The figure register: sources, labels and dates

Every figure carries exactly one of three labels, and the distinction between them is the most important thing on this page.

sourced means the number is printed in the cited document, either in a sentence (quoted exactly) or in a table (shown as that row’s cells as printed, transcribed in reading order). The quote, the address and the date the source was first read are recorded for each sourced figure, and on 5 October 2026 every cited document was fetched again and saved byte for byte. Where the live page refuses automated readers, the figure was read from a named Internet Archive copy of that exact address, and the copy’s date is printed beside the figure as a historical estimate.

derived means we did arithmetic on sourced data. The arithmetic is written out further down this page. A derived figure inherits every uncertainty of its inputs and adds whatever the operation assumes.

assumed means it is ours, with no direct source behind it. The figure register holds 18 of these, and every one is named in its own section below, alongside the 6 structural choices that are also ours. Where an assumed figure drives a control or a result, the page says so beside it.

Register size: 120 figures, of which 80 are sourced, 22 derived and 18 assumed, drawn from 11 named outside sources plus our own labelled adjustments. Most sourced figures were first read on 2026-08-06; the dates for each are in the table. Figures older than 183 days from the date their source was read are re-checked before the site is updated.

Both cost publishers we cite returned an HTTP 403 to a plain automated request on 5 October 2026, so whether their live pages still print these figures is not known. Their figures were read from named Internet Archive copies of the exact cited addresses — HomeGuide’s mini-split guide dated December 9, 2025, HomeGuide’s circuit guide dated November 24, 2025 and Forbes Home’s guide dated August 8, 2025 — and every page that prints one of them labels it a historical cost estimate with that date. The electricity prices are EIA’s May 2026 figures, marked preliminary by EIA, read from its dated July 2026 issue because the rolling table page has moved on; they are kept as historical figures on purpose.

The named outside sources behind this site:

• 10 CFR 430.32(c)(5), Energy and water conservation standards
• City of San Diego Development Services, Information Bulletin 103, Fee Schedule for Mechanical, Electrical, Plumbing/Gas Permits (May 2026)
• ENERGY STAR (U.S. EPA), Air-Source Heat Pumps key product criteria
• ENERGY STAR (U.S. EPA), Room Air Conditioners
• Forbes Home, How Much Does It Cost To Install A Mini Split AC System? (audited and verified December 1, 2024)
• HomeGuide, Cost to install a dedicated circuit (October 15, 2025)
• HomeGuide, Ductless mini split installation cost (page dated October 8, 2025)
• Internal Revenue Service, Energy Efficient Home Improvement Credit
• NOAA National Centers for Environmental Information, Climate at a Glance, statewide cooling degree days, January-December 2025
• NOAA National Centers for Environmental Information, Climate at a Glance, statewide heating degree days, January-December 2025
• U.S. Energy Information Administration, Electric Power Monthly, July 2026 issue, Table 5.6.A (May 2026 data, preliminary)

The state tables

Electricity price by state: U.S. Energy Information Administration, Electric Power Monthly, July 2026 issue, Table 5.6.A (May 2026 data, preliminary) — Historical figures — EIA Electric Power Monthly, July 2026 issue (archived), Table 5.6.A, May 2026 data, marked preliminary by EIA (read 2026-10-05 and saved that day). These May 2026 values were first read on 6 August 2026 from EIA’s rolling table page, which has since moved on to later months and no longer shows them. EIA marks these values preliminary; all 51 rows match page 137 of that issue. Heating and cooling degree days by state: NOAA National Centers for Environmental Information, Climate at a Glance, statewide heating degree days, January-December 2025 (read 2026-10-05 and saved that day) and NOAA National Centers for Environmental Information, Climate at a Glance, statewide cooling degree days, January-December 2025 (read 2026-10-05 and saved that day). These are one calendar year’s weather (2025), not a climate normal. NOAA revises its most recent years: the values first read on 6 August 2026 differed in all 48 states from the ones NOAA served on 5 October 2026, so every value was replaced from that day’s download rather than patched one state at a time. Alaska, Hawaii and the District of Columbia are not in NOAA’s statewide series.

Note the concentration. Government and standards sources carry the sizing, the efficiency thresholds, the electricity prices, the degree days and the tax position. Installed cost has no government source anywhere, so it rests on two commercial cost aggregators, and that is the weakest part of this site by a distance. We cite both, print their disagreements rather than averaging them, and never blend a low end from one with a high end from the other.

Ceiling height scaling

The sizing table states an 8-foot baseline and, for higher ceilings, suggests a unit with a higher CADR; it gives no capacity formula. We scale in proportion to volume: factor = max(1, ceiling feet / 8). The floor at 1 is deliberate — a low ceiling never becomes a discount the source did not offer.

Indoor unit style premium

premium = style band − wall band, both from HomeGuide’s single-zone-system style table, so both operands were gathered the same way. Applying that difference once per head on a multi-zone job is our assumption — neither source publishes a per-head style premium.

Zone counts nobody published

HomeGuide’s zone table prints rows for one, two, three, five and eight zones only. For four, six and seven we interpolate linearly between the adjacent published rows and relabel the result derived. Past the top published row we return nothing.

