ENERGY STAR sizes a 270 square foot bedroom at 7,000 BTU per hour. Its own buying advice adds a sentence that is easy to skip: an oversized air conditioner costs more to purchase, wastes energy and does not provide better cooling. Worse, a unit that is too large cools the room before it has had time to remove the humidity, and the air ends up damp and clammy. So the bigger box on the shelf is not the safe choice. It is a different mistake.
This article follows one small project from start to finish: an apartment with three rooms to cool and light before a summer. Every number comes from a calculator run with the inputs listed below, so you can repeat each step with your own rooms. The rate of $0.20 per kWh is the example rate of this project, not a national figure; swap in the one on your bill.
The project: three rooms, one summer
The apartment has a bedroom with west-facing windows, a separate kitchen and a square living room. Three people sleep or work in the bedroom, and a desktop computer drawing about 200 W runs there in the evenings. The cooling season is planned at 8 hours a day for 100 days.
| Room | Size | Floor area | What matters |
|---|---|---|---|
| Bedroom | 15 x 18 ft | 270 sq ft | west windows, three people, a 200 W computer |
| Kitchen | 12 x 14 ft | 168 sq ft | cooking, two people |
| Living room | 20 x 20 ft | 400 sq ft | average sun, two people |
The brief was simple: one room air conditioner per room, sized properly, the cheapest to run that makes sense, and enough light in each room once the blinds go down in the afternoon.
Step by step
Stage 1: the first idea, 12,000 BTU everywhere
The first plan on the table was three identical 12,000 BTU units, one per room, on the theory that a bigger number is safer in July. It is also the plan the rest of this article takes apart. A 12,000 BTU unit is 1 ton of cooling, or 3.52 kW. On the ENERGY STAR chart that capacity belongs to rooms of 450 to 550 sq ft, and none of the three rooms is that big.
Stage 2: each room on the ENERGY STAR chart
The air conditioner size calculator takes the floor area, finds the chart row and then applies the four corrections ENERGY STAR lists: minus 10% for a heavily shaded room, plus 10% for a very sunny one, 600 BTU for each person beyond two and 4,000 BTU for a kitchen. The bedroom also has the computer, which the calculator counts at 3.412 BTU per hour for every watt.
| Room | Chart row | Corrections | Capacity needed |
|---|---|---|---|
| Bedroom, 270 sq ft | 7,000 | very sunny +700, one extra person +600, 200 W +682 | 8,982 BTU |
| Kitchen, 168 sq ft | 6,000 | kitchen +4,000 | 10,000 BTU |
| Living room, 400 sq ft | 10,000 | none | 10,000 BTU |
The bedroom is the interesting one. Its chart row says 7,000, but sun, a third person and the computer add up to almost 2,000 BTU more, so it lands at 8,982. The closest size on sale is around 9,000. The kitchen is small, 168 sq ft, yet needs as much as the 400 sq ft living room, because cooking adds a flat 4,000 BTU. Had the living room been very sunny, it would have moved to 11,000.
Stage 3: what a summer of cooling costs
Size decides comfort; the efficiency rating on the yellow EnergyGuide label decides the bill. The air conditioner electricity cost calculator divides the BTU by that rating to get the watts the unit draws, then multiplies by the hours and the rate. A 10,000 BTU unit rated 10 draws 1,000 W; rated 12.5 it draws 800 W. Over 800 hours of compressor time that is 800 kWh against 640 kWh.
| Unit | Rating 10 | Rating 12.5 | Saving a season | $60 more pays back in |
|---|---|---|---|---|
| Bedroom, 9,000 BTU | $144.00 | $115.20 | $28.80 | 2.1 seasons |
| Kitchen, 10,000 BTU | $160.00 | $128.00 | $32.00 | 1.9 seasons |
| Living room, 10,000 BTU | $160.00 | $128.00 | $32.00 | 1.9 seasons |
| Whole apartment | $464.00 | $371.20 | $92.80 |
A better rating saves about $29 to $32 a season per unit here, and $92.80 across the apartment. If the more efficient model costs $60 more, it pays that back in about two seasons. The calculator assumes the compressor runs all the hours you enter, which is the upper bound; a unit that reaches the set temperature and cycles off costs less, and a plug-in meter reading is the honest way to find out by how much.
Why not just keep the 12,000 BTU units and run them fewer hours? Because the oversized unit is the one ENERGY STAR warns about: it reaches the set temperature fast, stops, and leaves the moisture in the air. The cost table cannot show that, and that is why sizing came before the bill in this plan.
