Resistive Heating

Direct conversion of electricity into heat via the Joule effect: power, seasonal consumption, and comparison with higher-performance systems.

Conceptual context

Resistive heating is the most direct way to convert electricity into heat: all the electrical energy passing through the resistance is dissipated as heat through the Joule effect, with no intermediate thermodynamic cycle. It is the reference system against which more sophisticated technologies such as the heat pump are compared, precisely because it marks the lowest possible efficiency floor.

This leaf breaks down the consumption of a resistive system into its three pieces — emitter power, effective usage time, and the resulting electrical consumption — so the student understands why an almost-perfect conversion system can still be expensive to run, and what real levers exist to reduce its cost.

🟢 Essential level

To understand resistive heating, three concepts that are often confused must be clearly separated:

Power $P$ is the rate at which the emitter consumes electricity. A 1.5 kW radiator consumes 1.5 kilowatts every second it operates. It is a fixed characteristic of the equipment that does not change with use. If the thermostat disconnects the device, $P$ stops acting on the circuit; if it keeps it on continuously, $P$ is constant.

Effective time $t$ is the variable the user controls most freely. It is not measured in hours with the device switched on, but in hours with the resistance actually active. A radiator with a thermostat may be switched on for 8 hours but with the resistance active for only 5 of those hours. Those 5 hours are what determine the $E$.

Consumption $E$ is the system's result: how much electrical energy the emitter has used in the analyzed period. It is exactly what the meter reads and what appears on the bill. The relationship between these three quantities is the simplest law of applied thermodynamics: $E$ is the product of $P$ by $t$.

This relationship has a direct and powerful implication: in resistive heating, to halve consumption you can halve $P$ (buy a less powerful emitter) or halve $t$ (use it half the time). Both strategies are physically equivalent but have very different practical consequences for comfort.

The conversion efficiency of resistive systems is extraordinarily high, typically between 0.95 and 1.0, meaning that virtually all electricity consumed reaches the space as heat. This is what makes resistive heating transparent and predictable.

🔵 Formal level

Basic formulas

The fraction of $E$ that actually heats the room is the $Q$, obtained by applying the emitter's conversion efficiency.

The fundamental mathematical relationship is the definition of $E$ as the product of $P$ by $t$:

Electrical consumption

Once consumption is known, the $Q$ delivered to the room is obtained by applying the emitter's conversion efficiency:

Useful heat

In practice, for domestic resistive emitters, conversion efficiency ≈ 0.98, meaning $Q$ is virtually equal to $E$.

The $COP$ of the system is defined as the ratio between $Q$ and the electrical energy invested $E$:

COP of resistive heating

Real student questions

• If it converts almost 100% of electricity into heat, why is it expensive? Because $COP$ is equal to 1 means you need exactly 1 electrical kWh for every thermal kWh you want. A heat pump with $COP$ is equal to 4 only needs 0.25 electrical kWh for the same heat, making it 4 times cheaper in energy terms.
• Which matters more, $P$ or $t$? Both are mathematically equivalent, but in domestic use $t$ is easier to control (schedules, thermostat) and usually offers more room for savings.
• Can I compare resistive and heat pump heating by nominal power? No. You must compare the $E$ needed to deliver the same $Q$ under the same conditions.

Final synthesis

Resistive heating is the most transparent system for understanding the relationships between $P$, $t$, and $E$. Its $COP$ = 1 is not a flaw but its most pedagogically valuable characteristic: it clearly defines the energy floor from which any other heating technology must demonstrate its advantage. Understanding resistive heating well is the first step to rigorously evaluating the efficiency of any alternative system.