Generator Sizing Guide: How to Calculate the Right Generator Size for Your Business
Introduction
Power interruptions can really stop a business from working. Imagine an office that cannot use its systems a restaurant that has to worry about its refrigeration or a factory that has to delay its production. In these situations picking the backup generator is a very important decision for a business.
This generator sizing guide explains how to calculate the right generator size for your business in South Africa, avoid common sizing mistakes, and understand the factors that influence generator performance. By the end of this guide, you’ll know how to estimate your power requirements, allow for future growth, and choose a diesel generator that delivers reliable backup power without unnecessary operating costs.
Why Generator Sizing Matters
Many business owners assume that buying the biggest generator they can afford is the safest option. Others try to save money by choosing the smallest generator that appears capable of powering their equipment. Both approaches can lead to expensive problems.
A correctly sized diesel generator should:
- Supply enough power for all essential equipment.
- Handle the starting current of motors and compressors.
- Operate efficiently under normal load conditions.
- Allow room for future business expansion.
- Reduce fuel consumption and maintenance costs.
- Extend engine and alternator lifespan.
Getting the right size generator is about making sure it works well and is reliable and does not cost much to run for a long time. It is not about getting a generator with the highest power rating. The right size diesel generator is important, for all these reasons.
What Happens If a Generator Is Too Small?
Undersized generators are one of the most common causes of poor backup power performance.
When demand exceeds the generator’s capacity, you may experience:
- Voltage drops
- Frequent overload trips
- Difficulty starting electric motors
- Equipment shutdowns
- Excessive engine strain
- Reduced generator lifespan
- Unexpected production downtime
For businesses that need to keep running all the time these problems can cost a lot of money. It is better to get the right size generator from the start because this can save you money in the run.
Can a Generator Be Too Large?
Yes.
Oversizing is often overlooked, but it can create its own problems.
A diesel generator that consistently operates at very low loads may experience:
- Higher fuel consumption
- Wet stacking caused by incomplete combustion
- Carbon build-up in the exhaust system
- Increased maintenance requirements
- Lower operating efficiency
Most diesel generators perform best when operating at approximately 60% to 80% of their rated capacity during normal use.
The aim is not to buy the largest generator available but to select one that comfortably handles your business’s electrical demand while operating within its optimal load range.
Once you’ve established the correct generator capacity for your business, exploring our diesel generators for sale can help you compare models that match your power requirements, operating environment and future growth plans.
Understanding Generator Capacity
Before calculating your power requirements, it’s helpful to understand how generator capacity is measured.
Most commercial diesel generators in South Africa are rated in kilovolt-amperes (kVA) rather than kilowatts (kW).
kVA
kVA measures the generator’s total apparent power.
kW
kW measures the usable power available to operate electrical equipment.
Because electrical systems have power losses, these values are not identical.
For most commercial applications, generators are sized using a power factor of 0.8.
Simple example:
- 20 kVA × 0.8 = 16 kW
- 50 kVA × 0.8 = 40 kW
- 100 kVA × 0.8 = 80 kW
It’s really important to understand the difference, between kVA and kW when you are comparing different generators or figuring out how much power your business needs. You need to know about generator capacity and how it works. Generator capacity is a thing to consider when you are thinking about kVA and kW.
Before calculating your electrical load, it’s also worth understanding the differences between single-phase and three-phase power. Our single-phase vs three-phase generator guide explains which system is best suited to different business applications.
Step 1: Identify Every Essential Electrical Load
The first step in any generator sizing guide is determining exactly what needs to remain operational during a power outage.
Instead of estimating, create a complete inventory of all equipment that will run from the generator.
Typical business loads include:
Office Equipment
- Desktop computers
- Laptops
- Servers
- Network switches
- Internet equipment
- Security systems
- Printers
- Lighting
- Air conditioning
Retail Businesses
- Point-of-sale systems
- Refrigeration units
- Display lighting
- Security cameras
- Payment terminals
- Automatic doors
Restaurants and Hospitality
- Cold rooms
- Freezers
- Kitchen equipment
- Ventilation systems
- Ice machines
- Lighting
- Extraction fans
Farms
- Borehole pumps
- Irrigation systems
- Milking equipment
- Cold storage
- Security systems
- Feed processing equipment
Industrial Facilities
- Conveyor systems
- Compressors
- Pumps
- Production machinery
- Welding equipment
- Ventilation systems
- Emergency lighting
Industrial equipment often has high starting loads and continuous power demands, making accurate sizing even more critical. If your operation relies on heavy machinery or uninterrupted production, our range of commercial diesel generators includes reliable backup power solutions designed for demanding industrial environments.
Create a spreadsheet listing each item, its power rating, and whether it needs to operate during a power outage. This provides a reliable starting point for calculating your generator size.
