before selecting it.

Proper generator sizing is not about choosing the biggest unit available.
It is about matching capacity to real demand, including startup behaviour and usage pattern.

This is why structured assessment matters more than rough estimates.

How To Think About Generator Size Without Doing the Maths

You do not need to be an electrical engineer to approach generator sizing properly.

Instead of trying to calculate everything manually, work through four simple questions.

1. What equipment needs power?

Start by listing every item that must run.

This includes obvious equipment such as lighting, refrigeration, and tools, but also less obvious loads such as pumps, control panels, battery chargers, and site offices.

If it needs power, it needs to be included.

2. Will everything run at the same time?

Total load is not just about what is connected.
It is about what operates simultaneously.

On some sites, equipment cycles on and off.
On others, everything runs together during peak periods.

Understanding simultaneous demand is critical to choosing the right generator size.

3. Are there motors or high start loads?

Motors behave differently from simple electrical loads.

Compressors, welders, air conditioning units, refrigeration systems, and pumps can draw significantly higher current when starting.

If these surge loads are not considered, the generator may struggle even if the running load appears acceptable.

4. Is this backup power or primary power?

Backup generators typically run intermittently and may only support essential circuits.

Power outages and grid disruptions remain an operational risk across Australia, particularly during extreme weather events.

Primary power generators often run continuously and support full site operations.

Continuous use requires stable capacity and careful planning to avoid stress on the equipment.

Thinking through these four questions will often highlight whether your sizing assumptions are realistic.

At this point, a structured sizing tool or professional assessment becomes far more reliable than estimation.

Generator Size by Application

Generator size is not universal.
The right capacity depends heavily on how and where it will be used.

Below are general application categories to help frame your thinking. These are ranges and use cases, not exact recommendations.

Small Homes and Light Use

For light domestic use or emergency backup of essentials, power demand is usually limited to:

• Refrigerator
  
• Basic lighting circuits
  
• Internet and communication devices
  
• Small kitchen appliances

In these situations, the goal is maintaining critical services rather than powering the entire property.

Businesses and Small Commercial Sites

Small commercial premises typically require more consistent load capacity.

This may include:

• Lighting systems
  
• Air conditioning
  
• Office equipment
  
• Point of sale systems
  
• Small machinery or tools

Commercial environments often require more stable output and may need additional headroom to manage startup demand.

Construction Sites and Industrial Use

Industrial and construction applications usually involve higher loads and surge behaviour.

Common equipment includes:

• Welders
  
• Compressors
  
• Large pumps
  
• Site cabins
  
• Three phase machinery

These environments often require three phase power and careful consideration of simultaneous motor starts.

Events and Temporary Installations

Events and temporary setups have unique requirements.

Typical loads may include:

• Audio visual equipment
  
• Lighting rigs
  
• Catering equipment
  
• Ticketing systems
  
• Temporary structures

Noise control, reliability, and fuel planning become just as important as raw capacity.

The same generator that works perfectly for a small home may be completely unsuitable for a construction site.

Application context matters as much as total wattage.

Is a 10kVA Generator Enough?

This is one of the most common questions.

• Is a 10kVA generator enough?
  
• Is 20kVA enough?
  
• Is 100kVA too much?

The honest answer is the same in every case.

It depends.

A 10kVA generator may be more than enough for a small site office with lighting and light equipment.
That same 10kVA unit may struggle immediately on a site running compressors and pumps.

A 20kVA generator might comfortably support a small commercial building.
It may be completely unsuitable for industrial machinery with high starting loads.

Capacity alone does not determine suitability.
Load type, simultaneous demand, and startup behaviour are just as important.

This is why assumptions based on a single number often lead to problems.

The same generator can be perfect in one situation and inadequate in another.

Instead of asking whether a specific kVA rating is enough, a better question is:

What is my real load, including surge demand and usage pattern?

Once that is clear, the correct size usually becomes obvious.

Why Oversizing and Undersizing Both Cost You

Choosing the wrong generator size rarely saves money.
It usually creates it.

When a Generator Is Too Small

An undersized generator may appear to work at first.
But under real load, problems surface quickly.

• Voltage drops can occur under peak demand.
  
• Protection systems may trip.
  
• Motors may fail to start properly.
  
• Sensitive equipment can shut down.

In construction or industrial environments, this can lead to lost productivity and downtime.

