Heat Load Units: BTU/h, Watts, kW, and Tons

A heat load calculation unit describes thermal power: BTU/h, watts, kilowatts, or tons of refrigeration. These describe heat transferred per unit of time. BTU and kWh describe energy totals. A correct conversion must also distinguish thermal capacity from the electricity the equipment consumes.

What each unit actually means

A heat load is a rate, not a quantity of stored energy. That is why the honest units here are energy per hour (or, for the watt, energy per second built into the unit). Mixing up a rate with a total, for instance confusing BTU with BTU per hour, is among the most common unit errors in load work.

BTU per hour (BTU/h)

The British thermal unit is a heat-energy unit historically related to warming water. On its own a BTU is an amount of energy; HVAC capacity is a rate, so labels that casually say “12,000 BTU” generally mean 12,000 BTU/h. Verify the specification. Many residential results, including a Manual J output, are reported in BTU/h.

Ton of refrigeration

A “ton” in HVAC has nothing to do with equipment weight. It is a historical cooling rate based on melting one short ton (2,000 lb) of ice over 24 hours, which works out to 12,000 BTU/h. Tonnage therefore refers to cooling rate. A 3-ton nominal cooling capacity equals 36,000 BTU/h of cooling, not necessarily 36,000 BTU/h of outdoor heat rejection, because the outdoor unit also rejects heat associated with compressor input.

Watt and kilowatt

The watt is the SI unit of power: one joule per second. It is used two ways in HVAC. Thermal kW describes the heat flow itself, where 1 kW is about 3,412 BTU/h. Electrical kW describes the power the equipment draws. A heat pump can deliver several kW of heat per kW of electricity, so the two numbers are not interchangeable.

Kilocalorie per hour (kcal/h)

Legacy kcal/h ratings also describe thermal power, but calorie definitions differ slightly. Using the thermochemical kilocalorie, 1 kcal/h is approximately 1.1622 W or 3.9657 BTU/h. Identify the convention if precision matters and use one convention throughout. It is simpler to compare current specifications in BTU/h or thermal kW.

Core conversions

Conversion matrix

Each row below expresses one unit in all the others. The BTU/h column is the common anchor, and kcal/h figures use the thermochemical calorie.

Worked conversions

• 24,000 BTU/h ÷ 12,000 = 2 tons; 24,000 ÷ 3,412 ≈ 7.034 kW thermal.
• 5 kW thermal × 3,412 = approximately 17,060 BTU/h, or 1.422 tons.
• A 10 kW resistance heater converts approximately 34,120 BTU/h of electrical input to heat at the heater; delivered room heat depends on distribution.
• A 36,000 BTU/h cooling label is nominally 3 tons. Actual design-condition capacity may differ.
• A 3,000 kcal/h rating is about 3,000 × 3.966 ≈ 11,900 BTU/h, roughly a 1-ton machine.

Energy versus power

Check a mixed-unit equipment schedule

Suppose a hypothetical report shows 30,000 BTU/h total cooling, including 24,000 sensible and 6,000 latent. Its equivalent total is 30,000 ÷ 3,412 ≈ 8.793 kW thermal. A candidate listed at 9.2 kW total and 7.5 kW sensible at the same conditions provides approximately 31,390 BTU/h total and 25,590 sensible. Its latent remainder is 1.7 kW, or about 5,800 BTU/h. The total and sensible figures exceed the example loads, but latent capacity is short. Converting only the headline capacity would hide that mismatch.

Add columns for the original unit, converted unit, operating conditions, and whether the value is total, sensible, latent, or input. Preserve the original figures so another reader can reproduce the conversion. Keep extra digits while calculating and round the displayed answer afterward. Reporting more decimal places does not make an estimated building input more accurate.

Energy totals use different units: 1 kWh equals about 3,412 BTU, and 1 therm equals 100,000 BTU. These are amounts, not rates. If an appliance draws a constant 2 kW for 3 hours, it uses 6 kWh. A 2 kW rating alone is not a daily energy total. Heat pump input changes with conditions and speed, so rated input multiplied by all operating hours may misrepresent annual consumption.

Which unit gets used where

Thermal kW is not electrical kW

If a heat pump delivers 9 kW thermal at COP 3, the corresponding electrical input is 9 ÷ 3 = 3 kW for the same operating boundary and condition. Converting 9 kW thermal to about 30,708 BTU/h does not establish electrical input, circuit size, or energy use. Use the manufacturer’s electrical ratings for installation. In metric markets a mini-split advertised as “3.5 kW cooling” is describing thermal output, about 1 ton (3.5 × 3,412 ≈ 11,900 BTU/h), while its electrical input will be a smaller number that depends on efficiency and conditions.

Airflow has different units

Temperature and assembly units must match the equation

Convert an actual Fahrenheit temperature with °C = (°F − 32) × 5/9. A temperature difference has no 32-degree offset: a 20°F difference equals 20 × 5/9 ≈ 11.11°C, or 11.11 K. Subtracting 32 from a temperature difference creates a false load. In a conduction equation, use U in W/(m²·K), area in m², and difference in K to obtain watts. With U in BTU/(h·ft²·°F), use ft² and a °F difference to obtain BTU/h.

For a hypothetical metric assembly, 0.30 W/(m²·K) × 50 m² × 20 K = 300 W, or approximately 1,023.6 BTU/h. That is the surface conduction term, not a whole-building load. Avoid mixing a metric U-factor with square feet even if the temperatures look reasonable. Similarly, insulation resistance in SI and US customary units has different numerical values; invert an R-value only after identifying its unit system and whether it describes the whole assembly.

At a 20°F room-to-supply temperature difference, 10,800 sensible BTU/h ÷ (1.08 × 20) = 500 CFM. Do not substitute total cooling load containing moisture removal into this sensible-only formula. The CFM calculator applies the same relationship.

Convert first, select second

Make all candidate capacities comparable, then check design-condition performance and sensible/latent loads. Do not round every conversion up to the next half ton; a nominal label is not a substitute for Manual S selection. For example, a 28,800 BTU/h load estimated with the BTU calculator converts to 2.4 tons, and whether a 2-ton or 2.5-ton nominal unit is appropriate depends on actual capacity at your design conditions. The unit converter and AC tonnage calculator handle the arithmetic in both directions.

Continue with: Tonnage conversion; CFM estimate; Load formulas.

References and verification

ASHRAE Handbook—Fundamentals; ACCA Manual S.