Updated
Generator sizes
Generator Sizes We Repair, Service & Maintain
Generator sizes run from about 1 kVA portable sets to 1,000 kVA and larger standby plant, read from the kVA on the data plate: at the typical 0.8 power factor, a 20 kVA set supplies 16 kW and a 100 kVA set 80 kW. Your plate and the load you need to carry decide the right size.
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- Portable through large plant
- Single-phase and three-phase
- Onsite across Gauteng
How size is read
What generator size do I have?
Your generator’s size is the kVA rating on its data plate, the controller screen or the specification sheet that came with it. Canopy size and fuel tank volume are poor guides. A photo of the plate is all we need to book a visit.
kVA is apparent power and kW is real power. Multiply kVA by the power factor to get kW. Generator ratings typically use 0.8, so a 100 kVA set delivers 80 kW, not 100 kW. If your plate shows a different power factor, use that figure.
Sizing a set for a building starts with the load: what must run at the same time, the starting current of motors, pumps and compressors, single-phase or three-phase supply, sensitive loads such as UPS systems, and room for later equipment. Standby or prime duty changes the rating you size from.
Size bands
Common generator size bands
We repair, service and maintain sets across all of these bands. A generator between two bands is booked from its plate just the same.
Portable and small
About 1 kVA to 10 kVA. Often petrol, used at a house, a small site or as a backup frame.
Residential and small business
About 10 kVA to 30 kVA. House standby and smaller shops, sometimes diesel, sometimes petrol.
Commercial
About 30 kVA to 100 kVA. Offices, retail and sites with several circuits and motor loads.
Large commercial
About 100 kVA to 500 kVA. Bigger buildings, plant rooms and sets that carry a wider load.
Industrial
About 500 kVA to 1,000 kVA and above. Heavy standby plant. Access, lifting and test gear are part of the visit plan.
Large industrial and standby plants
1 MVA and above. We plan these as plant work, with access, lifting and test equipment arranged ahead of the visit.
What changes the job
Why correct generator sizing matters
A generator that is too small for the starting load trips, overheats or drops frequency. A set that is far larger than the load can also run poorly if it never sees a meaningful load.
Starting current
Motors, pumps, lifts and compressors draw more current at start than they do once they are running.
Single-phase or three-phase
The plate and the building supply have to match. A three-phase set is not a cure for a single-phase board, or the reverse.
Sensitive loads
UPS systems, refrigeration controls and electronics need stable voltage and frequency, not only enough kVA on paper.
Future equipment
A set sized only for today’s lights can be short once more plant is added.
Standby or prime
A set kept for outages is used differently from one that runs as the main supply. Service and loading follow that duty.
The plate still leads
Size a set from the kVA rating on its plate. Canopy size, noise and fuel tank volume tell you little about output.
How size changes the work
Small, commercial and industrial generator work
Small generator repairs
Portable and house sets usually need a starting, fuel, battery or output repair. We handle petrol frames as petrol generator repairs.
Commercial generator repairs
Office and retail sets add controllers, changeover and a larger fuel system. The fault is still tested before parts are listed.
Industrial generator servicing
Larger diesels need filters, coolant, batteries and a running check planned around access. We follow the engine maker’s schedule, such as the 500-hour minor service Perkins publishes for its diesels. Our industrial servicing and maintenance page sets out that programme.
Large standby generator maintenance
Readiness on a set that sits is batteries, heaters, coolant, fuel condition and a start under a real or test load. Standby generator maintenance covers how those checks are scheduled.
Generator size and load testing
A start in the yard does not prove the set can carry the building. When capacity is the question, we arrange a load bank test.
Generator size and ATS systems
The transfer switch has to suit the generator and the board. We treat a controller alarm or a failed transfer as an electrical fault and test the changeover itself.
Choosing a size
How to choose a generator size
Choose a size from the load it has to carry, not from the price or the canopy. Follow these steps in order. The generator size calculator does the arithmetic.
- List what must run during an outage. Leave out what can wait, such as a geyser or a stove, unless it really must run.
- Note the running watts of each item from its rating plate or manual.
- Mark the items with a starting surge (motors, pumps, compressors, air conditioners) and note their starting watts or a typical multiple.
- Decide what runs at the same time. The running load is the total of those items; the largest single surge is added on top.
- Convert to kVA with the power factor (typically 0.8 for generator ratings) and add headroom; our calculator uses 25% by default.
- Allow for altitude and heat. Gauteng sits high above sea level and engines lose power with altitude; manufacturers publish derating figures.
