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Incentives for installing insulation and for buying energy-efficient appliances like refrigerators, washing machines, and air conditioners are often available from local and state governments and utilities. You can see what's available at DSIRE, Energy.gov, and Energy Star.

Related sites:

Home Power Magazine. All about renewable energy for the home.

No-Impact Man. Blog about a family striving to have no net impact. (i.e., What little they use, they offset.) Inspirational.

Off-Grid. News and resources about living without being connected to a utility company.

Mr. Electricity in the news:

"Michael Bluejay runs the outstanding Saving Electricity site that I've mentioned many times before." —J.D. Roth, Get Rich Slowly

Deep Green (book) by Jenny Nazak, 2018
Small Steps, Big Strides: Building Sustainability Habits at Home (book), Lucinda F. Brown, 2016
How much money you'll save with these common energy-saving strategies, Lifehacker, Sep. 28, 2015
Radio interview about saving electricity, Newstalk 1010 (Toronto), April 21, 2015
How much does your PC cost in electricity?, PC Mech, Nov 21, 2013
How Much Electricity Do Your Gadgets Really Use?, Forbes, Sep. 7, 2013
Can my bicycle power my toaster?, Grist, June 10, 2013
Six summer debt traps and how to avoid them, Main St, June 5, 2013
To convert to gas or electric?, Marketplace Radio (NPR), July 20, 2012
8 Simple Ways to Reduce Household Waste, Living Green Magazine, June 29, 2012
Why is my electric bill so high?, New York Daily News, Mar. 27, 2012
Fight the Power, CTV (Canada's largest private broadcaster), Mar. 23, 2012
How to Cut Your Electric Bill, Business Insider, Mar. 20, 2012
Tips to save energy when using your computer, WPLG Channel 10 (Miami, FL), Feb. 23, 2012
How long will it take an energy-efficient washer/dryer to pay for itself?, Christian Science Monitor, Oct. 29, 2011
10 Easy Ways to Lower Your Electric Bill, Forbes, August 23, 2011
18 ways to save on utility bills, AARP, July 9, 2011
How to Save $500 Worth of Energy This Summer, TIME magazine, June 28, 2011
Hot over the energy bill? Turn off the A/C, just chill, Chicago Tribune, June 24, 2011
Cool Site of the Day, Kim Komando (syndicated radio host), May 29, 2011
This calculator shows how much you spend washing clothes, Lifehacker, May 6, 2011
What you pay when you're away, WCPO Channel 9 (Cincinatti), May 5, 2011
Spotting energy gluttons in your home, Chicago Tribune (CA), Apr. 7, 2011
Walnut Creek author has tips for livng a thrifty life, Contra Costa Times (CA), Jan. 24, 2011
Do space heaters save money and energy?, Mother Jones, Jan. 10, 2011
Energy steps to take for a less pricey winter, Reuters, Nov. 10, 2010
Should you shut down your computer or put it to sleep?, Mother Jones, Nov. 1, 2010
Energy saving tips for fall, Chicago Tribune & Seattle Times Nov. 7, 2010
10 ways to save money on your utility bill, Yahoo! Finance, Oct. 2, 2010
Mr. Electricity Ranks Refrigerators & Electrical Wasters, Green Building Elements, Sep. 8, 2010
The case against long-distance relationships, Slate, Sep. 3, 2010
10 household items that are bleeding you dry, Times Daily (Florence, AL), July 27, 2010
Cold, hard cash, Kansas City Star, June 22, 10
Stretch your dollar, not your budget, Globe and Mail, May 18, 2010
Auto abstinence, onearth magazine, Winter 2010
2010 Frugal Living Guide, Bankrate.com
Energy-saving schemes yield €5.8m in savings, Times of Malta, Dec. 20, 09
Four ways to reduce your PC's carbon footprint, CNET, Dec 2, 09
The day I hit the brakes, onearth magazine, Fall 2009
How Much Do You Really Save By Air-Drying Your Clothes?, The Simple Dollar, 2010
Enjoy the mild weather, low electricity bills, Detroit Free Press, Jul 18, 09
The most energy-efficient way to heat a cup of water, Christian Science Monitor, Jun 16, 09
Ten ways to save energy, Times of Malta, Jan 3, 09
Measuring your green IT baseline, InfoWorld, Sep 4, 08
Bald Brothers Breakfast (MP3), ABC Adelaide, March 27, 2007
Net Interest, Newsweek, Feb 12, 07
The Power Hungry Digital Lifestyle, PC Magazine, Sep 4, 07
Net Interest, Newsweek, Feb 12, 07
Answers to all your electricity questions, Treehugger, Jul 11, 08 Going Green, Monsters and Critics, Jan 6, 2007
A hunt for energy hogs, Wall Street Journal Online, Dec 18, 06

