Generator Size Calculator from Your Load List (HP or kW)

Enter every load you want on the generator, in HP or kW, with the starter used on each motor. The calculator returns the smallest common Indian kVA rating that carries your running load within 80%, survives the worst motor start, and allows for heat and altitude. It works at 415 V three-phase or 230 V single-phase, 50 Hz, and shows every step of the arithmetic so your electrician or consultant can check it.

1. List your loads

The tool opens with an apartment-block example. Change the figures, add your own loads, or press Clear to start empty.

Add a load Borewell submersible pump Water transfer pump Lift (VVVF drive) Fire pump Air compressor Split AC, fixed speed Inverter AC UPS / inverter Welding set Lighting and sockets Heater / oven Other load

Add load

2. Site and supply (Hyderabad defaults; open to change)

Supply Three-phase, 415 V, 50 Hz Single-phase, 230 V, 50 Hz

Site altitude (m)

Peak ambient temperature (°C)

Set in an acoustic canopy or closed room (adds 5 °C to intake air)

Alternator X′d (%), optional

Voltage dip limit (%)

Maximum running loading (%)

Non-linear oversize factor

Maker’s altitude factor (0–1), optional

Maker’s temperature factor (0–1), optional

Leave the optional boxes empty to use the built-in screening values. Enter X′d from the alternator data sheet to add a voltage-dip check.

Calculate generator size Load example Clear

How to use the calculator

1. List each load once. Use the quantity box for identical units, such as two lifts or four air conditioners. Take the rating from the nameplate: motors in HP or kW output, other equipment in kW input.
2. Pick the type and starter. Mark motors as motors and choose their starter. Mark UPS units, drives, inverter ACs and welding sets as non-linear.
3. Add start steps if your panel brings loads on in stages. Leave the step box empty and the tool assumes the largest motor starts last, with everything else already running.
4. Check the site settings. Altitude and peak temperature default to Hyderabad conditions. Tick the canopy box if the set sits in an acoustic enclosure or a closed room.

The result updates as you type. Open Show the working to see each step, and use Print or save as PDF to keep a copy with your enquiry.

How the size is worked out

Every answer comes from the same five steps. The larger of the running case and the starting case sets the size; derating and the dip check can only push it up.

1. 1Your load listEach load in HP or kW, with power factor, motor efficiency, starter and start step
2. 2aRunning caseSum of running kVA, non-linear loads × 1.5, divided by 80% maximum loading
   2bStarting caseLargest motor start plus everything else running, or the worst start step
3. 3Take the larger, then derateDivide by the altitude and intake-temperature factors (canopy adds 5 °C)
4. 4Voltage dip checkWith alternator X″d entered, raise the size until the biggest start stays inside your dip limit
5. 5Next Indian rating, then four checksStarting covered · running load ≤ 80% · engine kW covers running kW · dip within limit

What the calculator does

Each load is turned into the kVA it draws from the alternator. For a motor, the nameplate output is divided by efficiency to get electrical input, then by power factor to get kVA.

Input kW = HP × 0.746 ÷ efficiency · Running kVA = input kW ÷ power factor

Two cases are then compared. The running case is the sum of all running kVA, with non-linear loads multiplied by an oversize factor (1.5 by default), divided by the maximum loading you allow (80% by default). The starting case is the largest motor’s starting kVA plus the running kVA of everything else. If you set start steps, it is instead the worst step: everything already running plus the block being switched on.

Starting kVA of a motor = running kVA × starting multiple
Required kVA = larger of the two cases ÷ (altitude factor × temperature factor)

The answer is rounded up to the next common rating. If you enter the alternator’s subtransient reactance (X″d), the tool also estimates voltage dip when the largest block starts, and raises the size if the dip would exceed your limit.

Voltage dip ≈ X″d × r ÷ (1 + X″d × r), where r = starting kVA ÷ generator kVA

For the theory behind each step, see motor starting, voltage dip and UPS and non-linear loads.

Starting multiples used

A motor’s starting kVA depends far more on its starter than on its size. These screening values are built into the tool; choose Own figure if you have the motor’s locked-rotor data.

The star-delta figure assumes the motor reaches near full speed before changeover. A badly timed changeover can draw close to DOL current for a moment.

Common generator ratings in India

The tool picks from the ratings commonly sold in India, from 5 kVA single-phase sets up to 2500 kVA. Above that, the load needs two or more sets in parallel. Exact availability depends on the engine make and emission norm.

