Buying a generator sounds simple until you hit the most confusing question: What size should I get? Too small, and it trips, overheats, or can’t start your equipment. Too big, and you waste money on fuel, maintenance, and a higher upfront cost. This guide breaks generator sizing down practically, so you can choose a capacity that actually fits your home, shop, office, factory, or project site in Bangladesh.
Start with the real goal: what are you trying to power?
Before you calculate anything, decide what “backup power” means for you.
Some people only want lights, fans, Wi‑Fi, and a fridge. Others need full business continuity, meaning lifts, HVAC, IT rooms, pumps, production machines, or an entire building load.
The size you need depends on two things: running load (what keeps operating) and starting load (the extra surge needed when motors start). Most wrong generator selections happen because people ignore the starting load.
Step 1: List your loads (and separate motor loads)
Make a simple list of everything you want to run during the outage. Then mark which items have motors or compressors. Motor loads are the ones that usually need a surge allowance.
Typical motor/compressor loads include ACs, refrigerators, pumps, lifts, air compressors, and many workshop machines.
If you want a quick way to structure it, use this:
- Non-motor loads: lights, computers, Wi‑Fi router, TV, POS system, CCTV, basic electronics
- Motor loads: AC, fridge, water pump, lift, compressor, industrial machines
Step 2: Calculate total running watts (or kW)
For each item, find its watts (W) or kilowatts (kW) on the nameplate. Add them up to get your total running load.
If something is listed in amps (A) instead of watts, you can estimate:
Single-phase: Watts ≈ Volts × Amps
In Bangladesh, many loads are around 220–230V single-phase.
For three-phase industrial loads, it’s best to have an engineer calculate based on actual power factor and operating conditions because the “simple” formula often misleads.
Step 3: Add starting surge (the part people forget)
The starting surge depends on the type of motor and its starting method. As a rough guide:
- Small motors (fridge, small pump): 2–3× starting surge
- Large motors (big pump, compressor, heavy machinery): 3–6× starting surge (sometimes more)
You do not always add surge for every motor at once. Instead, we look at the biggest motor that will start while others are running, or we plan a start sequence to reduce the surge requirement.
If you want your generator to feel “effortless” under load, don’t size it on running watts alone. Size it so motor starting does not cause voltage dip, nuisance tripping, or equipment stress.
Step 4: Convert kW to kVA (and understand why it matters)
Generators are often rated in kVA, while your loads are often discussed in kW.
The relationship depends on the power factor (PF):
kW = kVA × PF
So, kVA = kW ÷ PF
A common planning PF is 0.8 for mixed commercial and industrial loads. That means:
kVA ≈ kW ÷ 0.8 (or kW × 1.25)
Example: If your running load is 20 kW, the estimated generator size is 25 kVA, before adding surge and safety margin.
Step 5: Add a safety margin (don’t run at 100%)
In real life, loads grow. Temperature rises. Dust builds up. Voltage fluctuates. Fuel quality varies. If you run a generator constantly near full capacity, it wears faster and behaves less stably.
A practical margin is 15–25%, sometimes more if you expect future expansion or heavy motor loads.
That’s why a good selection feels slightly “oversized,” but not wastefully oversized.
Typical sizing examples (to make it real)
A small apartment might only need lights, fans, a fridge, a TV, a router, and charging. A modest generator can do that comfortably if you avoid running heavy AC during an outage.
A medium office often needs lighting, networking, computers, printers, and maybe one or two AC units. The AC starting surge is usually what pushes the size up.
A commercial building or industrial facility is a different game. Lifts, fire pumps, HVAC, compressors, production lines, and panels require proper load study, diversity factor, and starting method review. In these cases, generator sizing is engineering, not guessing.
Don’t ignore these practical factors.
Fuel type matters. Diesel generators are common for higher loads and longer runtimes. Noise control matters in residential and commercial zones, so canopies and acoustic treatment can become part of “right sizing.”
Also consider whether you need single-phase or three-phase, whether you want manual changeover or ATS, and whether your setup needs synchronization or load sharing.
Most importantly, sensitive equipment like servers, lab instruments, and modern electronics benefit from stable voltage regulation and clean switching. A correctly sized generator paired with the right AVR/ATS setup prevents a lot of headaches.
The simple rule to remember
If you remember only one thing, remember this: Generator size is not just your total running load. It’s running load + motor starting surge + a realistic safety margin. When those three are handled properly, your generator feels stable, your equipment stays protected, and outages stop being emergencies.
FAQs
Q1. How do I know if my generator is undersized?
If lights flicker, breakers trip, voltage drops when a motor starts, or the generator sounds strained, it’s likely undersized. Frequent overloads also reduce generator life and harm connected equipment.
Q2. Is it better to oversize a generator for safety?
Slightly oversizing is smart, but extreme oversizing wastes fuel and increases cost. The goal is a balanced margin with proper surge handling, not buying the biggest unit available.
Q3. What’s the difference between kW and kVA in generator sizing?
kW is real usable power, while kVA is apparent power. The difference comes from the power factor. Many setups assume 0.8 PF, so kVA is usually higher than kW.
Q4. Can one generator run air conditioners and pumps together?
Yes, if the generator can handle the combined running load plus the starting surge of the largest motor. Start sequencing, and proper ATS settings can make a big difference in performance.
Q5. Do I need an ATS for my generator?
If you want automatic switching during outages, ATS is recommended. For simpler setups, a manual changeover works. The right choice depends on your load criticality and budget.
Q6. Where can I find reliable generators?
For those seeking reliable generators that meet specific needs and requirements, considering a reputable generator supplier is crucial.