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How to size a Mini Split or other HVAC system

  (aka air conditioner sizing, heating system sizing)

Last update: May 31, 2024

The Basics

Sizing simply means figuring out what size equipment you need to heat and/or cool your home or room.  We calculate how much heating/cooling we need, then we buy equipment that supplies that capacity.  That's true no matter what flavor of heating/cooling equipment you want (central HVAC, mini split, window unit, etc.).

Sizing is important because a unit that's too small won't heat and cool your space well, and a unit that's too big will cost more than necessary (and may have other possible problems).  Most contractors will try to sell you a system that's too big, either because they're trying to make more money, or because they don't know how to properly do the sizing work. (sources 1, 2, 3, 4)

The best way to size your system is to have a "Manual J" calculation done on your space.  Manual J is the gold standard for sizing, taking into account things like how much insulation you have, what kind of windows and what direction they're facing, and everything else.  Many utility companies will do this for free (check with them), and if not, you can hire an energy auditor.  Do not go with an HVAC contractor for the Manual J (who has an obvious conflict of interest in wanting to sell you a bigger system than you need), go with your utility or an energy auditor so you can trust that they did it right.  They'll tell you exactly what size you need, and then you can shop accordingly.

If you have ducts, you should also have your ducts tested for leaks, because installing properly-sized equipment won't do any good if you're going to gain or lose lots of BTUs through your ductwork.  The national average performance of ducts is only 57%. (source)  If your ducts are a problem, then have them serviced.  Of course, if your ducts are running through conditioned space, then duct losses are not a problem, because you "lose" the air right into the area that you were trying to condition in the first place.


The problems with oversizing are cost, and possibly comfort

Buying a system larger than what you need is just a waste of money.  That's why I steer you to get a Manual J report so you don't buy equipment that's too big.

Oversized systems might also be less comfortable, but that's becoming less relevant as modern units are moving to inverter technology, which can ramp the output up or down depending on the load.  Without inverter tech, the system might blast you with too-hot air in the winter, and they cause uneven temperatures throughout the house, because it won't run as much as a right-sized sytem, so the air isn't circulated as much.

If you're satisfied with that answer you can skip to the next section.  Otherwise, you can keep reading here about other potential problems with oversizing.

Conventional wisdom is that oversizing also creates other problems besides cost and less comfort, but I consider those to be mostly myths.

  1. MYTH: Oversized AC's don't control humidity as well.  The theory here is that oversized systems don't run long enough to dry the air.  This is only sometimes true.  First, modern units with inverters keep running even at low loads.  Second, a university study of homes where grossly oversized units were replaced with right-sized units didn't result in lower humidity.  The researchers theorized that the longer running time of the right-sized units meant that the fans they were drawing in more moisture into the house through the building envelope and the attic. (source PDF)  And Martin Holladay, an icon in the building science field, says, "Oversized air conditioners aren't really as terrible at dehumidification as many energy experts claim." (source)  However, I know of at least one case in which an oversized system reportedly did result in more humidity.