The tool also uses 5, 7.5, 10, 12.5, 35, 45, 50, 75, 140, 400, 600, 1750, 2250 and 2500 kVA. Full current values for every size are on the generator current calculator.

Worked example 1: apartment block in Hyderabad (illustrative)

A 40-flat building wants backup for common services: two 10 HP lifts on VVVF drives, a 7.5 HP borewell pump on DOL, a 5 HP transfer pump on star-delta, and 6 kW of common lighting. Site: 550 m altitude, 40 °C peak, open-air set.

1. Starting case: borewell 49.37 kVA + the other running loads 31.17 kVA = 80.54 kVA.
2. Running case: 39.40 ÷ 0.80 = 49.25 kVA. The starting case is larger.
3. No derating at 550 m and 40 °C, so the answer is the next rating above 80.54: 82.5 kVA, 48% loaded, 114.8 A per phase.

The better answer: fit a soft starter to the borewell pump. Its starting kVA falls to 24.68, the starting case to 55.85 kVA, and a 62.5 kVA set does the job at 63% loading. One starter on a 7.5 HP pump saves a full generator size, and the smaller set runs better loaded.

Worked example 2: small factory in an industrial area (illustrative)

A fabrication unit runs a 30 HP screw compressor on star-delta, four CNC machines of 7.5 kW each, a 15 HP hydraulic press on DOL, 8 kW of lighting and fans, and four 1.8 kW office ACs. Peak ambient is 45 °C and the set will sit in an acoustic canopy.

1. Running load is 82.37 kW, or 93.32 kVA. The CNC drives are non-linear, so their 33.33 kVA counts as 50.00 kVA: the running basis is 109.99 kVA, and at 80% loading 137.48 kVA.
2. The press on DOL is the worst start: 14.97 kVA × 6 = 89.81 kVA, plus 78.35 kVA already running = 168.16 kVA.
3. Intake air is 45 + 5 = 50 °C. That is two 5 °C steps above 40 °C, so the temperature factor is 0.96.
4. Required: 168.16 ÷ 0.96 = 175.17 kVA. The next rating is 180 kVA, 54% loaded, 250.4 A.

Put the press on a soft starter and the starting case drops to 121.92 kVA. The 80% running check now governs: 137.48 ÷ 0.96 = 143.21 kVA, so a 160 kVA set is enough. The tool also warns that over a quarter of this load is non-linear, so ask for an alternator with PMG excitation.

Worked example 3: hospital wing with staged starting (illustrative)

A 50-bed wing needs backup for a 40 kW UPS (ICU and operation theatres), 35 kW of lighting and sockets, two 25 kW chillers on VFDs, a 10 HP medical air compressor on DOL, and a 50 HP fire pump on star-delta. The AMF panel brings loads on in three steps: UPS and lighting first, chillers and the air compressor second, fire pump last. Peak 42 °C, canopy set, alternator X″d 15%.

1. Running load is 173.93 kW, or 193.44 kVA. With the UPS counted at 1.5×, the running basis is 214.49 kVA; at 80% loading, 268.12 kVA.
2. The worst step peaks at 241.14 kVA, so here the running check governs, not starting.
3. Intake air is 42 + 5 = 47 °C. That is 7 °C above 40 °C, which counts as two 5 °C steps, so the factor is 0.96. Required: 268.12 ÷ 0.96 = 279.29 kVA.
4. The next rating is 320 kVA, 63% loaded, 445.2 A. The biggest block switched on is step 2 at 145.10 kVA, which gives an estimated dip of 6.6%, inside the 15% limit.

For a hospital, a single set leaves no backup for the backup. Read N+1 redundancy before you decide between one 320 kVA set and two smaller sets.

Derating for Indian sites

Generator ratings assume clean, cool air. In Telangana and Andhra Pradesh, heat matters far more than altitude: most of the region sits between 0 and 700 m, but summer peaks of 44–46 °C are common, and an acoustic canopy adds roughly 5 °C to the air the engine breathes.

• Temperature: the tool removes 2% for each 5 °C step, or part of a step, above 40 °C at the air intake.
• Altitude: the tool removes 3% for each 300 m step, or part of a step, above 1000 m. This rarely applies in the Deccan, but it does in hill stations.
• Your maker’s figures win: enter the altitude and temperature factors from the set’s data sheet and the tool uses them instead.

These are screening slopes, not a substitute for the engine data sheet. See the derating assessment tool for a fuller check.

