Two kinds of household question
Almost everything in this category is either a quantity or a running cost, and the two behave differently.
Quantities -- paint, flooring, area, volume -- are geometry. The arithmetic is exact and the uncertainty sits entirely in the measurement and in the waste allowance. Getting these right saves a second trip to the store and, more usefully, stops you buying a second batch of paint that does not quite match the first.
Running costs -- electricity, solar payback -- are a rate multiplied by a quantity of time. The arithmetic is trivial; the difficulty is that almost nobody knows their own rate. Which brings us to the single most useful idea on these pages.
Find your real rate before anything else
The number printed on a tariff sheet is not what you pay per unit. A household bill also carries fixed charges, delivery or distribution charges, taxes and sometimes a tiered structure that raises the price above a threshold. Divide the total amount you were charged by the kilowatt-hours you used, and you get your effective rate -- usually noticeably higher than the headline figure.
Use that number, not an average. National and even state averages conceal enormous variation between utilities, tariffs and times of day, and a running-cost estimate built on somebody else's rate is arithmetic performed on the wrong input.
Watts, kilowatts and kilowatt-hours
This is the confusion that spoils more home energy calculations than any other, and it is worth stating plainly.
A watt is a rate of energy use, like a speed. A kilowatt is a thousand watts. A kilowatt-hour is a quantity of energy: one kilowatt drawn for one hour. Your bill charges you for kilowatt-hours, so a device's wattage tells you nothing about its cost until you multiply by the hours it actually runs.
This is why a 2,000 W kettle used for four minutes a day costs far less to run than a 50 W device left on permanently, and why "high wattage" and "expensive to run" are not the same claim.
What the models leave out
Nameplate wattage is a maximum, not a constant draw. Anything with a thermostat -- refrigerators, heaters, air conditioners -- cycles on and off, so real consumption is well below the label. Paint coverage on a can is a best case that assumes a smooth, sealed, similarly colored surface. Solar payback here is a simple cumulative model that ignores financing, net-metering and time-of-use rules, inverter replacement and roof condition, any of which can move the answer by years.
Each page says so explicitly, because an estimate you know the shape of is worth more than a precise-looking one you do not.