About Energy Unit Converter — kWh, BTU, Joules, Therms & kcal
The energy unit converter translates between the units that utility bills, equipment datasheets, and engineering calculations mix constantly: kilowatt-hours from electricity meters, therms from US natural-gas bills, BTU from heating equipment, joules and megajoules from SI calculations, kilocalories from older thermal literature, and foot-pound-force from mechanical work. Every conversion runs through the SI unit — the joule — using NIST SP 811 Appendix B factors: 1 kWh = 3.6 MJ exactly, 1 BTU (IT) = 1,055.056 J, and 1 US therm = 1.054804×10⁸ J exactly.
The anchors worth memorizing: 1 kWh ≈ 3,412 BTU (the number that links electric heaters to gas furnaces), 1 therm ≈ 29.3 kWh (the number that makes gas-vs-electric cost comparisons honest), and 1 kcal = 4.1868 kJ exactly. Energy is a quantity, not a rate — kWh, not kW — and the converter treats it as a magnitude, so negative entries are rejected as sign mistakes rather than converted.
How It Works
- Enter the energy value, pick the unit it is currently in ("from"), and pick the unit you need ("to"). The headline shows the converted value; the table beneath shows the same energy in all nine units at once.
- Every conversion is value × (joules per from-unit) ÷ (joules per to-unit). kWh, Wh, kcal (IT), and the US therm are exact by definition; BTU (IT) and ft·lbf carry the published NIST 7-significant-figure values.
- Displayed values are rounded to 6 significant digits; the conversion itself runs in full double precision, so converting back returns your original number.
- Keep energy and power separate: a kWh is an amount, a kW is a rate. A 1 kW heater running 10 hours uses 10 kWh — the converter handles the kWh; multiplying by hours is your step. For gas bills, note the therm is a US billing definition (≈100,000 BTU); the EC therm differs in the fifth digit.
Worked Example
An electric-vs-gas heating comparison needs both fuels in one unit. The electric side: 1 kWh × 3.6×10⁶ J/kWh = 3.6 MJ, and 3.6×10⁶ ÷ 1,055.056 J/BTU = 3,412.14 BTU — the chart's first row. The gas side: 1 therm = 1.054804×10⁸ J ÷ 3.6×10⁶ = 29.3001 kWh. So gas at $1.50 per therm costs $1.50 ÷ 29.3 = 5.1¢ per kWh of heat content — compare that against the electric rate (times 1/COP for a heat pump) before deciding which fuel heats cheaper.
kWh to BTU conversion chart (with MJ and kcal)
Common electrical energy amounts converted with the NIST SP 811 factors (1 kWh = 3.6 MJ exact; 1 BTU IT = 1,055.056 J; 1 kcal IT = 4,186.8 J exact), shown to 6 significant digits — exactly what the converter outputs for the same inputs.
| Energy | BTU | MJ | kcal |
|---|---|---|---|
| 1 kWh (billing unit) | 3,412.14 | 3.6 | 859.845 |
| 5 kWh (dryer load) | 17,060.7 | 18 | 4,299.23 |
| 10 kWh (EV commute charge) | 34,121.4 | 36 | 8,598.45 |
| 50 kWh (EV full charge) | 170,607 | 180 | 42,992.3 |
| 100 kWh (heavy household day) | 341,214 | 360 | 85,984.5 |
Formulas
- Conversion through the base unit
x = value × f_from ÷ f_to- Joules per unit (the factor table)
kWh 3.6×10⁶ · Wh 3,600 · BTU 1,055.056 · kcal 4,186.8 · ft·lbf 1.355818 · therm(US) 1.054804×10⁸- The everyday anchors
1 kWh ≈ 3,412.14 BTU; 1 therm ≈ 29.3001 kWh; 1 kcal = 4.1868 kJ (exact)
Standards & References
- NIST SP 811, Guide for the Use of the International System of Units, Appendix B — conversion factors (BTU_IT 1.055056×10³ J; ft·lbf 1.355818 J; therm (US) 1.054804×10⁸ J exact)
- Exact definitions: 1 kWh = 3.6×10⁶ J; 1 Wh = 3,600 J; 1 kcal (IT) = 4,186.8 J
- The therm here is the US (billing) therm; the EC therm (1.05506×10⁸ J) differs in the fifth significant digit
Frequently Asked Questions
How many BTU are in a kWh?
1 kWh = 3,412.14 BTU (3.6×10⁶ J ÷ 1,055.056 J per BTU). That is the bridge between electric and combustion heating: a 1,500 W space heater delivers 1.5 × 3,412 ≈ 5,118 BTU per hour, and a 40,000 BTU/h furnace output is equivalent to about 11.7 kW.
How many kWh are in a therm of natural gas?
One US therm = 1.054804×10⁸ J = 29.3001 kWh of heat content. To compare fuel prices, divide the gas price per therm by 29.3 to get a cents-per-kWh heat cost — $1.50/therm is about 5.1¢/kWh — then weigh it against your electric rate, remembering furnace efficiency (≈80–97%) on the gas side and COP (≈2–4 for heat pumps) on the electric side.
Which BTU and which calorie does this tool use?
The International Table (IT) versions, which are what NIST SP 811 lists for fuel and HVAC use: 1 BTU (IT) = 1,055.056 J and 1 kcal (IT) = 4,186.8 J exactly. The thermochemical variants are about 0.07% smaller — irrelevant on a utility bill, but the honest answer to why different references disagree in the fourth digit. Note the food-label "Calorie" is the kilocalorie, not the small calorie.
What is the difference between kWh and kW?
kW is a rate (power); kWh is an amount (energy) — the rate sustained for one hour. A 2 kW appliance running 3 hours uses 6 kWh; a 50 kW EV charger delivering 30 minutes transfers 25 kWh. Utility bills charge for kWh (and sometimes peak kW demand separately). This converter handles energy; for kW ↔ HP ↔ BTU/h, use the power converter — BTU/h is a rate too.
Why is ft·lbf in an energy converter when torque uses the same unit?
Because work = force × distance along the motion is an energy, and lifting 1 lbf through 1 ft does 1.355818 J of work — the same number as the torque factor because both come from pound-force times feet. The physical difference (moment arm vs displacement) is why the site keeps separate torque and energy converters: 100 J of work is meaningful, "100 J of torque" is a category error.