Energy / Sizing guide
What size generator do I need?
The honest answer is that four numbers decide it: your running load, your largest motor and how it starts, your duty, and how hot the site gets. Put them in below and the calculator will show you the requirement — and its working, so you can check it.
01Sizing calculator
Four inputs. One number.
This applies the same checks an engineer does on the back of an envelope: steady-state load, worst-case starting transient, ambient derating and duty rating. It is indicative — it gets you to the right order of magnitude and the right conversation.
Indicative requirement
230kVA
Governed by motor starting surge.
The working
- Running load100 kW ÷ 0.8 pf125 kVA
- Starting surgelargest motor × 3200 kVA
- Governing casemotor starting surge200 kVA
- Ambient derateat 50 °C4%
Closest set we stock
250 kVAPerkins 1306A-E87TAG3Indicative only. A firm recommendation needs your load schedule, the starting method per motor and the duty profile — send them over and an engineer will confirm.
02Get this wrong and it costs
Six sizing mistakes.
We see the same handful of errors on site every year. Two of them cost fuel, two cost reliability, and two cost a mid-programme equipment swap.
01
Oversizing “to be safe”
A set running at 20% load burns fuel inefficiently, glazes bores and wet-stacks — unburnt fuel and carbon build up in the exhaust. You pay more to hire it, more to fuel it, and more in maintenance. Oversizing is the most expensive kind of caution.
02
Adding up nameplates
Nameplate ratings assume everything runs flat out simultaneously. Real sites have diversity — not every motor, hoist and heater peaks at once. Sizing on the sum of nameplates routinely produces a set 40% larger than the site ever needs.
03
Ignoring the starting surge
A direct-on-line motor draws up to six times its running current for a few seconds. A generator sized only for steady-state load will sag or trip every time that motor starts — the single most common cause of “the generator keeps cutting out”.
04
Forgetting the ambient
Generators are rated at 25–40 °C. At 50 °C on a Musaffah site in July, usable output drops. A set that is exactly right on paper is undersized in August.
05
Confusing standby and prime ratings
Standby ratings assume intermittent use with no overload capability. If the set is your only supply and runs continuously, it must be sized on its prime rating — typically about 10% below the standby figure.
06
Sizing for today only
Fit-out load ramps as floors come online, and site loads grow as trades arrive. Some headroom, or a paralleled arrangement that can be added to, avoids swapping the set out mid-programme.
What we need for a firm quote
- —Load schedule or approved single-line diagram
- —Largest motors and their starting method
- —Duty profile — hours per day, load pattern
- —Site location and expected ambient
- —Connection arrangement at the changeover point
- —Hire duration and start date
Generator sizing
Sizing questions.
What size generator do I need for my site?
It depends on four things: the total running load in kW, the largest single motor and how it starts, the duty (standby or continuous) and the site ambient temperature. The calculator on this page applies all four and shows its working. As rough orientation: a site office and welfare cabins typically land around 30–60 kVA, a small construction site 60–125 kVA, a mid-size site 200–250 kVA, and data-hall or commissioning duty 500–1000 kVA or paralleled sets above that.How do I convert kW to kVA?
kVA = kW ÷ power factor. Most mixed site loads sit around 0.8, so a 100 kW load is about 125 kVA. Resistive loads such as heaters and load banks are close to 1.0. Generator sets are rated in kVA, which is why the conversion matters — quoting a kW figure without the power factor understates what you need.Why does the largest motor matter so much?
Because of inrush. A direct-on-line start can draw around six times running current for a few seconds. The generator has to absorb that transient without the voltage or frequency falling outside the equipment’s tolerance. Star–delta roughly halves it, a soft starter reduces it further, and a VFD nearly eliminates it — which is why the starting method changes the answer more than almost any other input.How much does UAE heat reduce generator output?
Sets are rated at a reference ambient, commonly 25–40 °C. Above that, air density falls and the cooling package works harder, so usable output drops. A conservative working figure is roughly 4% per 10 °C above 40 °C, before accounting for altitude or restricted airflow in a plant room. On a 50 °C site that is a real reduction, and it is why a set sized at reference conditions can disappoint in July.Should I size for prime or standby duty?
Standby if the set only runs when mains fails, for limited hours a year. Prime if it is the site’s only supply, running daily for months — which describes most construction and bridging-power hires. Prime duty means sizing above the standby figure, typically by about 10%, and it also changes the maintenance interval.Is it better to hire one large generator or two smaller ones?
Two smaller sets cost more per kVA but give you redundancy and let you run closer to efficient load as demand changes — you run one set at good load rather than one big set at poor load. For critical facilities, or where load ramps over a programme, paralleled sets are usually the better engineering answer. For a steady, non-critical load, a single correctly-sized set is simpler and cheaper.What information do you need to give a firm recommendation?
A load schedule or approved single-line diagram, the starting method for each significant motor, the duty profile (hours per day and expected load pattern), the site location and ambient, and the connection arrangement. With those an engineer can confirm the set, the cabling and the distribution rather than estimating.
Got a number?
Let an engineer confirm it.
Send the load schedule and we'll confirm the set, the cabling and the distribution — quote within 2 working hours.