About
How we check our numbers
Short answer
Every number comes from the people who set it: the maker's spec sheet or install manual, the building code, or a government data set. Each page lists its sources. The math runs in code that is tested every time we change it.
Where the numbers come from
We go to the first source. For a product, that's the maker's own spec sheet or install manual. For a rule, it's the code itself, such as the International Residential Code, or the agency that publishes it. For climate, prices and energy, it's the government data set: NOAA, the EIA, ENERGY STAR, the Census Bureau, USDA, or the National Laboratory of the Rockies' PVWatts model.
Every calculator and guide lists its sources under "Where the numbers come from". When a maker's site won't let us read its spec sheet, we leave that product out and say so, rather than copy a number from a reseller.
How the calculators work it out
Each calculator shows its math under the answer and explains it under "How we work it out". The formulas live in code with automated tests that check worked examples, so a change that breaks an answer is caught before it ships.
The answers are typical numbers. Your building department, your utility and the install manual for whatever you buy have the final say.
What the dates mean
"Updated" under a title is the last time that page or the math behind it changed. Some guides also say when we last checked the source itself.
The Amazon links
This site has no affiliate links. Our garage, yard and kitchen sites link some products on Amazon, and say so on the page.