Bigger is not better: sizing home systems
Almost everything else on this site rewards over-ordering. Buy ten per cent more concrete than you need and you have wasted ten per cent of the concrete. Buy an air conditioner twenty per cent bigger than you need and you have bought a worse air conditioner.
Building systems are sized rather than counted, and sizing has an answer that is wrong in both directions. That single difference is what makes this category behave unlike the rest.
Why an oversized air conditioner is worse
Cooling does two jobs: it lowers the air temperature and it pulls moisture out. The first happens fast and the second only happens while the coil is running and cold. An oversized unit hits the thermostat setpoint in a few minutes, shuts off, and never runs long enough to do the second job.
The result is a room that is cold and clammy at the same time, which feels considerably worse than a slightly warmer room that is dry. It also short-cycles the compressor — starting is the hardest thing a compressor does — so the equipment wears out faster while performing worse.
A 500 square foot room in a moderate climate wants about 10,000 BTU an hour, which is a little over three quarters of a ton. The per-square-foot rule that produces that figure is a starting point rather than an answer: the real method is a Manual J calculation, which accounts for insulation, air leakage, window type and which direction the house faces.
25 ft x 20 ft = 500 sq ft
Moderate climate = 10,000 BTU/hr
In tons (12,000 BTU) = 0.83
Hot water is a rate, not a volume
The number printed largest on a water heater is its tank size, and it is not the number that decides whether you run out. What matters is the first-hour rating: what it holds plus what it can reheat within the hour, which is on the yellow energy label rather than the front of the box.
That is why a forty gallon gas heater and a fifty gallon electric one deliver about the same amount in an hour. Gas recovers roughly twice as fast, so it makes up in reheating what it gives away in storage.
A household with two showers, a dishwasher, a load of laundry and a couple of hand washes in its busiest hour needs about sixty-one gallons. That is the figure to size against — the peak hour, not the daily total, because nobody runs out of hot water at the daily average.
Tankless units are rated differently again, in gallons a minute at a given temperature rise, and the rise is the part that catches people out. The same unit delivering four gallons a minute in a warm climate manages barely half that in January further north, because it has to lift the incoming water so much further.
Work it out: Water Heater Size Calculator →
Peak sun hours is not hours of daylight
Solar sizing turns on one number that almost nobody reads correctly. Peak sun hours is not how long the sun is up. It is the day's total energy expressed as an equivalent number of hours at full rated intensity — so a place with fourteen hours of summer daylight might still count only four and a half.
The other figure worth understanding is the derate. Panels are rated in a laboratory at 25°C, and a roof in July is far hotter than that. Add the inverter, the wiring, dust and any shading and the usable output is around eighty-five per cent of the rating on the box.
A household using 900 kWh a month at four and a half peak sun hours needs about an 8 kW array — twenty 400-watt panels, around 420 square feet of panel. The real roof needs more than that, because setbacks from the edges and an access path for firefighters are not optional.
900 kWh/month, 4.5 peak sun hours, 85% derate
Array size = 8 kW (20 x 400W panels)
Generates = about 930 kWh/month
Panel area alone = 420 sq ft, before setbacks
Work it out: Solar Panel Calculator →
Insulation: the gaps cost more than the R-value
Insulation is the one item here that is counted like a material, but it has its own trap. Studs at sixteen inch centres take up about a tenth of the wall and are not insulated, so the cavity area is meaningfully less than the wall area — 335 square feet of cavity in a 416 square foot wall once the openings and framing come off.
The bigger point is that fit beats specification. A batt sized for sixteen inch centres stuffed into a twenty-four inch bay leaves gaps at the edges, and gaps cost more performance than the difference between one R-value and the next. Compressing a thick batt into a shallow cavity does the same thing.
Coverage per bag is printed on the packaging and changes with the R-value, because a higher-rated batt is thicker and fewer of them fit in a bag. That is the number to use rather than a default.
Questions
What size air conditioner do I need?
For a moderate climate, roughly 20 BTU an hour per square foot — so about 10,000 BTU for a 500 square foot room. Treat that as a starting figure; a Manual J calculation is the real method.
Is a bigger air conditioner better?
No. An oversized unit reaches the setpoint quickly and shuts off before it has removed any humidity, so the room ends up cold and clammy. It also short-cycles the compressor, which is what wears it out.
What is a first-hour rating?
What a water heater can deliver in one hour — what it holds plus what it reheats while you are using it. It is printed on the energy label and it predicts running out of hot water far better than tank size does.
Why does a 40 gallon gas heater match a 50 gallon electric one?
Because gas recovers about twice as fast. Over an hour the faster reheating makes up for the smaller tank, so the two have similar first-hour ratings.
What are peak sun hours?
The day's total sunlight expressed as that many hours at full rated intensity. It is not hours of daylight — somewhere with fourteen hours of summer daylight may still count only four or five peak sun hours.
Why is my solar array rated higher than what it produces?
Panels are rated at 25°C in a laboratory. Heat, the inverter, wiring losses, dust and shading take roughly fifteen per cent off that before it reaches your meter.
Does insulation cover the whole wall?
No. Studs at sixteen inch centres take about ten per cent of the surface and are not insulated, so subtract framing as well as doors and windows when working out how much to buy.