Keeping cool in Chennai is non‑negotiable, but nobody wants bill shock. This practical guide shows you how to pick the right AC tonnage for your room, then calculate real‑world electricity costs in minutes. You will also find simple tips to cut power usage without sacrificing comfort.

Step 1: Pick the right AC tonnage for your room

Choosing correct capacity avoids two costly mistakes. Undersizing makes the AC run harder and longer, oversizing causes short cycling that wastes energy and leaves air humid.

Quick room‑size to tonnage guide for Chennai

Use this as a starting point, then adjust for sun exposure, insulation, occupancy and floor level.

Room size (sq ft)	Typical AC size	Go higher if	Go lower if
90 to 110	1.0 ton	West facing, top floor, many gadgets	Well shaded, low sun, excellent insulation
111 to 150	1.0 to 1.2 ton	Large windows, 3 to 4 people often	Shaded, 2 people, false ceiling insulation
151 to 200	1.5 ton	Open kitchen nearby, top floor, heavy sun	Minimal sun, fewer occupants, good insulation
201 to 260	2.0 ton	High ceilings above 10 ft, glass walls	Highly insulated, mostly evening use

Rule of thumb for apartments with standard 9.5 to 10 ft ceilings in Chennai: tonnage ≈ room area ÷ 120. For example, 150 sq ft ÷ 120 ≈ 1.25 ton. If a 1.2 ton model is not available, step to 1.5 ton. For villas, high ceilings or large glass areas, a professional heat‑load survey is recommended.

If you are comparing brands for a new purchase, our local guide on Chennai’s top performers can help you shortlist options: Best AC brand for Chennai heat 2025.

Step 2: Convert capacity to power usage

Two numbers on the label tell you how much electricity an AC is likely to use.

• Cooling capacity, shown as tons, BTU per hour, or kW. 1 ton equals 12,000 BTU per hour, which is 3.517 kW of cooling.
• Efficiency rating, ISEER or EER. Higher is better. In India, the BEE star label shows ISEER and an indicative annual electricity consumption. See the official program at BEE Star Label.

Basic formula for estimated power draw at rated conditions:

• kW input = cooling capacity in kW divided by ISEER

Example for 1.5 ton at ISEER 4.7: 1.5 ton equals 5.275 kW cooling. 5.275 ÷ 4.7 ≈ 1.12 kW at rated load.

Inverter ACs throttle down after reaching set temperature, so real‑world average power is usually 50 to 70 percent of the rated input depending on setpoint, insulation and usage.

Step 3: Turn kW into rupees

Use your electricity tariff from your TANGEDCO bill. Domestic tariffs in Tamil Nadu are slab based, so actual cost per unit varies by consumption band. Check the latest tariff schedule on the official site, TANGEDCO.

Step by step:

1. Find cooling capacity, convert tons to kW: cooling kW = tons × 3.517.

2. Estimate rated input: kW input = cooling kW ÷ ISEER. If your nameplate shows rated power in watts, use that number instead.

3. Adjust for real usage, especially for inverter ACs: average kW ≈ rated kW × duty cycle (0.5 to 0.7 for well sized inverter units, 0.7 to 1.0 for non‑inverter units).

4. Monthly units = average kW × hours per day × 30.

5. Monthly cost = monthly units × your ₹ per kWh.

Real‑world examples at ₹8 per kWh

These are illustrations. Replace with your actual tariff and hours.

Scenario	Input maths	Monthly units	Estimated monthly cost
1 ton, 5‑star (ISEER 5.0), 8 hours per day, inverter duty 0.6	Rated kW = 3.517 ÷ 5.0 = 0.70, average kW ≈ 0.70 × 0.6 = 0.42	0.42 × 8 × 30 ≈ 101 kWh	₹810
1.5 ton, 3‑star (ISEER 3.7), 10 hours per day, inverter duty 0.7	Rated kW = 5.275 ÷ 3.7 ≈ 1.43, average kW ≈ 1.43 × 0.7 ≈ 1.00	1.00 × 10 × 30 ≈ 300 kWh	₹2,400
2 ton, non‑inverter, EER 3.2, 12 hours per day, duty 0.9	Rated kW = 7.034 ÷ 3.2 ≈ 2.20, average kW ≈ 2.20 × 0.9 ≈ 1.98	1.98 × 12 × 30 ≈ 712 kWh	₹5,696

Quick reference, hourly running cost at rated load

Worst‑case figures, before inverter throttling, using ₹8 per kWh.

Tonnage	Example efficiency	Rated kW input	Cost per hour
1.0 ton, ISEER 3.6 (approx 3‑star)	3.517 ÷ 3.6 = 0.98 kW	₹7.82
1.0 ton, ISEER 4.7 (approx 5‑star)	3.517 ÷ 4.7 = 0.75 kW	₹5.99
1.5 ton, ISEER 3.6	5.275 ÷ 3.6 = 1.47 kW	₹11.72
1.5 ton, ISEER 4.7	5.275 ÷ 4.7 = 1.12 kW	₹8.99
2.0 ton, ISEER 3.6	7.034 ÷ 3.6 = 1.95 kW	₹15.63
2.0 ton, ISEER 4.7	7.034 ÷ 4.7 = 1.50 kW	₹11.97

Note, in practice inverter ACs consume less than the rated value once the room reaches the set temperature.

