Heating, cooling and solar equipment is a big investment, and the right size matters. An air conditioner that is too small never cools the room, while one that is too large short-cycles, wastes energy and leaves the air damp. Solar systems that are undersized do not cover your bills. These calculators give you a sensible starting estimate before you talk to an installer.
All HVAC & Energy Calculators
Choosing the Right Calculator
- Buying a window, portable or mini-split air conditioner? The BTU Calculator estimates the cooling capacity a room needs based on its size, sun exposure, occupants and use.
- Thinking about solar? The Solar Panel Calculator estimates system size in kW and the number of panels from your monthly electricity use and local sun hours.
What Is a BTU?
A British Thermal Unit (BTU) is the amount of energy needed to raise one pound of water by one degree Fahrenheit. Air conditioner capacity is rated in BTU per hour. A common rule of thumb is about 20 BTU per square foot of living space, adjusted for ceiling height, insulation, sunlight, the number of people in the room and heat from kitchens.
How Solar Sizing Works
A solar estimate starts with how much electricity you use, in kilowatt-hours per month, which you can find on your utility bill. Divide your daily use by the number of peak sun hours your location receives to get the system size in kilowatts, then divide by the wattage of a single panel to get the panel count. Real systems also account for inverter losses, shading and roof orientation.
Important Safety Note
These tools are for planning only. HVAC and solar installations involve electrical work, refrigerants and structural loads. In most areas they require permits and must be installed by licensed professionals. For a full assessment, ask an HVAC contractor for a Manual J load calculation or a solar installer for a site survey. See our disclaimer for more.
Preparing for outages or cutting bills? Size a backup generator with the Generator Size Calculator, find how much insulation your attic needs with the Insulation Calculator, and see what any appliance costs to run with the Electricity Cost Calculator.
Sizing heating, hot water or wiring? See the Furnace Size, Heat Pump Size, Water Heater Size, Duct Size and Wire Size calculators.