The arithmetic: sizing, running cost, and what SACC means

This section is the reference material the rest of the site is built on. Every cost figure, every sizing recommendation and every capacity claim elsewhere on Fan & Frost comes from one of the formulas here — and all of them are published rather than asserted, so you can check any number with a calculator.

That reproducibility is the whole point. We have not tested any of these products, and we say so on every page. What we do instead is compile published specifications and do arithmetic in public. A site you can check is worth more than a site that tells you it has a lab.

(watts ÷ 1000) × hours × your rate per kWh

We use 18.34¢/kWh, the US average residential price of electricity for June 2026, from the EIA Electric Power Monthly, Table 5.3. It is cited and dated on every block that uses it and refreshed monthly.

Your rate is almost certainly not the national average. US residential rates vary by more than a factor of two between states, which is precisely why we print the formula next to every answer rather than the answer alone. Take the rate off your own bill and substitute it.

Worked once, so the tables elsewhere are obvious:

• A 49 W fan for 8 hours is 49 ÷ 1000 × 8 = 0.392 kWh. At 18.34¢/kWh that is about 7.2¢. Thirty nights: $2.16.
• A 1,300 W portable air conditioner for the same 8 hours is 10.4 kWh — about $1.91, or $57 over thirty nights.

One important refinement. Where ENERGY STAR publishes a certified annual kWh figure — as it does for every certified window air conditioner — we use that instead of a nameplate calculation. A variable-speed compressor spends most of its life well below nameplate, so multiplying nameplate by hours would substantially overstate the bill. The certified figure already models a standard cooling season.

Not a formula so much as a table, and it is the official one. Most pages online give you “20 BTU per square foot”, which is close enough in the middle of the range and meaningfully wrong at both ends.

The relationship is not linear: 150 sq ft works out at about 33 BTU per square foot, while 2,500 sq ft is about 14. Small rooms need proportionally more, because the walls, ceiling and windows — where heat actually enters — do not shrink as fast as the floor area does. Since small rooms are the common case, the rule of thumb misleads most people who use it.

Then four adjustments, all quoted verbatim from ENERGY STAR:

• Heavily shaded room: reduce capacity by 10 percent
• Very sunny room: increase capacity by 10 percent
• More than two regular occupants: add 600 BTU for each additional person
• Used in a kitchen: increase capacity by 4,000 BTU

The kitchen adjustment is the one people skip and the largest. An oven and cooktop are space heaters that happen to make dinner, and a 250 sq ft open-plan kitchen-diner needs 11,000 BTU rather than 7,000.

Do not round up to be safe. ENERGY STAR: an oversized air conditioner “costs more to purchase, wastes energy and does not provide better cooling.” It reaches temperature before it has removed humidity, shuts off, and leaves the room cold and clammy. The full walkthrough with four worked rooms is here.

A portable air conditioner’s box figure is measured under a standard that ignores the heat its exhaust duct radiates back into the room and the outdoor air that infiltrates to replace what it blew out. SACC — the DOE-rated Seasonally Adjusted Cooling Capacity, defined at 10 CFR part 430, appendix CC — subtracts both.

Size a portable from SACC, not from the box. And note that the reduction is not a fixed ratio, which is the part almost nobody publishes: two units both sold as 14,000 BTU are DOE-rated at 12,000 and 9,500 by their own manufacturers — a 14% haircut against a 32% one. The mechanism and the evidence are here.

Every figure on this site is one of three things — a published manufacturer specification, an official standards or certification figure, or arithmetic we performed and showed. There is no fourth category. These are the official sources:

• ENERGY STAR, Room Air Conditioners — the sizing table and its four adjustments.
• ENERGY STAR Certified Room Air Conditioners dataset — tested CEER, modeled annual kWh, dimensions, weight, refrigerant. 392 US models available from 2024 onward.
• ENERGY STAR Certified Ceiling Fans dataset — certified CFM per watt, per-speed and standby wattage. 987 US models from 2023 onward.
• 10 CFR part 430, subpart B, appendix CC — the DOE test procedure defining SACC.
• EIA Electric Power Monthly, Table 5.3 — the electricity rate in every cost calculation.

Two gaps on this site are deliberate, and both are worth knowing about because they are the kind of thing other pages fill in with a guess.

The SACC weighting factors. SACC is a weighted average of adjusted cooling capacity measured at two test conditions. Two independent attempts to confirm those weighting factors and test temperatures against the primary regulation returned conflicting figures, and the government sites that hold the authoritative text block automated access. So the SACC page publishes the mechanism and cites appendix CC, and states no weighting number at all.

Wattages nobody publishes. Vornado publishes no wattage or CFM for its air circulators; Lasko publishes none for its 18-inch pedestal fan; Midea publishes none for the Duo portable AC. Where that happens, our spec tables say “not published” and no running cost is computed. We would rather have a visible hole in a table than a plausible-looking number nobody stated.

If you can point us at a published figure we recorded as missing, we would genuinely like to hear it — those blanks are the least satisfying thing on the site.