Generators Under 15 kVA

In short. A generator under 15 kVA is chosen by the hardest load it must start, not by the total wattage of everything connected. In this class a single borewell pump, refrigeration compressor or air-conditioner can demand more at start-up than the whole running load, and single-phase 230 V versus three-phase 415 V decides the whole installation. Define the load, the phase, the runtime and whether automatic start is needed, and the right set follows from a short list of candidates.

Where small generators are used

Below 15 kVA the typical buyers in and around Hyderabad are households wanting lights, fans and a lift, clinics and diagnostic centres, shops and small offices, petrol pumps, ATMs, telecom shelters, poultry sheds, dairy chilling points and farms with a borewell. The loads look modest but are rarely resistive, and the set has no spare capacity to absorb a bad assumption.

These sets are usually water-cooled single, two or three cylinder diesel engines at 1,500 rpm with a brushless alternator, supplied in a canopy that meets the CPCB noise limit. All new diesel sets up to 800 kW, including this class, must meet CPCB IV+ emission norms, so a new set carries the certification mark and the price reflects the electronic fuel system it needs.

Define the load before looking at catalogues

• Single-phase or three-phase supply at the premises, because a three-phase set feeding single-phase loads must have those loads balanced across its phases.
• The largest individual load and how it starts, particularly pumps, compressors and air-conditioners.
• Whether there are electronic loads such as computers, medical equipment or inverters that need stable voltage and frequency.
• Required runtime per outage and per day, which sets the fuel tank and the duty rating.
• Portable, trolley-mounted or stationary installation.
• Manual start, or automatic start and changeover through an AMF panel.

The load inventory page gives a simple format for listing loads, and it is worth doing even for a 7.5 kVA set.

Why total wattage misleads

A 6 kW resistive load and a 6 kW motor-driven load do not create the same generator requirement. A direct-on-line induction motor draws roughly six to seven times its full-load current for the first second, and a small generator must supply that from an alternator with limited transient capability. Single-phase capacitor-start motors can be worse. A compressor that restarts against pressure after a short outage starts under load and takes longer to accelerate.

A set in this class can typically accept a single step of 70 to 100 percent of its rating if the step is mostly resistive, but a motor start that momentarily demands twice the set’s kVA pulls voltage down far enough to trip electronic loads or stall the motor. The method for checking this is on the motor starting page.

Current, phase and the arithmetic

I = (kVA × 1000) / V for single phase, and I = (kVA × 1000) / (√3 × V) for three phase

Here I is the rated current in amperes, kVA is the generator rating and V is the line voltage, 230 V single-phase or 415 V three-phase in India. A 7.5 kVA single-phase set delivers about 32.6 A, and a 10 kVA three-phase set delivers about 13.9 A per phase (illustrative). The three-phase figure explains a common disappointment: a 10 kVA three-phase set cannot run a 7 kVA single-phase load, because each phase is rated for one third of the total. If most loads are single-phase, buy a single-phase set, or a three-phase set with a manufacturer-rated single-phase output, and balance the rest. The single-phase generator page explains the trade-offs. Converting kW to kVA uses the power factor, and 0.8 is the usual assumption for a mixed small load, so a clinic with 6 kW of measured demand needs 7.5 kVA of running capacity before any starting allowance.

Runtime, fuel and duty

Small sets consume roughly 0.3 to 0.4 litres of diesel per kWh delivered at useful loads, which is higher per unit than larger engines. A 10 kVA set carrying 6 kW uses around 2 to 2.5 litres per hour (illustrative), so a base tank of 25 to 40 litres gives a working day. Running a diesel set for long periods below about 30 percent load causes incomplete combustion, carbon build-up and oil dilution, so a set bought for one 5 HP pump should not idle all day feeding two tube lights. For telecom sites that run many hours a day, ask for the prime rating rather than the standby rating and plan service by hours run, as set out on maintenance planning.

Automatic start and installation

An AMF panel senses loss of DISCOM supply, starts the set, transfers the load after warm-up and transfers back when supply returns. For lifts, clinics and shelters it is worth the cost; for a shop it may not be. The changeover must be interlocked so the set can never feed back into the DISCOM network. Even a small canopied set needs a level base, clearance for cooling air and service, an exhaust that does not discharge toward a window, an earth electrode connected to the alternator frame and neutral as the supply authority requires, and correctly sized cables with an MCB or MCCB at the set. For a one-off event, see small rental generators.

Common mistakes

• Adding up appliance wattages and buying the next size. The borewell pump will not start, or it starts and drops the computers every time.
• Buying a three-phase set for a single-phase house because it was available. Only one third of the rating is usable on any phase and the set runs badly unbalanced.
• Running the set for hours at a tenth of its load. Carbon builds up, the exhaust smokes and the engine wears early.
• Omitting the earth connection on a portable set. A fault on an appliance leaves the metalwork live with no path to trip the breaker.
• Connecting the set with a plug and socket rather than a changeover. Back-feed endangers DISCOM linemen and the set can be destroyed when supply returns.

What to send us

Send a list of the loads you want to run together, the rating and starting type of the largest motor or compressor, whether the premises has a single-phase or three-phase connection, the hours per day the set will run and a photograph of the proposed location. If you have an existing set, a nameplate photograph tells us the make, rating and phase. Submit it through the generator sales enquiry form and we will recommend a size and a changeover arrangement.

Questions engineers ask

Generators by rating

• 15–50 kVA generators
• 50–125 kVA generators
• 125–250 kVA generators
• 250–500 kVA generators
• 500–1000 kVA generators
• 1–2 MVA generators
• 2 MVA and above generators

Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.