Generator Sizing Guide South Africa | Choose the Right Generator 

Why Choosing the Right Generator Size Matters

Buying a generator isn’t simply about choosing the biggest machine your budget allows. In fact, one of the most expensive mistakes we see is purchasing a generator that’s either too small or far larger than necessary.

An undersized generator struggles to handle electrical demand. Lights may flicker, motors can fail to start, circuit breakers may trip repeatedly, and the generator itself is forced to work beyond its intended capacity. Over time, this shortens engine life and increases maintenance costs.

On the other hand, an oversized generator isn’t automatically the better option. Running a large diesel generator on a very light load for extended periods can reduce fuel efficiency and contribute to engine issues such as wet stacking. It also means paying significantly more for equipment that your property may never fully utilise.

The goal is to choose a generator that comfortably supports your electrical load while leaving enough spare capacity for future growth and unexpected power demands.

Whether you’re protecting a family home during load shedding, keeping a retail shop open, operating a farm, running a factory or safeguarding critical equipment, selecting the correct generator size is one of the most important decisions you’ll make.

This guide explains everything you need to know about generator sizing in South Africa, including how to calculate your electrical load, understand kVA ratings, account for motor starting currents, and avoid common buying mistakes.

Understanding Generator Ratings: kW vs kVA

One of the biggest areas of confusion for generator buyers is the difference between kilowatts (kW) and kilovolt-amperes (kVA).

Although the two measurements are closely related, they are not the same.

A generator’s kVA rating represents its apparent power, while its kW rating represents the actual usable power available for running equipment.

Most diesel generators sold in South Africa use a power factor of 0.8, meaning:

kVA × 0.8 = kW

For example:

Understanding this relationship helps you compare generators correctly and prevents buying equipment that cannot support your intended electrical load.

For a more detailed explanation of electrical power ratings, refer to the guidance published by International Electrotechnical Commission.

Step 1: Make a List of Everything You Want to Power

Before selecting any generator, write down every appliance, machine or electrical system that needs backup power.

For homeowners, this might include:

• Lighting
• Refrigerator
• Freezer
• Television
• Wi-Fi router
• Security system
• Electric gate
• Alarm system
• Borehole pump
• Air conditioning
• Computers

For businesses, the list becomes much larger:

• Office computers
• Servers
• Network equipment
• POS systems
• Warehouse lighting
• Air compressors
• Refrigeration
• Production machinery
• Pumps
• Conveyor systems
• Security systems
• HVAC equipment

Industrial facilities often require a complete electrical load assessment before generator selection.

Creating a complete inventory ensures nothing important is overlooked.

Step 2: Determine the Running Power of Each Appliance

Every electrical appliance has a running power requirement.

This information can usually be found on:

• Manufacturer labels
• User manuals
• Equipment specification sheets
• Electrical nameplates

For example:

Keep in mind that these are approximate figures. Actual consumption depends on the manufacturer, efficiency rating, age of the appliance and operating conditions.

Step 3: Don’t Forget Starting Power

Many people calculate only the running power of their appliances.

Unfortunately, motors don’t start using running power.

Electric motors require a much higher surge of electricity during startup.

This is known as:

• Starting Current
• Starting Load
• Surge Power
• Inrush Current

Motor-driven equipment may briefly require 2 to 7 times its normal running power.

Examples include:

• Water pumps
• Air compressors
• Refrigerators
• Freezers
• Air conditioners
• Workshop machinery
• Industrial motors

Imagine a borehole pump that normally uses 1.5 kW.

During startup, it may temporarily require over 5 kW.

If the generator cannot supply that extra power, the pump may fail to start even though the generator appears large enough on paper.

This is one of the most common reasons homeowners believe their generator is faulty when, in reality, it was simply undersized.

Why Load Shedding Has Changed the Way South Africans Buy Generators

A decade ago, many South Africans viewed generators as emergency equipment used only during occasional outages.

Today, backup power has become an essential investment for households, farms, offices, medical facilities and manufacturing businesses.

With frequent load shedding and unexpected grid interruptions, many properties rely on generators for extended operating hours rather than occasional emergencies.

This shift has changed the way generators should be sized.

Instead of choosing a generator that only powers a few lights, many buyers now want systems capable of running:

• Entire homes
• Office buildings
• Retail stores
• Restaurants
• Farms
• Workshops
• Medical practices
• Guest houses
• Warehouses

Because generators are expected to operate for longer periods, correct sizing has become even more important. A properly sized generator improves fuel efficiency, reduces wear on the engine and helps ensure reliable performance whenever the grid fails.

Residential Generator Sizing Guide

Every household has different power requirements.

A small apartment may only need backup for lighting, a refrigerator and internet equipment, while a large family home could require power for air conditioners, borehole pumps, electric fencing, security systems and multiple appliances running simultaneously.