Step 2: Record the Power Rating of Every Appliance
When you have found all the things that need power write down how much power each thing uses.
Most things that use electricity have a label that says how much power they use.
Look for:
- Watts (W)
- Kilowatts (kW)
- Amps (A)
- Voltage (V)
If only the current (amps) is listed, you can calculate the approximate power requirement using the appropriate electrical formula. For larger commercial installations, it’s advisable to have these calculations verified by a qualified electrician to ensure accuracy.
Making a list of all your equipment is an idea because it makes it easy to add new things later. You can see if your generator can handle the things you want to add. This way you can plan for the future. Make sure you have enough power, for everything.
Step 3: Add the Total Running Load
After recording the power requirements for each piece of equipment, calculate the total running load by adding them together.
For example:
| Equipment | Running Load |
|---|---|
| Office Lighting | 2 kW |
| Computers | 3 kW |
| Air Conditioning | 5 kW |
| Server Room | 2 kW |
| Security Systems | 1 kW |
| Refrigeration | 4 kW |
| Total Running Load | 17 kW |
This figure represents the continuous power your generator must supply during normal operation.
However, the calculation isn’t complete yet.
Many businesses make the mistake of selecting a generator based only on the running load, overlooking the additional power required when motors start. Ignoring this factor can result in nuisance trips, voltage drops, and unreliable performance, even if the generator appears correctly sized on paper.
In the next section, we’ll look at motor starting currents, explain the difference between running load and starting load, and show you how to convert your total demand into the correct generator size for your business.
Expert Tip
One of the most common issues we encounter when helping businesses choose backup power is incomplete load calculations. Customers often total the wattage of their equipment but forget to include items such as air conditioners, compressors, pumps or future electrical additions. It is really important to make a list of the power you need before you buy a generator.
This can save you a lot of money and trouble on. You will not have to pay for upgrades or deal with operational problems. Taking the time to do this now can really help you with your power system, like a generator.
Step 4: Account for Motor Starting Loads
Adding together the running power of your equipment is only part of the calculation. Many commercial appliances require significantly more power for a few seconds when they start. This temporary surge is known as the starting load or inrush current.
If your generator cannot supply this additional demand, you may experience:
- Voltage drops
- Equipment failing to start
- Generator overload alarms
- Frequent circuit breaker trips
- Premature wear on the generator
This is why a generator sizing guide should always consider both running loads and starting loads.
Equipment That Typically Has High Starting Loads
Some of the biggest contributors include:
| Equipment | Typical Starting Load |
|---|---|
| Air compressors | 2–3 × running load |
| Borehole pumps | 2–3 × running load |
| Refrigeration compressors | 2–3 × running load |
| Air conditioning units | 2–3 × running load |
| Conveyor motors | 2–3 × running load |
| Industrial fans | 2–3 × running load |
| Welding machines | Varies depending on model |
For example, a refrigeration compressor that normally consumes 5 kW may briefly require 10–15 kW during start-up. If this additional demand isn’t considered, the generator may struggle even though the continuous running load appears to be within its capacity.
Step 5: Convert Your Running Load from kW to kVA
Commercial diesel generators are generally specified in kVA, while most electrical equipment is rated in kW. Converting between the two is an essential step when following a generator sizing guide.
For most commercial installations, the calculation is:
kVA = kW ÷ Power Factor
A power factor of 0.8 is commonly used for diesel generators.
Worked Examples
| Running Load | Calculation | Generator Capacity |
|---|---|---|
| 16 kW | 16 ÷ 0.8 | 20 kVA |
| 24 kW | 24 ÷ 0.8 | 30 kVA |
| 40 kW | 40 ÷ 0.8 | 50 kVA |
| 80 kW | 80 ÷ 0.8 | 100 kVA |
These calculations provide a starting point, but they should always be adjusted to account for motor starting currents and any anticipated future expansion.
Step 6: Allow for Future Business Growth
Many businesses expand over time by adding machinery, refrigeration, office equipment or production lines. Selecting a generator that only meets today’s demand can lead to expensive upgrades later.
It is an idea to have a little extra power, like 20% to 30% more than you need so your business has room to grow and your generator can still run well. This way you can add things to your business without having to worry about your generator not being able to handle it.
For example:
| Current Load | Recommended Capacity |
|---|---|
| 30 kVA | 40 kVA Generator |
| 45 kVA | 60 kVA Generator |
| 80 kVA | 100 kVA Generator |
This reserve capacity can accommodate additional equipment without immediately replacing your backup power system.
If you’re still weighing up different generator capacities and features, our generator buying guide will help you compare the available options and choose a solution that suits your business’s current and future power requirements.
Business Generator Sizing Examples
The following examples illustrate typical backup power requirements for different business types. These are general estimates only; every installation should be assessed individually.