  2. MYTH: Oversized systems wear out faster because of short-cycling.  The theory here is that big systems heat or cool the house so quickly that they shut off after just a few minutes, and the constant on/off cycling is hard on the equipment.  There are a few problems with that:  First, with modern inverter tech, cycling is even less than older systems that are sized perfectly.  Second, it's hard to believe that the extra cycles would really make a big difference in equipment life.  The AC will likely be up for replacement just because it's old before it dies completely.  Third, any stress added by extra cycles should be balanced by the fact that the equipment doesn't run as long.

  3. MYTH: Oversized systems use lots of extra energy.  The theory is that AC's need to run for several minutes to get efficient.  But the study above found that they don't use more energy overall.  However, for mini-split systems, especially heaters, oversized systems can indeed use a lot more energy. (source)

BTU: The measurement of heat

Heat is measured in BTU's (British Thermal Units).  For example, a heater might supply 24,000 BTUs per hour to a house when it's operating.  To size a heating system, we figure how many BTUs of heat the house loses to the outside, and then get a system big enough to replace the BTUs that were lost.

We use BTUs for cooling, too, to measure how much heat an air conditioner removes from a house or a room.  For example, a small window-unit AC will remove about 5000 BTU per hour from a room.  When we size for cooling, we'll be figuring out how many BTUs of heat enter the house, and then get a system that can remove that much heat.

AC equipment is also measured in tons.  This has nothing to do with the weight of the equipment, it's just a measurement of heat.  One ton is 12,000 BTU.  So a 2.5-ton system would be 2.5 tons x 12,000 BTU/ton = 30,000 BTU.



Match to the “rated” capacity or the “maximum” capacity?

Confusingly, equipment specs show both a "rated" BTU capacity as well as a "maximum" capacity.  For example, a mini split I bought has a "rated" cooling volume of 6000 BTU, and a "max" volume of 11,184 BTU. (source)  Which figure do you use when shopping for equipment to match your Manual J?

The short answer is that you can use either, but I'd favor going for max.  If that satisfies you, skip to the next section.  Otherwise, keep reading this one.

Some things to know about equipment efficiency:

  1. “Efficiency” refers to how electricity used per unit of heating/cooling delivered.  One way it's measured is COP.  The higher the COP, the less electricity used for the same amount of heating/cooling.
  2. Modern equipment uses inverters which can ramp the output up or down depending how much heating/cooling you need.  That means, they run harder when they need to make a big difference in temperature, and slower when they're close to the set temperature.
  3. Equipment is most efficient (unit of electricity used per unit of heating/cooling) when running at the rated volume.  For example, at 95°F, the 6000-BTU unit mentioned above has COP of 3.66 at the rated level, but only 2.73 at the max level.  Likewise running below rated volume is also less efficient than running at rated.  The penalty isn't the same, though:  running above rated is generally worse than running below rated.
  4. Manual J is based on season extremes.  Not the absolute extreme that has ever happened, but covering 99% of the most extreme hours based on a 30-year average. (source)  So, if you buy equipment based on its rated volume, then most of the time (i.e., not extreme weather), your equipment will run below rated volume, which is less efficient than running at rated volume.
  5. Because temperatures vary a lot during the heating or cooling seasons, there is no way to get ideal efficiency.  Whether you match your Manual J to the rated or to the max volume, sometimes your system will run higher or lower than rated, which is less efficient than running exactly at rated. 

A table probably makes that last concept easier to understand.  Say your Manual J comes back at 12,000 BTU.  You're looking at two different models, one that's 7,000 BTU rated and 12,000 BTU max, and another that's 12,000 BTU rated and 20,000 BTU max.  

Choosing equipment for 12,000 BTU Manual J

7/12k BTU eqp. 12/20k BTU eqp.
Most days More efficient Less efficient
Extreme temp. days Less efficient More efficient

Let's walk through that table.