Rather than trying to power every circuit in the house, many homeowners choose to back up only their essential loads. This approach reduces the required generator size, lowers installation costs and improves fuel efficiency.

As a general guide:

These figures are only estimates. The correct generator size depends on the actual electrical load, the number of appliances operating at the same time and whether any equipment has high startup currents.

If you’re unsure, it’s always better to calculate your load carefully or consult an experienced supplier before making a purchase.

Once you’ve determined your power requirements, you can browse our diesel generators for sale to compare models suitable for homes, businesses, farms and industrial applications.

Commercial Generator Sizing Is More Complex

Sizing a generator for a business involves more than simply adding up the wattage of electrical equipment.

Commercial properties often include a combination of lighting, office equipment, refrigeration, HVAC systems, machinery and motors that do not all operate continuously or start at the same time.

A supermarket, for example, may have refrigeration systems cycling on and off throughout the day, while an office building experiences peak demand during working hours. A factory may have several large motors that require substantial startup current before reaching normal operating speed.

These varying load profiles mean that commercial generator sizing should account for both the total running load and the highest expected demand at any given time.

An experienced supplier will often perform a detailed load assessment before recommending a generator. This helps avoid unnecessary costs while ensuring the generator can reliably support business operations during power outages.

 Selecting the correct generator is only half the job. Professional generator installation services ensure your backup power system is installed safely, complies with South African electrical regulations, and performs reliably when you need it most.

How to Calculate the Correct Generator Size

Once you’ve listed the appliances or equipment you want to power, the next step is to calculate the total electrical load. While this might sound technical, the process is straightforward when broken down into a few simple steps.

The aim is to determine not only how much power your equipment uses during normal operation, but also how much additional power is required when motors and compressors start up. Ignoring startup loads is one of the main reasons people end up with undersized generators.

Step 1: List Every Electrical Load

Start by creating a list of everything you want the generator to power. Include both essential and non-essential equipment so you can decide later whether everything needs backup power.

For a typical South African home, your list may include:

Commercial premises should also include:

• Office computers
• Printers
• Point-of-sale systems
• Air conditioning
• Cold rooms
• Compressors
• Pumps
• Warehouse lighting
• Production machinery
• CCTV systems
• Network equipment

The more accurate your equipment list, the more accurate your generator sizing will be.

Step 2: Add the Running Load

Once you know the running wattage of each appliance, add everything together.

Example:

Total Running Load = 2,500 Watts (2.5 kW)

At first glance, you may think a 3 kVA generator is sufficient.

Not necessarily.

Step 3: Account for Starting Loads

Motors require significantly more electricity during startup than during normal operation.

For example:

If several motors start at the same time, the generator must be capable of supplying these temporary surges without experiencing voltage drops or shutdowns.

Step 4: Add a Safety Margin

A generator should never operate continuously at 100% capacity.

Running at full load all day can:

• Increase fuel consumption
• Shorten engine life
• Increase maintenance costs
• Reduce reliability
• Leave no room for future expansion

Most generator specialists recommend sizing a generator to operate at approximately 70–80% of its rated capacity during normal use.

This provides a comfortable reserve for:

• Future equipment purchases
• Higher seasonal demand
• Startup surges
• Unexpected power requirements

Example: Sizing a Generator for a Home

Imagine a homeowner wants to power:

• Lights
• Refrigerator
• Freezer
• Television
• Wi-Fi
• Alarm system
• Borehole pump

The total running load is approximately 4.8 kW.

The borehole pump has a startup requirement of approximately 6.5 kW.

Rather than choosing a generator based solely on the running load, it’s wise to select a model that comfortably handles both the startup demand and future expansion.

In this case, a generator in the 7–8 kVA range may be suitable, depending on the specific equipment and whether appliances start simultaneously.

Knowing your required generator size makes the buying process much easier. Before making a purchase, we recommend reading our generator buying guide to understand the differences between residential, commercial and industrial generators, as well as the features that matter most.

Generator Size Guide by Property Type

The following table provides a general indication of generator sizes commonly used in South Africa. Every property is different, so these recommendations should be confirmed with a proper load assessment.

These are general estimates only. Factors such as simultaneous loads, motor starting currents and future expansion can significantly influence the correct generator size.

Generator Sizing for Businesses

Businesses often have more complex power requirements than residential properties.

For example, an office building may need to support:

• Computers
• Servers
• Printers
• Air conditioning
• Security systems
• Lighting
• Elevators
• Access control systems

A restaurant might require:

• Cold rooms
• Freezers
• Ice machines
• Kitchen equipment
• Ventilation systems
• Point-of-sale terminals
• Lighting

Manufacturing facilities may include:

• Conveyor belts
• Hydraulic systems
• Pumps
• Compressors
• Welders
• Large electric motors

Because these loads vary throughout the day, commercial generator sizing should always consider peak demand rather than average consumption.