- Match the supply: single-phase or three-phase, and on three-phase sets, how the load is spread across the phases.
- Worked example: 6 kW of running load plus a 3 kW starting surge. Running: 6 ÷ 0.8 = 7.5 kVA, plus 25% headroom = 9.4 kVA. Peak: 9 ÷ 0.8 = 11.25 kVA. The larger figure wins, so the next common size up is 13 kVA.
Connecting a generator to a building must go through a changeover switch so it can never feed back into the mains. That work is for a registered electrician, who can issue a Certificate of Compliance. Never plug a generator into a wall socket to power the house.
Common Questions
Generator size questions
What size generator do I need?
Add up the running load of everything that must run together, add the largest motor starting surge, and convert to kVA at the typical 0.8 power factor with headroom. For example, 6 kW running plus a 3 kW surge points to about 13 kVA. Our generator size calculator does the arithmetic.
What size generators do you repair?
From about 1 kVA portable sets through commercial machines to industrial standby plant of 1,000 kVA and more. We book the visit from the kVA on the plate, the fuel type and the fault.
Can you service generators above 500 kVA?
Yes. Send the kVA and a photo of the installation when you book, so we plan access, lifting and test equipment for the visit.
How do I find my generator's kVA rating?
Read the data plate on the alternator or canopy, or the rating shown on the controller. A clear photo of that plate is the fastest way to send it.
What is the difference between kVA and kW?
kVA is apparent power and kW is real power. Multiply kVA by the power factor, typically 0.8, to get kW: a 100 kVA set gives 80 kW.
Can you service both single-phase and three-phase generators?
Yes. Tell us which one it is, because the output checks and the building supply are different.
Can a generator be too small for a building?
Yes. If motors, lifts or the full board pull more than the set can start and hold, it trips, slows or overheats. That is a sizing problem, not only a repair.
Can you test whether my generator can carry the building load?
A running test shows it starts and produces voltage. A load bank test is the separate check when you need to know whether it can carry a defined load.
What if my generator sits between two size bands?
We book it from its plate and its fault, like any other set. The bands simply describe the range of sets we work on.
Still unsure what the fault is? Call or WhatsApp 082 057 4605 for onsite diagnosis.
Related Pages
Repairs, servicing and load testing
- Generator Size Calculator Estimate kVA from your appliances
- Generator Repairs Fault diagnosis on site
- Generator Servicing Oil, filters and inspection
- Generator Maintenance Keeping a standby set ready
- Load Bank Testing Capacity under a test load
- Diesel Generator Repairs Standby and commercial diesels
- Petrol Generator Repairs Portable and small petrol sets
- Gas Generator Repairs Supported gas and dual-fuel sets
- kVA to kW Apparent power and usable power
- Single-Phase vs Three-Phase Match the generator to the board
- Standby vs Prime What the duty rating means
- Fuel Consumption Litres per hour at your load
- Commercial Generator Repairs Business premises and standby sets
- Industrial Generator Repairs Larger diesel and high-load sets
- Industrial Servicing & Maintenance Servicing larger diesels by running hours
- Standby Generator Maintenance Readiness on a large standby set
- Contact Send the plate and the fault
Service Areas
Onsite visits across Gauteng
We come to your generator anywhere in Gauteng, 24/7. Choose your region for local suburbs, call-out details and the faults we see most there.
What to send before we attend
These details help us plan the visit and bring the right parts. Send what you can on WhatsApp; if you can’t find the model number, we still book the visit.
- Location
- The suburb or site, so the visit can be booked in the right area.
- Make and model
- The generator brand and model, if they are known.
- kVA
- The size printed on the data plate.
- Fuel type
- Diesel, petrol, LPG, natural gas, or dual-fuel.
- Controller model
- The module name on the panel, when it is visible.
- Fault or alarm
- What the set does, and any text on the screen.
- Photos
- The generator, the data plate, and the controller or alarm. Send these on WhatsApp.
Book a Visit
Send the kVA and the fault
A photo of the data plate, the fuel type and what the generator is doing is enough to arrange a visit across Gauteng.
- Call
- 082 057 4605
- Message us on WhatsApp
What happens next
- We reply with questions about the generator, or to book a visit.
- A technician comes to you. For a breakdown we arrive within 1 hour.
- The fault is diagnosed on site and you get a written quote. No work starts until you accept it.
Charlie, lead technician: Degree in Mechanical Engineering, Degree in Electrical Engineering, Diesel Plant Fitter. Generator specialists since 2000.