How to read the BEE label and nameplate correctly

• Check ISEER and the star rating. Higher ISEER means lower electricity for the same cooling.
• Look for Annual Energy Consumption on the label. This is a standardised estimate for typical usage. Use it as a benchmark between models.
• On the indoor or outdoor unit nameplate, note the rated power input in watts. This is your quickest number for cost maths if ISEER is not available.

For more on star labels and testing methods, visit the Bureau of Energy Efficiency’s portal, BEE Star Label.

7 proven ways to cut your AC electricity cost in Chennai

• Set at 24 to 26°C, not 18 to 20°C. The Ministry of Power recommends 24°C as a default setpoint for efficiency. Even a 1°C higher setting can save meaningful units over a month. See MoP’s advisory via the Government of India’s Press Information Bureau.
• Use Dry mode on very humid days when you need dehumidification more than rapid cooling.
• Clean filters every 2 to 4 weeks during peak season. Clogged filters can add 5 to 15 percent to power draw.
• Shade the outdoor unit and keep coils clean. Poor airflow and dirty coils force higher compressor load.
• Seal gaps around doors and windows, use curtains or reflective blinds in west‑facing rooms.
• Pair the AC with a ceiling fan at low speed. You can raise the setpoint by 1 to 2°C and feel the same comfort.
• Service the AC before summer. A preventive check catches low refrigerant, failing capacitors and blocked drains. If you prefer worry‑free upkeep, consider an AC AMC in Chennai.

Multi‑room or office? Consider VRF for lower lifecycle costs

If you are cooling several rooms or floors, a properly designed VRF or VRV system can match load precisely across zones, which reduces wastage, improves humidity control and often lowers long‑term energy costs compared to multiple standalone units. Explore options with a specialist: VRF and VRV AC dealer in Chennai.

When capacity is wrong, bills suffer

• Too small, the AC runs flat out, takes longer to dehumidify and may never reach setpoint on very hot days.
• Too large, the AC short cycles, air stays clammy and inverter benefits are reduced because the compressor does not get enough steady run time at efficient part load.

If you are unsure, ask for a quick heat‑load assessment. Techno Power Aircons has been advising homes and offices in Chennai since 1982 and can size, install and maintain systems across leading brands. If you are price‑shopping, you can also check ongoing offers here: Low price AC in Chennai.

Frequently Asked Questions

How do I quickly estimate the tonnage for my bedroom or living room? Start with room area divided by 120 for standard 9.5 to 10 ft ceilings. Adjust up for strong sun, top floor, more people or heat‑generating appliances. For example, 150 sq ft ÷ 120 ≈ 1.25 ton, so pick 1.2 or 1.5 ton depending on sun and usage.

How many units does a 1.5 ton AC consume per hour? At rated conditions, a 1.5 ton 3‑star unit might draw about 1.4 to 1.5 kW, and a 5‑star unit around 1.1 kW. Multiply by your tariff to get cost per hour. In real use, an inverter often averages 50 to 70 percent of its rated draw once the room reaches setpoint.

What is ISEER and why does it matter? ISEER is the Indian Seasonal Energy Efficiency Ratio. It measures cooling output over a season divided by energy input over that season. Higher ISEER means lower electricity for the same cooling. It is displayed on the BEE star label.

Do inverter ACs always save money? Yes when correctly sized and used at sensible setpoints. They run the compressor at variable speeds to match load, which avoids wasteful on‑off cycles. Savings fall if the unit is oversized or if you routinely set very low temperatures.

Does setting 24°C really reduce bills compared to 20°C? Yes. Each degree higher reduces compressor workload, so units consumed drop over time. The Ministry of Power has recommended 24°C as a default for energy efficiency.

Is a stabiliser required in Chennai? Many modern inverter ACs work across a wide voltage band. If your area has frequent low or high voltage events, a branded stabiliser with the right kVA rating protects electronics and can prevent expensive failures. Check your model’s manual or ask your dealer.

How do I get an accurate running‑cost estimate for my home? Note your room sizes, usage hours, preferred setpoint and tariff slab, then share the AC model’s ISEER or rated input with a dealer. A quick load check plus your star label data gives a precise monthly estimate.

Get a right‑sized AC and a predictable bill

If you want a no‑guesswork sizing and running‑cost estimate tailored to your home or office in Mylapore or anywhere in Chennai, talk to Techno Power Aircons. Since 1982 we have helped customers choose the correct tonnage, install it properly and keep it efficient with dependable after‑sales support and AMC options. We carry leading brands and advanced systems including VRF and VRV.

Contact us to schedule a consultation, and ask about current offers such as zero‑cost EMI and seasonal discounts. When your AC is sized right and installed right, comfort is high and electricity costs stay under control.