This guide is for a site manager, electrician or small contractor who has a list of equipment to power and must tell a supplier what size to quote. Follow the four steps with one worked example. The start of your largest motor often decides the size, so Step 2 covers it. You finish with a kVA figure and a short list of details the supplier needs to check it.
What is the difference between kVA and kW?
kW is real power: the part that does the work, such as turning a motor or lighting a lamp. kVA is apparent power: the total the generator has to supply, including a part that does no work but still loads the set.
The link between them is the power factor. kW = kVA x power factor. Generator sets are normally rated at a power factor of 0.8, so a set rated at 100 kVA delivers about 80 kW of real power.
A "100 kW" request and a "100 kVA" request are not the same, so always write the unit.
Note. Equipment plates may show kW, kVA or amps. Convert everything to the same unit before you add it up.
Step 1: How do I list everything the generator must power?
Make a table with one row for each machine, light circuit or cabin. Take the running kW from the nameplate or the data sheet. On a motor, the plate kW is usually the shaft output, so the electrical input is somewhat higher. Where the data sheet gives input kW, amps or kVA, use that instead. If you have only the plate kW or amps, ask your electrician to convert it.
Then ask one question for each row: can this be on at the same time as the others? Add up the running kW of everything that can be on together. Do not add every item on site. Do add what you expect to connect in the next few weeks.
Common mistake: adding every tool in the store, which gives a set that is too big. The fix: add only the loads that share the same working hours. For example, a cabin air conditioner and the site lights run together overnight, but a welder that works only by day does not overlap with them.
Step 2: Which loads draw extra current when they start?
A generator that carries the running total easily can still stall or drop its voltage when one big motor starts.
| Load type | Examples | Starting behaviour | What to tell the supplier |
|---|---|---|---|
| Resistive | Lamps, heaters | Starting equals running | Total kW only |
| Induction motors | Pumps, fans, cabin air conditioners | Several times running current for a moment | Largest motor size and how it starts |
| Compressors | Air compressors, chillers | High starting demand, often under load | Whether it starts loaded or unloaded |
| Welders | Arc welding sets | Heavy, uneven demand | Number of welders working together |
| Electronic loads | Variable-speed drives, UPS units, some LED drivers | Can distort the supply | That they are on site, and what they power |
How a motor starts matters as much as its kW. As typical ranges, a direct-on-line start draws roughly five to seven times full-load current. A star-delta starter cuts that sharply, and a soft starter or variable-speed drive cuts it further. The exact figure depends on the motor and the starter, so take it from the data sheet.
Start big motors one at a time, not all together. Tell the supplier the largest motor's kW and its starter type. Drives and UPS units can distort the supply and may need a larger set or a different alternator, so say so in the request. For the air side of a compressor (CFM and pressure), see air compressor sizing.
Step 3: How do I turn the total kW into kVA?
Divide the total running kW by 0.8. The result is the running kVA your request starts from. It is a planning figure, and the supplier confirms the final set.
For three-phase equipment that shows full-load amps and volts on the plate, apparent power is the square root of 3, times volts, times amps, divided by 1,000. That gives kVA directly.
For example, with invented, round numbers, say a site has these loads that can run together:
| Load | Running kW |
|---|---|
| Lighting and sockets | 6 |
| Two cabin air conditioners (5 kW each) | 10 |
| Water pump | 7 |
| Welder | 10 |
| Small power tools | 7 |
| Total | 40 |
40 kW divided by 0.8 is 50 kVA. That is the running need only. Name the pump and the air conditioners as the motor loads, with the pump as the largest. Suppose the 7 kW pump starts direct-on-line. Ask the supplier to confirm the set can start a motor of that size and what voltage dip to expect. At the Step 2 range, its start is a short surge of several times its 7 kW running load, on top of the other loads. Step 4 adds headroom.
Step 4: How much headroom should I leave?
Leave headroom so normal load sits comfortably below the set's rating. This also covers loads you add later. Ask the supplier what load is normal for the set and your duty. The answer depends on the engine, the alternator and the rating type, covered in prime, standby and continuous generator ratings.
Back to the example. Say the supplier advises a normal load of no more than about 70 percent of the rating for your duty. This figure is only for the example, and it differs by set and rating type. A 50 kVA need then points to a set of roughly 71 kVA or more, and the supplier offers the next size above that. The starting check from Step 2 is separate from this headroom: the supplier still confirms that the set starts the pump.
Tip. Ask the supplier to name the set size, the rating type and the largest motor it can start, all in writing.
Single-phase or three-phase, and which voltage and frequency?
Saudi Arabia's public grid runs at 60 Hz, but equipment brought from other countries may be 50 Hz. Check that the set's frequency, voltage and number of phases match the equipment it feeds. A mismatch can damage motors or leave the set unusable.
Do not assume the voltage on your site. Confirm it from the equipment plates, the site electrician or the site owner. If any motor, pump, compressor or welder is three-phase, you need a three-phase set, and single-phase loads can be taken from it. Spread single-phase loads evenly across the phases, because one overloaded phase limits the set before the total kVA is reached.
Connection to the site system is work for a qualified electrician. Never connect a generator to a building's wiring without a proper changeover arrangement, because feeding power back into the grid can kill people working on it. For the switch itself, see automatic transfer switch and paralleling.
What do heat and altitude do to the generator's output?
Engines can lose output in high ambient heat and at altitude, and cooling gets harder in the heat. Sets are rated at a stated reference ambient temperature and altitude. A set that gives its full figure on the nameplate may give less on a hot summer site, or at altitude, for example in the high areas of the south-west.
Ask what reference conditions the rating assumes and what output the supplier expects at your site, not only the nameplate figure. Give the city, the season and whether the set will run in a closed or shaded space.
Watch out. Exhaust contains carbon monoxide. Never run a generator in a closed room, cabin, tent or tunnel. Only a purpose-built plant room with engineered ventilation and exhaust led outside is acceptable. Keep exhaust away from air intakes and sleeping areas.
How to do this on Maedah
Post one request in generators with your kVA figure. Put these in the details, or raise them in the chat on each quote:
- the largest motor and how it starts
- any drives or UPS units
- phase, voltage and frequency to be confirmed
- site city and conditions
Sealed quotes line up side by side: compare set size, rating type and what is included. To hire, see how to rent a generator for a site; for fuel, generator fuel planning. More machines: power equipment.