Stage 4: the light in the same rooms
Lighting looked like an afterthought until the numbers came in. The lumens calculator uses the light levels that IES residential guidance gives for each room, as listed in Penn State's energy course: 10 to 20 foot-candles for living rooms and bedrooms, 30 to 40 for general kitchen light, 50 to 100 over the stove and sink. One foot-candle is one lumen per square foot, so the floor of the bedroom needs 2,700 to 5,400 lumens. Lamps lose part of their light in shades, fixtures and on walls; with typical planning factors of 0.7 and 0.8 (an assumption, not a rule from that table) the lamps should give 4,821 to 9,643.
| Space | Level | On the floor | From the lamps | 800 lm bulbs | LED watts |
|---|---|---|---|---|---|
| Bedroom, 270 sq ft | 10 to 20 fc | 2,700 to 5,400 | 4,821 to 9,643 | 7 to 13 | 54 to 108 |
| Kitchen, 168 sq ft | 30 to 40 fc | 5,040 to 6,720 | 9,000 to 12,000 | 12 to 15 | 101 to 135 |
| Counter by the stove and sink, 24 sq ft | 50 to 100 fc | 1,200 to 2,400 | 2,143 to 4,286 | 3 to 6 | 24 to 48 |
| Living room, 400 sq ft | 10 to 20 fc | 4,000 to 8,000 | 7,143 to 14,286 | 9 to 18 | 80 to 161 |
The kitchen is the surprise: its general light needs more lumens than the much larger living room at the low end, 9,000 against 7,143, simply because the level is three times higher. And the counter by the stove and sink, only 24 sq ft, wants up to 4,286 lumens of its own. That is not a reason to light the whole kitchen to the task level; Penn State's course calls increasing the light everywhere a waste of energy. It is a reason for under-cabinet lights on the counter and a calmer level everywhere else.
The settlement: first idea against the calculated plan
Put the first plan next to the calculated one and the difference is not small. The three identical units would have installed 36,000 BTU of cooling where the chart asks for about 29,000.
| Room | First idea | Calculated | Difference |
|---|---|---|---|
| Bedroom | 12,000 BTU | 8,982 BTU | 3,018 BTU too much |
| Kitchen | 12,000 BTU | 10,000 BTU | 2,000 BTU too much |
| Living room | 12,000 BTU | 10,000 BTU | 2,000 BTU too much |
| All three | 36,000 BTU | 28,982 BTU | 7,018 BTU too much |
Every row of that table is oversizing of the kind ENERGY STAR warns about. The bedroom is the worst case even after its sun, people and computer are counted. The lighting went the other way: the kitchen alone needs 12 to 15 bulbs of 800 lm for its general light, before the counter gets its own.
Four lessons from three rooms
Lesson 1: the chart row is only the starting point
In the bedroom the corrections were worth almost 2,000 BTU, more than a quarter of the chart value. Sun, people and equipment are where a sizing estimate goes wrong, so they deserve the most honest answers on the form.
Lesson 2: bigger is a different mistake, not a safer one
ENERGY STAR is blunt about it: an oversized unit costs more, wastes energy and leaves the room clammy. Pick the size closest to the calculated figure, not the next one up.
Lesson 3: the rating pays, but on a scale set by hours
Going from a rating of 10 to 12.5 cut the cooling bill by a fifth, $92.80 a season for the whole apartment at 800 hours. Run the units half as long and the saving halves too, so the payback of a pricier model depends more on your summer than on the label.
Lesson 4: the kitchen counts twice
It needed 4,000 BTU of extra cooling for the cooking and three times the light level of the bedroom, plus a task level over the counter. In both calculators, the smallest room was the most demanding per square foot.
Starting over
If this apartment were planned again from zero, the order would be the same as this article, only without Stage 1. Measure the rooms and size each unit from the chart and its corrections; compare ratings with your own hours and rate before buying; then plan the light room by room, with the counter as its own zone. The plan that comes out is smaller than the bigger-is-safer guess, and by ENERGY STAR's own reasoning cheaper to buy, cheaper to run and less clammy.
Tools discussed in this article
- Air Conditioner Size Calculator - BTU for a room from its dimensions or floor area, with the ENERGY STAR chart and its sun, occupant and kitchen corrections, in BTU, kW and tons.
- Air Conditioner Electricity Cost Calculator - cost per hour, day, 30 days and season from the BTU and the CEER or EER on the label, and the payback of a more efficient unit.
- Lumens Calculator - lumens for a room from its floor area and the recommended foot-candles, with losses in fixtures, the number of bulbs and the LED watts.
Ready-made calculations from this article
How many BTU for a room of this size? 100 sq ft, 150 sq ft, 200 sq ft, 250 sq ft, 300 sq ft, 350 sq ft, 400 sq ft, 450 sq ft, 500 sq ft, 550 sq ft, 600 sq ft, 650 sq ft, 700 sq ft, 750 sq ft, 800 sq ft, 850 sq ft, 900 sq ft
What size AC for these rooms? 12x12 room, 10x12 room, 12x14 room, 14x14 room, 15x15 room, 16x16 room, 24x24 room, 40 m², 50 m²
How many watts does this air conditioner use? 5,000 BTU, 6,000 BTU, 8,000 BTU, 10,000 BTU, 12,000 BTU, 14,000 BTU, 15,000 BTU
How many lumens for this room? 12x12 bedroom, 10x10 bedroom, 10x10 kitchen, 12x12 kitchen, 20x20 garage, 24x24 garage
Lumens per square foot by room: bedroom, kitchen, bathroom, garage, home office
Foot-candles to lumens: 10 fc, 20 fc, 30 fc, 50 fc, 100 fc
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