After calculating your business’s electrical load, you can browse our commercial diesel generators to compare models based on power output, fuel efficiency, runtime and application requirements.

Generator Sizing for Farms

Reliable electricity is essential for modern farming operations.

Depending on the type of farm, backup power may be needed for:

• Irrigation pumps
• Borehole pumps
• Dairy equipment
• Poultry houses
• Ventilation systems
• Feed mixers
• Cold storage
• Security systems
• Workshop equipment

Motor-driven agricultural equipment often has high startup currents, making proper generator sizing especially important.

Selecting a generator with sufficient capacity helps ensure uninterrupted operations during load shedding, protecting livestock, crops and productivity.

Industrial Generator Sizing

Industrial facilities often require a detailed load analysis performed by experienced professionals.

Factors that influence industrial generator selection include:

• Total connected load
• Peak demand
• Motor starting sequence
• Harmonic distortion
• Power factor
• Continuous versus standby operation
• Future expansion
• Automatic Transfer Switch (ATS) integration
• Synchronisation with multiple generators

Choosing the right generator for an industrial application involves more than matching a kVA rating. A thorough assessment ensures the system can support critical operations safely, efficiently and reliably.

If your facility requires dependable backup power for critical operations, browse our industrial diesel generators to compare models built for high-performance, long-duration operation in demanding environments.

Signs Your Generator Is Too Small

An undersized generator often shows warning signs before it fails completely.

Watch out for:

• Lights dimming when equipment starts
• Circuit breakers tripping
• Generator overload alarms
• Difficulty starting motors
• Frequent engine stalling
• Excessive engine strain
• Voltage fluctuations
• Unexpected shutdowns

If you notice any of these issues, your generator may not be adequately sized for your electrical load.

Signs Your Generator Is Too Large

Many buyers assume that bigger is always better.

In reality, an oversized generator can create its own problems, including:

• Higher purchase costs
• Increased fuel consumption under light loads
• Reduced operating efficiency
• Wet stacking in diesel engines
• Higher maintenance costs
• Larger installation footprint

The ideal generator is one that closely matches your actual power requirements while allowing room for moderate future expansion.

10 Common Generator Sizing Mistakes to Avoid

Choosing the right generator involves more than matching a kVA rating to your estimated power requirements. Over the years, we’ve seen many buyers make the same avoidable mistakes, often resulting in higher costs, poor performance, or premature equipment failure.

Understanding these common pitfalls can help you make a more informed investment.

1. Choosing a Generator Based Only on Price

It’s tempting to choose the least expensive generator available, but the cheapest option isn’t always the best value. A low-cost generator that can’t handle your electrical load or requires frequent repairs may end up costing far more over its lifetime than a properly sized, high-quality unit.

2. Ignoring Starting Loads

Many appliances, particularly those with electric motors, require significantly more power during startup than during normal operation.

Equipment such as borehole pumps, air conditioners, compressors and refrigeration systems can draw several times their running current for a short period. If this isn’t considered during sizing, the generator may struggle to start the equipment or shut down due to overload.

3. Forgetting About Future Expansion

Your power requirements today may not be the same in two or five years.

Businesses often expand by adding machinery, offices or production lines, while homeowners may install additional air conditioners, security systems or electric vehicle chargers.

Allowing for moderate future growth can help you avoid replacing your generator sooner than expected.

4. Trying to Power Everything

Not every appliance needs backup power.

Instead of sizing a generator for your entire property, consider identifying only your essential loads. This approach often results in a smaller, more fuel-efficient and cost-effective generator.

5. Overloading the Generator

Running a generator continuously at or near its maximum capacity increases engine wear, raises fuel consumption and shortens service intervals.

For most standby applications, operating at around 70–80% of the generator’s rated capacity provides the best balance between efficiency, performance and reliability.

6. Buying a Generator That’s Too Large

A generator with excessive capacity may seem like a safer choice, but diesel generators operate most efficiently when carrying an appropriate load.

Lightly loaded generators can experience reduced fuel efficiency and, over time, issues such as wet stacking. Choosing a correctly sized generator is generally a better long-term investment.

7. Overlooking Fuel Storage Requirements

The larger the generator, the greater its fuel consumption.

Before purchasing a generator, consider:

• Daily operating hours
• Fuel tank capacity
• Refuelling intervals
• Safe fuel storage
• Fuel availability during prolonged outages

Planning ahead helps ensure uninterrupted operation during extended power cuts.

8. Ignoring Site Conditions

The installation environment can affect generator performance.

Factors such as:

• Ventilation
• Ambient temperature
• Altitude
• Dust
• Rain exposure
• Noise restrictions

should all be considered before installation.