  1. Let's say you buy the first unit, 7/12k BTU.  Is it undersized?  No.  The 12k from your Manual J is for the seasonal extreme temperature, most of the time you'll need less than 12k BTU.  So, on those less-extreme days, your equipment may run closer to its 7k rated volume, which is its most efficient.  So, your equipment will be more efficient on most days, and less efficient on the extreme days.  Also, if the temps happen to go way beyond the assumed extremes, then you might not have enough heating/cooling.
  2. Let's say you buy the second unit, 12/20k BTU.  Most of the time (i.e., not extreme temperatures), it'll be oversized, and running below rated volume, which is less efficient than running at the rated volume.  Buy on the extreme temperature days, it'll run close to the rated volume, which is more efficient.
So, damned if you do, and damned if you don't.  But you have to buy something, and neither choice is horrible compared to the other.  I do think it's usually preferable to match the max volume, because that's smaller equipment (which will cost less than bigger equipment), and it'll probably run closer to the rated (most efficient) volume more of the time than bigger equipment.


Easy mistakes to make when sizing gas heaters

First off, think twice before buying a gas heater, or replacing an old one.  Modern heat pumps almost always save energy versus gas, even in cold climates. (source)  However, if you're dead-set on going with gas, then keep reading.

Many homeowners who buy gas-fired equipment are bitten by a couple of big misunderstandings.  The first is that they don't know that gas furnaces are rated by their input, not their output.  So a "60,000 BTU" gas furnace that's 80% efficient will supply only 48,000 BTU to your home.  If you've figured that you need a 60k BTU of heating and you buy a 60k BTU gas furnace, you're going to wind up with only 48k BTU of heat which is not enough.  So you need to calculate the output when you buy a gas furnace.  (Oil furnaces, on the other hand, are rated by their output, so a 60,000 BTU oil furnace will supply 60,000 BTUs.)

The other mistake is to ignore the efficiency when replacing old equipment.  Let's say you have a 70k BTU gas furnace at the end of its useful life.  Your contractor points out that your old unit was only 80% efficient, but he can sell you a new 70k unit that's 95% efficient and it will cost only $350 more, promising that you'll make the difference back quickly with the energy savings.

But regardless of the energy savings, the unit is bigger than what you need, so if you buy it you'll be paying more than you need to.  Your old unit put out 70k BTU x 80% = 56k BTU.  The new one puts out 70k BTU x 95% = 66.5k BTU.  So the new one is effectively 10.5k BTU larger than what you need—and you'll be paying for it.  Here again, you must shop by output, not input.  If you need only 56k BTU of heating and you go with a 95% efficient unit, then you should buy one in the neighborhood of (56k BTU ÷ 95% = ) 59k BTU.




Methods of sizing HVAC equipment


METHOD 1:  Go by square feet alone

The worst way to size a system is by square feet, though many contractors do just that.  The first problem is that it doesn't take into account the climate (hot or cold), how much insulation you have, whether the house is shaded, how leaky the house is, etc.  The other problem is that it estimates only cooling; there's no rule of thumb for heating based on only square feet, because such an estimate would often be even farther off.

But for what it's worth, which is not much, the basic formula for cooling is 1 ton of cooling for every 600 s.f. of house.  So, for example, a 2400 s.f. home would need 2400 s.f. x 1 ton/600 s.f. = 4 tons.  Newer homes that are insulated well and aren't leaky would be closer to around 1 ton for every 1000 s.f.  You could invert the figures if that's more comfortable.  Another way of saying 1 ton for every 600 s.f., is 20 BTU per square foot.  Another way of saying 1 ton per 1000 s.f. is 12 BTU per square foot.