Where the point estimate sits inside a band

The midpoint is 0.5. Each named complexity factor — a long line-set run, awkward mounting, a ceiling-mounted indoor unit — moves the point estimate 0.125 of the band further toward the top, capped at 0.875 of the band. The weighting and the cap are ours; the band is not.

Low-ambient capacity and draw

Cold Climate certification minimum at 5 F = heating capacity at 47 F × 0.7 from the certification floor, and draw = capacity / (COP × 3.412142) using the watt-to-BTU identity. Neither step involves a judgement call.

Running cost

draw in kW = head size in BTU/h / (SEER2 or HSPF2 threshold × 1000), then cost per 1,000 hours = draw × 1000 × state price per kWh. There is no run-hour assumption in this pair; it does assume the unit performs exactly at the selected threshold and that its heating capacity equals the head size.

These are the 18 registered figures that no source backs. Each one is disclosed at the point it is used, not only here.

6 further assumptions are structural rather than registered figures and belong in the same list:

• The open-plan rule drops exactly 1 head when you say the main space is open, the conservative version of a rule with no published form.
• Zone counts HomeGuide does not print (4, 6 and 7) are interpolated linearly between the published rows either side.
• The style premium is applied once per indoor head on a multi-zone job; neither source publishes a per-head premium.
• The installed-cost and zone worksheets size every room with two occupants, average sun and an 8-foot ceiling, and split the floor area equally across the rooms.
• The running-cost worksheet treats the chosen efficiency threshold as the unit’s actual performance, and the head size as its heating capacity.
• Annual run hours are degree days times our coefficients; the hourly figures carry no run-hour assumption.

The oversizing caution threshold (25 percent), the point-estimate step (0.125) and cap, and the zone comparison’s cut-off are registered above as assumed figures.

Our own labeled adjustment. The ENERGY STAR table has no insulation term; a room whose envelope is much better or worse than typical will sit outside these multipliers. If a contractor has run a Manual J load calculation on your house, that number is a better guide than this one.

Our own labeled assumption converting a state's degree days into equivalent full-load run hours. We could not obtain a citable equivalent-full-load-hours table at a stable public address. If your utility publishes one, its number is a better guide than ours. The cost-per-hour figures on the same page use no run-hour assumption, but they do assume the unit performs exactly at the efficiency threshold you pick.

Line set beyond the standard run. The source states the direction only. We could not obtain any published dollars-per-foot price for line set beyond the standard run from a source we were willing to cite, so this site prints none. A long run moves the estimate toward the top of the sourced band and is listed as an itemise-it-in-writing question for the contractor.

A main panel or service upgrade. Unresolved source conflict. This site does not pick a winner and does not put a panel upgrade in any total. Both figures appear on the permits and electrical page so you can see the disagreement rather than take our word for it.

Permit fees outside one city. Permit schedules are local, published in different formats, and structured differently from place to place. We publish the one we read. Adding a national average would be inventing a number that describes nowhere.

An annual run-cost figure for Alaska, Hawaii or the District of Columbia. Their electricity prices are published; their statewide degree days are not, in the source we use. The per-hour figures work; the annual estimate does not exist for them.

State and utility rebates. Accurately tracking fifty states of programme rules is a full-time job and a stale rebate figure does real financial damage. We track the federal position, which is a single document, and say plainly that we do not track the rest.

The sizing table is a table. It knows your floor area and, through its adjustment rules, a little about sun and occupancy. It does not know your window areas, your orientation, your infiltration rate or your neighbours’ shade. A room-by-room load calculation run on your actual house is a better guide than it, and than this whole site.

The installed-cost figures are national survey data from commercial publishers, one of which describes its method as surveying cost databases and the other as tracking estimates homeowners report. Neither is your local market, and neither publishes its underlying data. Their article dates are older than the dates we read them, and we read them from archived copies: Forbes Home’s page is marked audited and verified on December 1, 2024, and HomeGuide’s is dated October 8, 2025. That lag is real and is the reason the cost pages talk in bands rather than in prices.

The electricity prices are statewide averages of what residential customers actually paid, for May 2026, marked by EIA as preliminary estimates. If you are on a time-of-use tariff, your rate at some hours may be well above this average; check your tariff’s peak hours.

The degree days are one calendar year, statewide. A single year is not a normal, and a statewide figure averages a mountain county with a coastal one.

Corrections are welcome and are the fastest way to improve this. If a figure here is wrong, or a source has moved, the contact page explains what to send and the editorial policy explains what happens next.

5 October 2026. Every cited document was fetched again and saved byte for byte. NOAA’s 2025 degree days had been revised in all 48 states and were replaced from that day’s download, which moves every annual running-cost estimate slightly. The electricity table now cites EIA’s dated July 2026 issue (same May 2026 values). Each cost-band row now cites its own row of the publisher’s table; HomeGuide and Forbes Home figures are labelled as historical estimates with the date of the archived copy read. The San Diego fee now cites the HTML bulletin the old PDF address redirects to. The standard head sizes and four thresholds were added to the register as assumed figures, and several sentences that went beyond what the sources say were corrected or removed.