AC Tonnage Calculator
Tonnage from the actual heat gain — area, roof, glass, occupants and equipment — then the electrical side of the answer: running current, starting surge and what it costs to run.
Room
Glass and gains
Unit and running cost
- Envelope 60 %
- Glass 20 %
- People 13 %
- Equipment 7 %
A simplified heat-gain method, calibrated for Indian design conditions. Right for one room and one split unit; a commercial building needs an hourly load calculation.
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AC Tonnage Calculator · Heat gain method · 1 TR = 12,000 BTU/hr · EnergyCalcHQ · energycalchq.com
Preliminary calculation. The figures behind it are representative values for the stated conditions, not a substitute for the current edition of the standard or the manufacturer's published data. Verify before issuing for construction. Not a substitute for a qualified engineer or a protection study.
For page numbers, keep Headers and footers ticked under More settings in the print dialog.
Why “one ton per 100 square feet” misses
It is the rule every showroom quotes, and it is wrong in both directions at once. A shaded ground-floor bedroom in Pune and a top-floor living room in Chennai with a west window can have the same floor area and cooling loads that differ by more than double. Area is one term of five.
The four that the rule ignores are the ones you can do something about. An exposed roof adds roughly 30 %. West-facing glass gains three times what shaded north glass does, square foot for square foot. Four people in a room add 2,400 BTU/hr — a fifth of a ton — and a rack of equipment adds its full wattage as heat, because every watt drawn ends up in the room.
Read the split, not just the total
The bar showing where the heat comes from is the more useful output. If glass is a third of your load, an external shade or a solar film is cheaper than the next size up and it keeps saving after the unit is in. If the envelope dominates on a top floor, roof insulation pays back across every summer the building stands.
Oversizing is a real fault, not a safety margin
An air conditioner that is too large cools the room quickly and switches off before it has removed the moisture. The result is a space that is cold and clammy, with the compressor short-cycling — which wears it out and wastes energy on every restart. Humid coastal locations suffer this badly.
This is why the tool reports the exact tonnage alongside the standard size, and how far the rounding pushed you over. If a 1.0 ton unit is 40 % more than the room needs, look at whether 0.8 ton fits before accepting the round number.
The electrical side, which is what actually gets specified wrong
A 1.5 ton fixed-speed unit draws around 1,700 W, about 8 A at 230 V — and roughly 40 A for a second or two every time the compressor starts, because it is thrown directly across the line and pulls locked-rotor current until it reaches speed.
That transient is what decides three things the tonnage figure never mentions: the breaker curve, the cable size, and whether an inverter or generator can carry the unit at all. It is the most common reason an installation that looked correct on paper trips whenever the compressor cuts in. Feed the running figure into the load calculator to see it against the rest of the house, and the breaker sizing calculator for the protection.
An inverter compressor ramps instead of slamming, so its surge is nearer twice running current than five times. That, rather than the star rating, is usually the stronger argument for one on a weak supply.
How far to trust this
The envelope figures are per-square-foot heat gains calibrated against Indian design conditions and accepted practice, not a full ISHRAE or ASHRAE hourly cooling-load calculation. For one room and one split unit that is the right level of detail, and it will land you on the correct standard size. Anything centrally air conditioned, or any building where the plant is a capital decision, needs a proper hourly load study with real wall constructions and a weather file.
What the star label measures, and what it does not
Indian air conditioners are rated by ISEER — the Indian Seasonal Energy Efficiency Ratio — which divides the total cooling delivered over a notional season by the electricity consumed to deliver it. That seasonal framing is the important part. The older EER measured a single operating point at full load, and a machine optimised for that point can be mediocre everywhere else.
ISEER assumes a 1600-hour cooling season and a temperature distribution drawn from Indian conditions, weighting part-load performance heavily because that is where the hours actually are. A unit spends very little of its life at the design outdoor temperature and most of it somewhere well below.
Two caveats worth carrying. The star thresholds are periodically tightened, so a five-star unit from several years ago may rate three stars under the current table without anything about the machine having changed. And the test conditions assume a correctly sized, correctly installed unit with a clean filter and a sensible refrigerant charge — none of which the label can promise about the one on your wall.
Inverter or fixed speed, at the load you will actually run
A fixed-speed compressor has two states. It runs at full output until the room passes the set point, stops, and restarts when the room drifts back. Every restart draws locked-rotor current, and the room temperature swings either side of the thermostat by a degree or more.
An inverter unit varies compressor speed instead, settling at whatever output matches the heat entering the room. It reaches set point faster because it can overshoot upward at start, then throttles down and stays there. Running continuously at 40 per cent is substantially more efficient than cycling between 100 and 0 for the same average, because the heat exchangers are oversized for the reduced load and the compressor is not paying the restart penalty repeatedly.
Which decides the sizing argument on this page. An oversized fixed-speed unit short-cycles, and short cycling means the coil never runs long enough to condense much moisture — a cold, clammy room and a high bill. An oversized inverter unit simply runs slower, so it tolerates a sizing error far more gracefully. It does not make oversizing free: the machine still costs more, and every inverter has a minimum speed below which it cycles like any other.
Questions people ask
- Is one ton per 100 square feet a reliable rule?
- It is wrong in both directions at once. A shaded ground-floor bedroom in Pune and a top-floor living room in Chennai with a west window can have the same floor area and cooling loads that differ by more than double. Area is one term of five, and the other four are the ones you can do something about: an exposed roof adds roughly 30 per cent, west-facing glass gains three times what shaded north glass does square foot for square foot, four people add 2,400 BTU/hr — a fifth of a ton — and equipment adds its full wattage as heat, because every watt drawn ends up in the room.
- Is a slightly oversized air conditioner the safe choice?
- No — oversizing is a real fault, not a margin. A unit that is too large cools the room quickly and switches off before it has removed the moisture, so the space ends up cold and clammy while the compressor short-cycles, wearing itself out and wasting energy on every restart. Humid coastal locations suffer this badly. That is why the exact tonnage is reported alongside the standard size and how far the rounding pushed you over: if a 1.0 ton unit is 40 per cent more than the room needs, look at whether 0.8 ton fits first.
- What current does a 1.5 ton air conditioner draw?
- A fixed-speed unit draws around 1,700 W, about 8 A at 230 V — and roughly 40 A for a second or two every time the compressor starts, because it is thrown directly across the line and pulls locked-rotor current until it reaches speed. That transient is what decides the breaker curve, the cable size and whether an inverter or generator can carry the unit at all. It is the commonest reason an installation that looked correct on paper trips whenever the compressor cuts in.
- Inverter or fixed speed on a weak supply or an inverter backup?
- Inverter, and the reason is the surge rather than the star rating. A fixed-speed compressor has two states: full output until the room passes the set point, then off, with every restart drawing locked-rotor current. An inverter compressor ramps instead, so its starting surge is nearer twice running current than five times. On a generator, a home inverter or a weak rural supply that is usually the stronger argument for one — the efficiency is a bonus rather than the deciding factor.
- What does the star rating actually measure?
- ISEER — total cooling delivered across a notional 1,600-hour season divided by the electricity used to deliver it. The seasonal framing is the point: the older EER measured one operating point at full load, and a machine optimised for that point can be mediocre everywhere else, while ISEER weights part-load performance heavily because that is where the hours are. Two caveats worth carrying. The thresholds are periodically tightened, so a five-star unit from several years ago may rate three stars under the current table without anything about it having changed. And the test assumes a correctly sized, correctly installed unit with a clean filter and a sensible refrigerant charge, none of which a label can promise.