Small Office
Typical equipment:
- Computers
- Internet equipment
- Lighting
- Air conditioning
- Printer
- Security system
Estimated running load:
8–12 kW
Recommended generator:
15–20 kVA
Retail Shop
Typical equipment:
- Refrigeration
- POS systems
- Display lighting
- CCTV
- Automatic doors
Estimated running load:
12–18 kW
Recommended generator:
20–30 kVA
Restaurant or Café
Typical equipment:
- Freezers
- Cold rooms
- Kitchen appliances
- Ventilation
- Lighting
- Payment systems
Estimated running load:
20–35 kW
Recommended generator:
30–50 kVA
Farm
Typical equipment:
- Borehole pump
- Irrigation
- Security systems
- Cold storage
- Feed processing
Estimated running load:
20–40 kW
Recommended generator:
30–60 kVA
Workshop
Typical equipment:
- Compressors
- Welding equipment
- Machinery
- Lighting
- Ventilation
Estimated running load:
30–45 kW
Recommended generator:
50–60 kVA
Manufacturing Facility
Typical equipment:
- Production machinery
- Conveyor systems
- Pumps
- Compressors
- HVAC
- Lighting
Estimated running load:
60–100+ kW
Recommended generator:
100–150+ kVA
Quick Business Generator Sizing Guide
| Business Type | Estimated Load | Suggested Generator |
|---|---|---|
| Small Office | 8–12 kW | 15–20 kVA |
| Retail Shop | 12–18 kW | 20–30 kVA |
| Restaurant | 20–35 kW | 30–50 kVA |
| Farm | 20–40 kW | 30–60 kVA |
| Workshop | 30–45 kW | 50–60 kVA |
| Small Factory | 60–100 kW | 100–150 kVA |
These examples are intended as a starting point. Actual generator requirements depend on equipment specifications, starting currents, duty cycle and future expansion plans.
Factors That Influence Generator Size
Two businesses with similar electricity consumption may require different generator sizes because of differences in how they use power.
Consider the following when applying this generator sizing guide:
- Number of motors starting simultaneously
- Single-phase or three-phase power requirements
- Continuous versus standby operation
- Altitude and ambient temperature
- Future expansion plans
- Sensitive electronic equipment
- Automatic transfer switch (ATS) requirements
- Fuel storage capacity and expected runtime during prolonged outages
Looking beyond the total electrical load helps ensure the selected generator delivers reliable performance under real operating conditions.
Should You Use a Generator Size Calculator?
Online generator calculators can provide a useful estimate, but they should not be treated as the final answer.
Many calculators cannot account for:
- Motor starting currents
- Power factor variations
- Future business growth
- Simultaneous equipment operation
- Site-specific conditions
- Generator duty ratings
For larger commercial and industrial installations, a professional load assessment remains the most reliable way to determine the correct generator capacity.
Expert Advice
After helping businesses across South Africa choose backup power systems, one of the most common mistakes we see is relying solely on the total running wattage shown on equipment labels. In practice, generators must also accommodate starting currents, future expansion and the way equipment operates together during peak demand.
If you take the time to really think about what you need you will probably get a backup power system. This means you will not have to spend much money in the long run. You will also not have to deal with your equipment stopping all the time using much fuel or having to replace it too soon. It is better to take your time and figure out what you need before you buy a backup power system, like a generator. This will save you a lot of trouble with your power system, like generators in the long run.
Common Generator Sizing Mistakes to Avoid
When you are trying to figure out the size of a generator for your business you can still make big mistakes. This is true even if you have a guide to help you. The thing is, picking the generator is not just, about adding up the power that all your equipment uses. You really need to understand how all your equipment works when it is actually being used.
1. Choosing a Generator Based Only on Price
A lower-priced generator may seem attractive initially, but selecting a unit purely because it fits the budget can lead to higher operating costs, reduced reliability and expensive upgrades later.
Instead, consider:
- Total power requirements
- Fuel efficiency
- Build quality
- Service support
- Spare parts availability
- Long-term operating costs
The cheapest generator is rarely the most economical over its lifetime.
2. Ignoring Motor Starting Currents
Many businesses calculate only the running load and forget about the extra power required when motors start.
This commonly affects:
- Air compressors
- Borehole pumps
- Refrigeration systems
- HVAC equipment
- Conveyor belts
- Industrial machinery
If these starting loads aren’t included, the generator may trip under load even though it appears large enough on paper.
3. Forgetting Future Expansion
Businesses rarely remain the same size for long.
Adding machinery, refrigeration, additional offices or production equipment can quickly exceed the generator’s available capacity.
Allowing for moderate expansion today is usually far more cost-effective than replacing an undersized generator in a few years.