Workplace power failures and unsafe electrical setups can also create compliance risks under national workplace safety guidance from Safe Work Australia.

In commercial settings, it can interrupt operations and damage reputation.

When a Generator Is Too Large

Oversizing feels safer.
But it carries its own costs.

Hire rates are typically higher for larger units.
Fuel consumption increases.
The generator may run inefficiently at low load.

Over time, those unnecessary operating costs add up.

The goal is not the biggest generator available.
It is the right generator for the real demand profile.

Accurate sizing protects both equipment and budget.

Oversizing can also increase hire costs unnecessarily. If you are budgeting for temporary power, review how much it costs to hire a generator before selecting capacity.

When a Generator Size Calculator Is Essential

There are situations where estimating is not enough.

If your setup includes multiple appliances, motor driven equipment, or business critical operations, a structured calculation becomes important.

You should strongly consider using a calculator if:

• You are running several items at once
  
• You have compressors, pumps, welders, or refrigeration units
  
• The generator will operate for extended hours
  
• Power failure would disrupt operations or cause loss
  
• You are unsure whether you need single phase or three phase supply

A generator size calculator removes guesswork.

It accounts for running load and starting surge.
It helps avoid oversizing and undersizing.
It provides a clearer recommendation before you commit to hire or purchase.

If you want an accurate starting point, use the Power Generator Size Calculator to estimate your required capacity.

For complex or high load sites, expert assessment is still recommended.
Real world conditions such as power factor, runtime profile, and site layout can affect final sizing decisions.

A calculator gives you structure.
Professional input gives you confidence.

Expert Advice vs Guesswork

Online calculators are useful.

They provide structure.
They help prevent obvious sizing errors.
They create a clear starting point.

But real world generator sizing is rarely just a number.

If you want to understand what a structured commercial hire arrangement includes beyond sizing, read our guide on how generator hire works.

Experienced specialists look beyond basic wattage totals.
They consider:

• How the load behaves over time
  
• Whether equipment starts sequentially or simultaneously
  
• Environmental conditions such as temperature and altitude
  
• Cable lengths and voltage drop
  
• Future expansion or temporary load increases

Two sites with identical equipment lists can require different generator capacities depending on how that equipment is used.

Guesswork often sounds like this:

“It should be fine.”
“That size worked last time.”
“Let’s go slightly bigger just in case.”

Those assumptions can lead to unnecessary fuel costs or performance issues.

Professional assessment reduces that risk.

Electrical infrastructure and generator installations must align with applicable Australian Standards.

If you are unsure after reviewing your load or using a calculator, speaking with a generator specialist can clarify:

• Correct kVA rating
  
• Single phase or three phase requirement
  
• Fuel planning considerations
  
• Backup redundancy needs

Sizing correctly at the start avoids operational problems later.

 

Frequently Asked Questions

What size generator do I need?

Your required generator size depends on the total running load of all equipment plus the highest starting (surge) load. Listing everything you want to power and understanding how loads behave together is the first step toward an accurate sizing decision.

How do I choose the right size generator?

Think about what equipment needs power, whether everything runs simultaneously, and if there are motors or compressor loads with high startup demand. Once you have that list, use a structured tool like the Power Generator Size Calculator to estimate the right capacity.

How much power do I need to run a generator?

You need enough capacity to cover the sum of running watts for every device plus any extra needed for the highest starting load. This means looking at both steady state and surge requirements, not just individual appliance ratings.

What is a generator size calculator?

A generator size calculator is a tool that helps you estimate the correct capacity by adding up running and surge loads for all the equipment you plan to power. For an accurate starting point, use the Power Generator Size Calculator.

What happens if a generator is too small?

If a generator is undersized, it may struggle under load, experience voltage drops, trip protection systems, or fail to start equipment properly. This can lead to shutdowns and equipment damage.

Making the Right Power Decision

There is no one size fits all answer to generator sizing.

The correct capacity depends on load, application, runtime, and risk tolerance.

If you are still unsure after reviewing this guide, the safest next step is to use a structured sizing tool. Use the Power Generator Size Calculator to estimate the correct capacity for your application.

If your site includes complex loads, three phase equipment, or business critical operations, speaking with a specialist can prevent costly mistakes.
You can contact our team directly to discuss your requirements here.

Getting the size right at the planning stage protects your equipment, your budget, and your operations.

When generator sizing is handled properly, power becomes predictable.

And predictable power keeps projects moving.