Mistakes that give the wrong size

• Adding nameplate HP and calling it kVA. A 10 HP motor at 0.85 PF and 90% efficiency draws about 9.8 kVA running, and up to 59 kVA starting on DOL.
• Forgetting the starter. The same 7.5 HP borewell pump needs 49 kVA of starting capacity on DOL, but about 25 kVA on a soft starter.
• Treating UPS and drive loads as ordinary loads. Their harmonic current heats the alternator. Undersized sets show voltage distortion and UPS units that refuse to sync.
• Sizing for everything at once when the panel staggers starts. If your AMF panel brings loads on in steps, enter the steps; the answer is often a size smaller.
• Buying far too large. A set that runs below 30% load for long hours wet-stacks: unburnt fuel collects in the exhaust and the engine glazes. The tool warns you when this is likely.
• Ignoring the canopy. A set boxed in a closed room or a tight canopy breathes hotter air than the weather report says.

Load survey worksheet

Print this table and fill one row per load before you use the calculator, or attach it to your enquiry. The figures you need are on each motor’s nameplate and in your panel drawings.

For a site with many loads, the load list builder helps you separate connected load from running load first.

What to send us for a firm recommendation

• This calculator’s printout, or your completed worksheet.
• Photos of the main panel and of the nameplates of the three largest motors.
• Your supply: three-phase or single-phase, and the present transformer or EB connection size.
• Where the set will stand: open yard, canopy, basement or terrace, and the distance to the panel.
• Whether you want a new, rental, used or refurbished set, and whether you need an AMF panel for automatic changeover.

Send these to info@reddygenerators.com or on WhatsApp to 98852 01608. A service engineer replies with a recommended size and, if you ask, a site visit. We cover all of Telangana and Andhra Pradesh and every district within about 300 km of Hyderabad.

When a calculator is not enough

• Sets running in parallel, or any load above 2500 kVA.
• 11 kV generation or step-up transformers, where energising inrush can exceed motor starting.
• Heavy non-linear loads, such as data rooms or large VFD banks, where harmonic distortion must be measured, not estimated.
• Large motors starting from a small set, where frequency dip and engine block-load acceptance matter as much as voltage dip. See the load step and transient screening tool.
• Hospitals, fire systems and other life-safety loads, where the design must be signed off by the responsible electrical engineer.

For these, book a load study. We log the real demand on site before recommending a set.

Basis of the screening values

Every default in the tool follows a published standard or a widely used engineering convention. Where a site has better data, such as a motor’s locked-rotor current or the engine’s own derating table, that data should replace the default.

The three worked examples on this page were run through the same calculation code that powers the calculator, so the tool and the text always agree.

Questions people ask

How do I convert HP to kVA for a generator?

Multiply HP by 0.746 to get kW output, divide by motor efficiency to get input kW, then divide by power factor. A 10 HP motor at 90% efficiency and 0.85 PF draws about 9.75 kVA running. Its starting kVA is that figure times the starter’s multiple.

What size generator do I need for a 7.5 HP borewell pump alone?

On DOL, the pump needs about 49 kVA of starting capacity, so a 50 kVA set is the practical minimum when the pump runs alone. With a soft starter, a 25 kVA set will usually start it. Other loads running at the same time add to both figures.

Why does the calculator suggest a bigger set than my running load?

Usually because one motor’s start needs more kVA than everything running. Open Show the working: if the starting case line is the larger one, a softer starter on that motor, or starting it first, may cut the size.

Is 80% loading a rule?

It is a sensible design margin, not a law. It leaves room for growth, hot days and fuel quality. You can change it in the site settings.

Can a generator be too big?

Yes. Below about 30% load for long periods, diesel engines wet-stack and carbon up. If your normal load is much smaller than your peak, ask about two smaller sets or a regular load-bank run.

Do inverter ACs need a bigger generator?

Inverter ACs start gently, so they add little starting load. But their electronics draw distorted current, so the tool counts them as non-linear and adds an allowance to the running load.

Does the calculator work for single-phase homes and shops?

Yes. Choose single-phase, 230 V in the site settings. The tool then gives single-phase full-load current and can suggest sets from 5 kVA.

Can I buy a set on this answer alone?

Use it to shortlist and to compare quotations. Before you buy, have a service engineer confirm the size against the actual motors, the panel and the set’s data sheet.

About this tool. Method and screening values reviewed by D. Srinivas Reddy, service engineer and Managing Partner, Reddy Generators, in the generator trade since 2001. Results are a first estimate for planning and quotation. Final selection must be checked against the engine and alternator data sheets and approved by the person responsible for the installation.