4. Powering Everything Instead of Essential Loads
During load shedding or unexpected outages, not every circuit needs to remain energised.
Ask yourself:
- Which equipment is essential?
- What can remain switched off temporarily?
- Which systems protect revenue or safety?
Prioritising essential circuits often reduces the generator size required while lowering both purchase and operating costs.
5. Overlooking Professional Installation
A correctly sized generator still requires proper installation to perform safely and reliably.
Incorrect cable sizing, inadequate ventilation, poor earthing or an incorrectly configured Automatic Transfer Switch (ATS) can all compromise system performance.
Once you’ve selected the correct generator size, proper installation is the next step in ensuring reliable performance and electrical safety. Our professional generator installation services ensure your backup power system is correctly connected, thoroughly tested and installed in accordance with South African electrical standards, giving you confidence that it will perform when it’s needed most.
Expert Advice
After helping businesses across South Africa select backup power solutions, one pattern appears repeatedly: customers often focus on the generator itself before understanding their actual electrical demand.
A detailed site assessment almost always leads to a better outcome than choosing a generator based on assumptions or recommendations from similar businesses. Every installation is different, even within the same industry.
Another common oversight is failing to consider how equipment starts. Compressors, pumps and large electric motors can briefly draw two to three times their normal running current. If this isn’t factored into the sizing calculation, the generator may struggle during startup despite having sufficient continuous capacity.
Our recommendation is straightforward: prepare an accurate load schedule, identify your essential circuits, allow for sensible future growth and have the calculations reviewed before committing to a purchase. This approach reduces the risk of costly mistakes and helps ensure your generator continues to meet your business needs for years to come.
Buying Advice: Choosing the Right Diesel Generator
Once you’ve established the correct generator size, there are several additional factors worth considering before making a final decision.
Generator Duty Rating
Select a generator with a duty rating that matches its intended use.
- Standby generators are designed for emergency backup during power outages.
- Prime power generators are suitable for sites where the generator operates as the primary source of electricity for extended periods.
Choosing the duty rating, for your diesel generator helps it last longer and be more reliable.
Fuel Efficiency
Fuel is one of the largest operating costs over the life of a diesel generator.
Compare generators based on:
- Fuel consumption at different load levels
- Fuel tank capacity
- Expected runtime
- Ease of refuelling
- Service intervals
A more fuel-efficient generator can significantly reduce long-term operating expenses.
Noise Levels
If your generator will operate near offices, residential properties or customer-facing areas, consider a canopy designed to minimise noise.
These special covers can really help make the generator quieter. Some generators are super quiet. These are usually the best choice when you need to keep the noise level low. Silent or super-silent diesel generators are what people usually pick when they need to be careful, about noise.
Service and Spare Parts
Before purchasing, confirm that replacement parts and servicing are readily available in South Africa.
If you have help after you buy something it can really make a difference. You will not have to wait long to get things fixed and your generator will last a lot longer. This is because you can get the Service and Spare Parts for your generator when you need them. So it is an idea to think about Service and Spare Parts and the help you will get after you buy your generator.
Automatic Transfer Switch (ATS)
An Automatic Transfer Switch detects a power failure and starts the generator automatically before transferring the electrical load.
For businesses like facilities, data centres and manufacturing plants that really need to keep working all the time the Automatic Transfer Switch is very helpful. It helps these businesses keep running without stopping when the power goes out. The Automatic Transfer Switch can really make a difference, for these businesses.
Frequently Asked Questions
1. What size generator does my business need?
The correct size depends on your total running load, motor starting requirements and expected future expansion. A professional load assessment provides the most accurate result.
2. Is it better to oversize a generator?
Not necessarily. Excessive oversizing can reduce fuel efficiency, increase maintenance requirements and contribute to wet stacking. The goal is to match the generator closely to your expected operating load while leaving reasonable capacity for future growth.
3. How do I calculate generator size?
Start by listing all equipment that requires backup power, total the running load, account for motor starting currents, convert kW to kVA where necessary and include an allowance for future expansion.
4. What is the difference between kW and kVA?
kW measures usable electrical power, while kVA represents apparent power. Diesel generators are generally rated in kVA, making it the standard unit used when comparing generator capacities.
5. Can one generator power my entire business?
Yes, if I get a generator that’s the right size, for my business. Some businesses choose to power the most important things so they can get a smaller generator and save money.
6. Should I buy a single-phase or three-phase generator?
This really depends on how my businesss set up and what kind of equipment I need to power. Big businesses and factories usually use three-phase power.. Smaller offices or shops might use single-phase power. My business needs to get a generator that matches its system.
Not sure which power supply your business has? Our single-phase vs three-phase generator guide explains the key differences, typical applications and how to determine which system is best suited to your electrical installation before choosing a generator.