Factors such as ventilation, earthing, exhaust routing and Automatic Transfer Switch (ATS) integration should all be considered during installation. Our generator installation services ensure every component is installed correctly to maximise safety, performance and long-term reliability.

9. Not Performing Regular Maintenance

Even a perfectly sized generator requires routine servicing.

Regular maintenance helps:

• Improve reliability
• Extend engine life
• Reduce operating costs
• Detect issues early
• Ensure the generator is ready when needed

Even the best generator requires regular servicing to perform reliably. Our generator maintenance services include scheduled inspections, preventative maintenance and performance checks to help maximise uptime and extend the life of your equipment.

10. Guessing Instead of Calculating

Perhaps the biggest mistake is relying on estimates rather than actual electrical data.

Whenever possible, calculate your load accurately or consult an experienced generator specialist before purchasing.

A proper assessment can prevent expensive mistakes and ensure you choose the right solution for your application.

Fuel Efficiency and Generator Sizing

Fuel efficiency is another important reason to select the correct generator size.

An undersized generator works harder than intended, while an oversized generator may operate inefficiently under light loads.

Choosing the right capacity helps:

• Reduce diesel consumption
• Lower operating costs
• Extend engine life
• Reduce emissions
• Improve long-term reliability

If fuel costs are a concern, it’s worth comparing not only the purchase price of different generators but also their expected operating costs over time.

Selecting the right generator size is one of the most effective ways to improve fuel efficiency. For a detailed breakdown of diesel usage, operating costs and practical fuel-saving tips, read our diesel generator fuel consumption guide.

South African Installation Considerations

Installing a generator involves more than placing it on a concrete slab and connecting a few cables.

A safe, reliable installation should consider:

• Proper ventilation
• Safe exhaust routing
• Adequate fuel storage
• Electrical protection
• Earthing
• Automatic Transfer Switch (ATS) integration
• Local municipal requirements
• Applicable electrical standards

Professional installation helps ensure your backup power system operates safely and complies with relevant South African regulations and industry best practices.

For information on South African standards, readers can consult the official South African Bureau of Standards (SABS) website.

Generator installations should always comply with applicable South African standards and electrical best practices. For more information on national standards and certification, visit the South African Bureau of Standards (SABS)

Frequently Asked Questions

What size generator do I need for a house?

The correct size depends on the appliances you want to power. Many homes require between 5 kVA and 10 kVA, while larger properties with borehole pumps or multiple air conditioners may require 10–15 kVA or more.

Can one generator power an entire house?

Yes, provided it has been correctly sized and installed. Many homeowners, however, choose to back up only essential circuits to reduce costs and improve fuel efficiency.

Is a larger generator always better?

No. An oversized generator can be less fuel-efficient and more expensive to purchase and maintain. Choosing the correct size for your actual load is generally the better approach.

What’s the difference between kVA and kW?

kVA measures apparent power, while kW measures usable power. Most diesel generators operate with a power factor of 0.8, meaning a 100 kVA generator typically provides around 80 kW of usable power.

How do I calculate my generator size?

Add together the running power of all equipment you want to operate, account for startup loads, and include a reasonable safety margin for future expansion.

Can a generator run continuously?

Some generators are designed for continuous or prime power applications, while others are intended for standby use only. Always select a generator that matches your intended operating pattern.

Should I install an Automatic Transfer Switch (ATS)?

An ATS automatically starts your generator and transfers power when the grid fails, making it an excellent option for homes and businesses that require uninterrupted backup power.

How often should a generator be serviced?

Service intervals vary by manufacturer, but regular maintenance is essential to maintain reliability, fuel efficiency and warranty compliance.

Why Choose Power Stay Solutions SA?

At Power Stay Solutions SA, we understand that selecting the right generator is about more than choosing a machine with the highest kVA rating. It requires a clear understanding of your electrical load, operating environment and long-term power requirements.

We supply reliable diesel generators for residential, commercial, agricultural and industrial applications across South Africa. Whether you’re preparing for load shedding, protecting critical business operations or investing in long-term backup power, our team can help you identify a solution that matches your needs and budget.

From helping you calculate your power requirements to recommending the right generator and supporting professional installation, we’re committed to providing dependable backup power solutions backed by practical expertise and customer-focused service.

Conclusion

Choosing the correct generator size is one of the most important decisions you’ll make when investing in backup power.

A properly sized generator delivers dependable performance, improved fuel efficiency and a longer service life, while reducing the risk of overloads, unnecessary operating costs and unexpected downtime.

By carefully calculating your electrical load, considering startup currents, allowing for future expansion and seeking professional advice where necessary, you can confidently select a generator that will meet your needs for years to come.

If you’re unsure which generator is right for your home, business, farm or industrial facility, Contact Us. Our team is ready to help you assess your requirements and recommend a reliable backup power solution tailored to your application.