Electric heating: Science and limits
Why every heater is 100% efficient, yet some waste half
What this tab covers What physically happens in the conversion, step by step, the science behind it, and the hard limits that physics sets on how well it can ever work.Electric heating turns electricity into heat by pushing current through a resistance, inducing currents in metal, striking an arc, or shaking water molecules with microwaves. Nearly all the electricity becomes heat; the question is how much of it reaches the target.
The conversion, step by step
Resistive element (kettle, geyser, space heater)Electrical → Heat in the element · Essentially 100% of electricity becomes heat (first law); losses are heat escaping from the tank or pot2Induction hob: inverter and coil
Electrical (50/60 Hz AC) → High-frequency magnetic field, then eddy-current heat inside the pan · About 85% to the food, versus 75-80% for resistance hobs and about 32% for gas1Electric arc furnace
Electrical (via furnace transformer) → Arc and resistance heat melting scrap steel · 400-500 kWh of electricity per tonne of steel in modern furnaces, against a theoretical minimum of about 370 kWh/t (1.33 GJ/t, calculated) to melt scrap and heat it to 1,600 degrees C511Microwave oven: magnetron
Electrical → Microwaves at 2,450 MHz · About 70% maximum at 2,450 MHz (quoted figures sometimes higher)6Microwave absorption in food
Microwaves → Heat in water molecules · Whole domestic oven 50% or less, electricity to heat in food6
The science
When current flows through a material with resistance, colliding electrons pass energy to the atoms, which vibrate faster: that is heat (Joule heating, power = current squared times resistance). Kettles and water heaters use a metal-sheathed element; arc furnaces pass tens of thousands of amperes through graphite electrodes into the scrap, forming arcs hotter than the steel's melting point. Induction heating uses Faraday's law: a coil carrying high-frequency current makes a changing magnetic field, which induces swirling (eddy) currents inside an iron pan or metal charge, so heat appears in the metal itself rather than in a hot plate. Microwave ovens use a magnetron to make radio waves at 2,450 MHz; water molecules, which have positive and negative ends, try to turn with the rapidly reversing field and their jostling becomes heat. Heating 1 litre of water from 15 to 100 degrees C needs about 356 kJ, or 0.099 kWh (calculated from water's specific heat of 4.18 kJ per kg per degree).
Limits set by science
| Limit | Value | Why it exists | Best today |
|---|---|---|---|
| Conversion ceiling for resistance heating | 100% of electricity becomes heat2 | Energy is conserved, so a resistance heater cannot be more than 100% efficient. But the second law allows a heat pump to move extra heat from the outside air, delivering several units of heat per unit of electricity. | US rules from 2029 for common electric water heaters require heat-pump-level performance, more than doubling efficiency versus resistance tanks |
| Minimum energy to melt scrap steel | About 1.33 GJ, or about 370 kWh, per tonne (calculated) for scrap melted and heated to 1,600 degrees C115 | The heat needed to warm steel to its melting point (about 1,500 degrees C), melt it and superheat it for tapping. No furnace can use less, whatever the heat source. | Modern furnaces use 400-500 kWh/t of electricity; electricity supplies up to about 60% and oxygen and carbon reactions up to about 25% of total energy input. DOE put total practice at 2.1-2.4 GJ/t of liquid steel (2000). |
| Magnetron efficiency | About 70% maximum at 2,450 MHz; 88% at 915 MHz (industrial)6 | Some electron energy ends up heating the magnetron's anode instead of becoming microwaves. Solid-state amplifiers reach up to about 69% but deliver energy more precisely. | Domestic ovens deliver 50% or less of their electricity into food |
| Coupling to the target | Induction about 85% versus gas about 32%1 | The more of the heat produced directly inside the pan or food, the less escapes into the room. | Induction hobs |
Sources
- ENERGY STAR Emerging Technology Award 2021: Residential Induction Cooking Tops US EPA / DOE ENERGY STAR, 2021
- DOE finalizes efficiency standards for water heaters to save Americans over USD 7 billion on household utility bills annually (press release, 30 April 2024) US Department of Energy, 2024
- Article modelling electric arc furnace electrical energy consumption (University of Ljubljana authors), DOI 10.1007/s40831-021-00390-y Journal of Sustainable Metallurgy (Springer), 2021
- Energy issues in microwave food processing: a review of developments and the enabling potentials of solid-state power delivery (Atuonwu and Tassou, Critical Reviews in Food Science and Nutrition; author manuscript) Brunel University London repository, 2019
- Theoretical Minimum Energies to Produce Steel for Selected Conditions (Fruehan, Fortini, Paxton and Brindle, Carnegie Mellon University) US Department of Energy, Office of Industrial Technologies, 2000