METHOD 2:  Go by square feet + climate

HVAC System Sizing

Blue
Green Yellow
Orange
Red
Cooling
1.5 tons
700-1100 sf 700-1050 sf 600-1000 sf 600-950 sf
600-900 sf
2 tons
1101-1400 sf
1051-1350 sf 1001-1300 sf 951-1250 sf
901-1200 sf
2.5 tons
1401-1650 sf 1351-1600 sf 1301-1600 sf 1251-1550 sf
1201-1500 sf
3 tons
1651-2100 sf
1601-2000 sf 1601-1900 sf 1501-1850 sf
1501-1800 sf
3.5 tons 2101-2300 sf 2001-2250 sf 1901-2200 sf 1851-2150 sf 1801-2100 sf
4 tons 2301-2700 sf 2251-2700 sf 2201-2600 sf 2151-2500 sf 2101-2400 sf
5 tons 2701-3300 sf 2751-3300 sf 2601-3200 sf 2501-3100 sf 2401-3000 sf
Heating

40-60 BTU/sf 35-50 BTU/sf 30-45 BTU/sf 20-40 BTU/sf 15-35 BTU/sf

U.S. Heat Zone map
 Map from DoE.  Data from AC Direct with permission, with my modifications for heat.

By taking climate into account, you can estimate heating as well.  While some people will estimate cooling based on square feet alone even though it's not accurate, nobody will estimate heating by square foot alone, because the results can be even farther off.  At a minimum, heating estimates must take into account the climate as well as the area to be heated.  (A study of office buildings showed that a building in Chicago needed three times less cooling than one in Miami, but it needed 48 times more heating. (source))

You can see the estimates in the table at right.  To make this method a little less useless, I give it with ranges rather than a static number.  You'll need more powerful equipment if you don't have good insulation and windows, less if you do.




METHOD 3:  Size it to your existing system

If you're shopping for an HVAC to replace an existing system (rather than buying it for new construction), you can usually use the capacity of your existing system to gauge the size of the replacement system.  The main thing is to make sure your existing system is sized correctly.  A properly-sized system will run continuously (or nearly so) on the hottest and coldest days of the year.  If your unit is shutting off even in the most extreme weather, it's oversized.  Also, if you've installed energy efficiency upgrades (e.g., more insulation, double-paned windows) since you bought your system, it's probably oversized.

If your equipment doesn't appear to be oversized, just buy the same capacity equipment to replace it.  If your current equipment is oversized, you could go with one that's a little smaller, but it's better to have the Manual J calculation performed (see way above) find out exactly how much smaller you can go.

This method also works if you're replacing window units with a central or ductless mini-split system.
  I had a small house that could be cooled well by three window units, which were 5000, 5000, and 6400 BTU respectively.  So I didn't need more than 16,400 BTU of cooling capacity for the new system.  (And even though I told that explicitly to the contractors I invited to bid, they gave me quotes for 24,000- and 28,000-BTU systems.  Sadly typical.)


METHOD 4:  Online calculator


There are various calculators on the net that try to help you size your system, but I didn't like any of them enough to recommend them.  I'd like to build my own to offer to my readers, but I've been having a hard time finding the data to use for the calculations.  (Manual J is too big for this project; what I'm envisioning is more like a "Mini-Manual J".)  Any readers who have data to share are welcome to contact me.  Mostly, I need the formulas.  I can look up things like heating/cooling degree days, and U-values and R-values for various building materials, it's the formulas to assemble those into an answer that I lack.  I'd like to account for losses/gains from ceilings, walls, windows, doors, floors, occupants, electrical devices, ductwork, leakiness, and shading.


METHOD 5:  Manual J calculation


A "Manual J" calculation is the gold standard for sizing HVAC equipment.  It's not something a homeowner can do themselves (unless they're willing to throw down $500 for the software, and confident that they can use it accurately...garbage in = garbage out).  Many utility companies will do the calculation for you, and failing that, you can hire an energy auditor.  Don't rely on an HVAC contractor to do your Manual J, since contractors frequently churn out bad Manual J's by using the wrong inputs—either because they're trying to upsell bigger equipment, or because they sincerely don't know how to use the software properly.


