The working behind the number.
Every model, its sources, assumptions and professional limits.
What these estimates can tell you.
These are shopping and planning screens. Ranges show sensitivity to simplified assumptions; they are not statistical confidence intervals. Recommendations round upward to practical capacity steps. A professional calculation can fall outside the displayed range.
Generators: running demand plus one motor start.
Add concurrent running watts, then add the largest incremental startup demand: total start watts minus that appliance’s own running watts. Apply the selected reserve, 25% by default, and round up to 0.5 kW. The lower band covers peak demand with no reserve; the upper band uses 40% or the larger selected reserve.
Generac’s buyer guide gives approximate running and additional-start watts. We normalize additional-start watts to total-start watts. The RV AC example uses the portable worksheet’s approximate two-times-running start rule. Check actual nameplates, fuel-specific ratings and simultaneous starts. A service breaker rating is not a household demand measurement.
Central AC: a disclosed regional screen.
Area × climate-zone coefficient × insulation × ceiling height ÷ 8 × sun factor, plus extra occupants and kitchen heat. Zone 1 through 8 coefficients are 25, 24, 22, 20, 18, 17, 16 and 15 BTU/h per sq ft. These are SizeMyUnit assumptions, not DOE or ENERGY STAR central-AC sizing rules. Good insulation uses 0.8 and poor insulation 1.25. Two occupants, 8 ft ceilings and moderate sun are defaults.
One ton equals 12,000 BTU/h. Nominal capacity rounds upward by half a ton; the ±20% load sensitivity rounds outward. This is not a substitute for Manual J/S/D.
Window AC and mini-splits: room and zone cooling.
The ENERGY STAR room cooling chart supplies the baseline for each 100–2,500 sq ft zone. Heavy shade reduces capacity 10%; strong sun increases it 10%; each person beyond two per zone adds 600 BTU/h; a kitchen adds 4,000 BTU/h once. At shared chart boundaries we use the smaller preceding band. The current chart displays 21,000 BTU/h for 1,000–1,200 sq ft.
We add ceiling-height scaling relative to 8 ft and a ±10% single-zone screening band; those are our assumptions. Larger areas default to enough equal-area zones to keep each inside the published chart, then add their capacities. Equal-area partitioning and uniform conditions are our assumptions, not an official whole-home method. Combined capacity uses a ±20% band and is labelled total; actual rooms and equipment combinations need individual professional checks. We do not add a climate or insulation multiplier to the room chart. State selection changes only the EIA operating-cost example. Mini-split winter performance needs a separate heating check.
Furnaces: output and input are different.
Area × zone heating coefficient × insulation × height ÷ 8 estimates delivered heat. Zone 1 through 8 coefficients are 20, 25, 30, 40, 45, 50, 55 and 60 BTU/h per sq ft. These regional screening coefficients are ours, informed by manufacturer guidance, not a government-endorsed load calculation.
Divide by selected AFUE, 95% by default, as a seasonal planning proxy for input. Round input upward to 10,000 BTU/h, with a ±20% load band. Actual manufacturer output, staging, distribution and design conditions control final selection.
Heat pumps: check both seasons.
Calculate central cooling and heating screens separately. Divide winter heating demand by assumed retained capacity, then take the larger of that nominal capacity and cooling load. Round upward by half a ton. Default retention is 100%, 100%, 95%, 90%, 85%, 80%, 75% and 70% for zones 1–8. These are adjustable assumptions, not product performance data. Use the manufacturer curve at your design temperature and assess minimum summer output, defrost and backup heat.
Tank water heaters: the busiest hour.
The DOE worksheet reproduced by Georgia DCA uses 10 hot-water gallons per shower, 2 per shave, 4 per hand-dishwashing event, 6 per dishwasher cycle and 7 per clothes-washer load. Multiply by peak-hour events and compare the total with the label’s first-hour rating. The worksheet advises choosing a rating within 1–2 gallons of measured demand; our displayed band is ±2 gallons.
Default events are one shower per person, one shave and one hand-dishwashing event in the same hour. Tank-volume shopping bands are separate illustrative household heuristics: 30–40 gallons for 1–2 people, 40–60 for 3–4 and 50–80 for larger households. They are not first-hour-rating conversions.
Tankless: flow at the actual temperature rise.
Count simultaneous shower, faucet and dishwasher flow. Defaults are 2 GPM per shower, 1 GPM per faucet and 1.5 GPM per dishwasher. Flow × temperature rise × 500 estimates delivered BTU/h; divide by the assumed thermal efficiency for input. The 500 factor approximates 8.33 pounds per gallon × 60 minutes × water’s specific heat.
Rinnai’s regional groundwater map informs our illustrative inlet presets of 75, 70, 60, 55, 50, 45, 40 and 35°F by zone. They are not measured county temperatures. Defaults are 120°F delivery and 95% assumed efficiency. Compare actual manufacturer flow curves at the selected rise. The 90–115% flow band is our shopping sensitivity.
Solar: annual use and cached weather.
Monthly kWh × 12 × offset ÷ cached PVWatts kWh/kW-year gives DC system kW. Divide by panel wattage and round panel count upward. Every state uses one named representative city, not a statewide average. Snapshots use PVWatts v8, 1 kW standard modules, roof mounting, 30° tilt, 180° azimuth and 14% losses. Alaska uses the available TMY3 dataset; other states use NSRDB.
The production sensitivity is ±15%; it is not PVWatts statistical uncertainty. Default panels are 400 W with 21 sq ft footprint and 600 sq ft usable roof. Actual layout needs setbacks and access space. Floor-area mode scales the EIA 2024 state household average by area/2,000, an illustrative consumption assumption. Use 12 months of bills whenever available.
Dehumidifiers: current pint ratings.
Current ENERGY STAR guidance gives 20–30 pints/day for damp, 25–40 for very damp and 30–50 for wet rooms below 2,000 sq ft. We select the midpoint rounded to 5 pints. For 2,000 sq ft and above, published minimums are 30+, 40+ and 50+; our shopping upper bands of 50, 60 and 70 are assumptions. The larger band at exactly 2,000 sq ft is a conservative boundary choice.
These approximately 65°F portable ratings must not be mixed with historical 80°F AHAM tables. Correct moisture entry and verify operating temperature.
Climate zones, design temperatures and professional design.
County zones come from PNNL’s IECC 2021 snapshot, published in 2022. Historical Connecticut counties remain for compatibility with that source. The missing Kusilvak zone is repaired using PNNL’s report and the Census-confirmed Wade Hampton rename. This is a climate lookup, not a current county-boundary or code-adoption service.
State defaults use one disclosed county. No city design temperature is fabricated or inferred from the zone. A contractor uses measured assemblies, window orientation, air leakage, local summer/winter design conditions, duct gains and losses and ventilation loads. Manual J calculates residential loads, Manual S selects equipment and Manual D sizes ducts. DOE’s HVAC sizing strategy guide explains the professional process.
Operating costs.
Electric examples use the published 2024 EIA state residential average; no live tariff is assumed. Central cooling EER 12, room cooling EER 10, heat-pump COP 3 and gas at $1.50/therm are explicit illustrative assumptions. Runtime and actual manufacturer efficiency are required for a real budget.