Home Energy · 7 min read
How Much Electricity Does Your AC Actually Use?
Tonnage, star ratings and ISEER explained — and why the figure on the box rarely matches what lands on your bill.
Published
In most Indian homes with air conditioning, the AC is the single largest line item on the summer bill — frequently more than everything else combined. It is also the appliance whose consumption people estimate least accurately, because the numbers printed on it describe cooling capacity and test-condition efficiency rather than what it will draw in your room.
Tonnage is cooling capacity, not power draw
A one ton air conditioner does not consume one ton of anything. Tonnage is a measure of heat removal, inherited from the amount of cooling once provided by melting a ton of ice in a day. One ton is roughly 3.5 kW of cooling capacity.
The electrical power it draws to deliver that cooling is a different and much smaller number, and the ratio between the two is precisely what the star rating describes.
What star ratings and ISEER actually tell you
ISEER is the ratio of the cooling delivered over a season to the electricity consumed over that season. A higher ISEER means more cooling per unit. Star ratings are bands of ISEER, and the thresholds are revised periodically — which is why a five star model from several years ago may sit at three stars against current bands.
The important caveat is that ISEER is measured against a defined test profile. Your room, your setpoint, your insulation and your usage hours are not that profile. Treat the rating as a reliable way to compare two machines against each other, and an unreliable way to predict your own bill.
A realistic range for power draw
As a rough guide, a 1.5 ton split unit draws somewhere in the region of 1.0 to 1.8 kW while the compressor is running, with efficient inverter models sitting toward the lower end and older fixed-speed units toward the higher. A 1 ton unit scales down roughly proportionally.
These are running figures, not averages. The number that determines your bill is how many hours the compressor actually runs, which is where most estimates fall apart.
Why inverter ACs change the arithmetic
A fixed-speed compressor has two states: on at full draw, or off. It cycles to hold temperature, so its average consumption is the full draw multiplied by the fraction of time it is running.
An inverter compressor varies its speed instead. After pulling the room down to the setpoint it throttles back and runs continuously at reduced power. This is more efficient in most real conditions, but it also means that the instantaneous draw you might measure at any moment tells you very little — it could be at 30 percent or at 100 percent depending on what happened ten minutes earlier.
The setpoint matters more than most people expect
Every degree lower asks the machine to hold a bigger gap against outdoor temperature, and the work required grows with that gap. Raising the setpoint by a couple of degrees is one of the few changes that reduces consumption immediately, measurably, and without buying anything.
- A higher setpoint shortens compressor run time on a fixed-speed unit and lowers steady-state draw on an inverter unit
- Sealing the room matters as much as the setting — an open door or an uninsulated west-facing wall keeps the compressor working
- A dirty filter or blocked outdoor coil raises consumption for the same cooling
Working out what it costs you
The calculation is straightforward: average power in kilowatts, multiplied by hours of use, gives units consumed. Multiply that by your effective tariff to get cost.
So a unit averaging 1.2 kW over six hours a night consumes about 7.2 units, and across thirty nights about 216 units. Whether that is a large addition to your bill depends heavily on which tariff slab those units land in — because in a slab structure, the last units you consume are usually the most expensive ones you consume.
Why that estimate is usually wrong
Every input to that calculation is an assumption. Average power depends on outdoor temperature, setpoint and how well the room holds cold. Hours of use depends on habits nobody records accurately. The effective tariff depends on total household consumption, not the AC alone.
That last point is the practical one. Any change you make to how you run an air conditioner is worth making only if you can see whether it worked, and a monthly bill cannot attribute a change to a single appliance.
Common questions
How many units does a 1.5 ton AC consume per hour?
Roughly 1.0 to 1.8 units per hour while the compressor is running, depending on efficiency, outdoor temperature and setpoint. Inverter models average less because they throttle down after reaching the set temperature rather than cycling on and off at full power.
Does raising the AC temperature by a degree really save electricity?
Yes. The compressor works harder the larger the gap between the setpoint and outdoor temperature, so a higher setting reduces run time on a fixed-speed unit and reduces steady-state draw on an inverter unit. It is one of the few savings you get without spending anything.
Is an inverter AC always cheaper to run?
In most real usage it is, because it avoids the repeated full-power restarts of a fixed-speed compressor and can run continuously at reduced output. The advantage is largest with long run times in a well-sealed room, and smallest with brief use where the unit spends most of its time pulling the room down.
