How we size

ENERGY STAR's room air conditioner sizing chart: find the row for the room's square footage, then apply its adjustments. Heavily shaded, take off 10 percent. Very sunny, add 10 percent. Add 600 BTU for each regular occupant past two, and 4,000 BTU for a kitchen. The chart assumes an 8-foot ceiling.

The tool then picks the smallest mini split size that covers the result, because ENERGY STAR warns that bigger is not always better.

• ENERGY STAR, Room Air Conditioners: find the correct cooling capacity
• Unit specs: each maker's published spec sheet or the AHRI directory

Champion Power Equipment's "Choosing a Generator" sheet: add up the running watts of everything you'll power, then add the highest single starting watts. Generac publishes the same formula. It's a maker's method, not a government one, and the tool says whose it is.

Appliance watts start at the top of each range in Champion's wattage chart, so the answer isn't short; your appliance's nameplate replaces them. For a home standby unit, Champion's standby guide adds a 20 percent cushion. Champion's worked example there gives a smaller number than its own written rule, and the tool follows the written rule.

• Champion, Choosing a Generator (PDF)
• Generac, What Size Generator Do I Need?
• Champion, Generator Wattage Chart
• Champion, What Size Home Standby Generator Do I Need?
• Safety: CPSC Safety Alert 5123, CPSC 468 and CDC, Avoiding Carbon Monoxide Poisoning

Gallons are plain geometry: surface area times average depth, at 7.48 US gallons per cubic foot. Pump flow is gallons divided by the turnover time. No federal rule sets that time for home pools, so the tool shows two published limits and lets you pick: the International Swimming Pool and Spa Code (once every 12 hours) and California's energy code (6 hours or 36 gallons a minute, whichever is greater).

Heater size uses energy.gov's formula: pool area × temperature rise × 12. Horsepower isn't given, because it can't be read from gallons alone.

• ICC, summary of ISPSC Section 810
• California Energy Commission, 2025 standards, Section 150.0(p)
• energy.gov, Gas Pool Heaters, Heat Pump Swimming Pool Heaters and Efficient Swimming Pool Pumps (archived copies)
• Pump rules: 10 CFR 431.485, 10 CFR 431.465 and ENERGY STAR pool pumps

The U.S. Department of Energy's method: add up the flow, in gallons per minute, of everything you'd run at the same time, then work out the temperature rise. Unless you know otherwise, DOE says to assume 50°F incoming water and a 120°F target. Heaters are matched on each maker's own flow table at that rise.

• DOE, Sizing a New Water Heater and Tankless or Demand-Type Water Heaters (archived copies)
• Fixture flows: 10 CFR 430.32 and EPA WaterSense
• Safety: CPSC, Carbon Monoxide Questions and Answers

DOE's peak-hour worksheet: add up the hot water your home uses in its busiest hour, then buy a heater whose first-hour rating at least matches it. energy.gov warns that an oversized unit costs more to buy and to run, so the closest rating is listed first.

One correction: energy.gov's worked example lists rows that add to 65 gallons but prints a total of 66. The tool adds the rows as listed.

• DOE, Sizing a New Water Heater and Storage Water Heaters (archived copies)
• Heater specs: each maker's spec sheet or ENERGY STAR certified product data
• Safety: energy.gov, Lower Water Heating Temperature (archived) and CDC, Carbon Monoxide Poisoning Basics

The target is ENERGY STAR's recommended R-value for your climate zone, based on the 2021 International Energy Conservation Code. What you already have comes from ENERGY STAR's method: read the R-value on the batts, or measure the depth and multiply by the number for that material. Your zone comes from Pacific Northwest National Laboratory's county list.

Bags and depth come from each maker's coverage chart. The tool takes the smallest chart row that covers what you need and never estimates between rows.

• ENERGY STAR, Recommended Home Insulation R-Values
• ENERGY STAR, DIY Checks and Inspections
• ENERGY STAR, Attic Insulation Project and Attic Air Sealing Project
• PNNL-33270, Guide to Determining Climate Zones by County and its county data files

ENERGY STAR's dehumidifier chart, which asks how big the space is and how damp it is, and gives a minimum capacity in pints a day. In 2019 the Department of Energy changed the test, so the same machine now reports fewer pints. The tool reads the chart in today's pints and compares it with each model's capacity on ENERGY STAR's certified list, measured on today's test. Nothing is converted.

• ENERGY STAR, Dehumidifiers (sizing chart, placement, humidity)
• ENERGY STAR, Dehumidifier Testing and Capacity
• ENERGY STAR certified dehumidifiers list
• DOE test procedure, 10 CFR 430 Appendix X1
• EPA, A Brief Guide to Mold, Moisture and Your Home

The same ENERGY STAR room air conditioner chart and adjustments as SplitSizer. Window units come in set sizes, so the tool picks the smallest one that covers your number. The sizes come from ENERGY STAR's certified list: every capacity sold as at least 5 certified models from at least 2 brands.

• ENERGY STAR, Room Air Conditioners
• ENERGY STAR Certified Room Air Conditioners (U.S. EPA data)
• Safety: U.S. Fire Administration, preventing home electrical fires

One line in North Dakota's water softener code: gallons per person per day × people × hardness in grains per gallon × days between recharges. Daily use starts from EPA WaterSense's figure of 82 gallons a person, or your own bill. Penn State Extension does the same math in its softener guide. For clear-water iron, the tool uses the rule GE, Whirlpool and Morton print in their manuals, and says so.

• North Dakota Admin. Code ch. 62-04-08
• EPA WaterSense statistics
• Penn State Extension, Water Softening
• USGS, Hardness of Water
• UNL Extension NebGuide G1491

energy.gov's daily energy formula: watts × hours used a day. Refrigerators cycle, so energy.gov says to divide their plugged-in hours by three. The wattages filled in are the samples from energy.gov's appliance calculator, until you enter your own.

Some of a station's battery never reaches an AC outlet. Where a maker publishes how much does, the tool uses it; where it doesn't, the card says so. Surge is only checked when you enter starting watts from your appliance's manual or label. Two stations under an active CPSC recall on October 30, 2026 are left off the list.

• energy.gov, Estimating Appliance and Home Electronic Energy Use and its appliance energy calculator (archived copies)
• NREL, Technology Tips for Solar + Storage (2024)
• CPSC Recalls