Other Technical Topics


BTUs:  Technical definition

A BTU is the amount of heat required to raise the temperature of one pound of water by one degree from 60° to 61°F at a constant pressure of one atmosphere. (Wikipedia)  Different substances require different amounts of heat to change their temperatures because each substance has a different molecular makeup.  It takes about four times as much heat to change the temperature of water as it does for air.  The property of a substance that dictates how much heat is required to change its temperature is called specific heat.

BTUs to heat air

I wanted to know how much energy is required to heat and cool air, so I could figure things like the penalty for running a clothes dryer considering that the dryer is sucking conditioned air out of the house, and unconditioned air in, which then has to be heated or cooled.

Incidentally, the figure for the energy required to condition air isn't really useful for figuring the energy needed to heat or cool a home itself.  For that, you don't figure how much heat is required to change the temperature of the air in the home, because the inside air is generally always conditioned to a certain temperature.  What you need to know is how much heat is entering or leaving, which is all the HVAC system has to deal with.  And if the inside temperature is not maintained (e.g., you left the AC off all day and now you want to turn it on because it's very hot inside), I presume we'd have to figure the amount of heat to add or remove from all the objects in the house too, not just the air, which would be very difficult to estimate.  (We could assume X lbs. of stuff per 100 sf, and figure that the specific heat of the objects is around half that of water, but those are just guesses.)

So here's my stab at calculating the BTUs required to heat air, and this is what I got.

  • Air weighs 1.184 kg/m3 at room temperature (Engineering Toolbox)
  • So air weighs 2.6 lbs./m3  (1.184 kg x 2.2 lbs/kg)
  • So one pound of air is 1/2.6m3 = 0.385m3
  • So one pound of air is 13.59 ft3 (0.385m3 x 35.3 ft3/m3, Google calculator)

So it would take one BTU to raise the temperature of 13.59 ft3 of air by one degree if water and air had the same specific heat, but as we saw earlier, they don't.  The specific heat of water is 4.14 times greater than that for air.  So, we should be able to heat (13.59 ft3 x 4.14=) 56.3 ft3 of air by 1°F with one BTU.

I calculated that because I couldn't easily find a reference figure for how much air could be heated with one BTU.  But after doing the calculation I found a reference:  55 ft3, from this 1909 book by the American Technical Society.  That's extremely close to the 56.3 ft3 figure we calculated above.  Good for us.


Cost to remove heat generated by appliance use

It can be useful to know how much energy is required to remove heat from electrical appliances (like light bulbs), and how much it costs.

  • 1 kW = 3412 BTU
  • A 2.5-ton AC uses 3.5 kW and removes (2.5 tons x 12,000 btu/ton =) 30,000 BTU/hr
  • So it's 3.5 kWh per 30,000 BTU.  That's 3.5 kWh/30,000 BTU ÷ 3.5 = 1 kWh/8571 BTU
  • To remove the 3412 BTU generated by 1 kWh of appliance use, we'd use 1 kWh/8571 BTU x 3412 BTU = 0.4 kWh
  • 0.4 kWh x 16¢ kWh = 6.4¢ worth of AC to remove the heat generated by 1 kW of electricity use
  • If we wanted to be really thorough, we'd calculate the energy required to remove the heat we created by running the AC to remove the original heat, which is a recursive issue of course.

Converting horsepower to tons of cooling

By a moderator at Refrigeration Engineer:

  1. 16 EER means 16 Btu/hr of cooling for every watt of power used
  2. 1 watt = 3.412 Btu/hr, so
  3. 16 EER unit provides 16 ÷ 3.412 = 4.69 watts of cooling for every watt of power used.  This is the coefficient of performance (COP).
  4. 1 ton refrigeration = 4.712 horsepower, electric
  5. 1 hp electric = 4.69 / 4.712 = 0.995 tons refrigeration