Heating & Cooling Calculators.
Sizing the load, not the shelf label — 2 tools, all free.
Cooling load in BTU per hour and electric heat in watts, both from the same room facts: floor area, ceiling height, insulation, sun and climate. Sizing close to the real load matters more than buying big — an oversized air conditioner short-cycles and leaves a room cold and clammy.
AC Size
BTUs and tonnage for the space
Answer in BTU/hrElectric Heat
Watts, baseboard feet, breaker
Answer in wattsThe reasoning behind them
Guides that cover the judgement calls these tools can't make for you — what to measure, which assumption to change, and where the money goes when an estimate is wrong.
The rest of the wall
Interior Calculators, Outdoor & Yard Calculators, Electrical Calculators — or all 18 in one list.