Common mistakes in electricity: beware of confusions and approximations!*
Working with electrical quantities requires precision and method. With similar units, conversions not to be forgotten, and real phenomena sometimes overlooked, certain errors occur very frequently. Here is an overview of the main mistakes to avoid.
1. Confusing power, energy, and capacity
This is a very common confusion:
Watt (W) → power: it is the rate of energy per unit of time.
Example: a 100 W lamp consumes 100 joules of energy per second.
Watt-hour (Wh) → energy: it is a quantity of energy.
Example: a 100 Wh battery can power a 100 W device for 1 hour.
Ampere-hour (Ah) → capacity of a battery, independent of the voltage.
Example: a 12 V – 10 Ah battery contains approximately 120 Wh of energy (12 × 10).
Common mistake: believing that a 10 Ah battery will have the same autonomy regardless of the voltage. However, it is the product U × Ah = Wh that matters!
2. Forgetting the conversion between speed and angular speed
In electromechanics, mechanical power is expressed as:
where C is the torque (in N·m) and ω is the angular speed in rad/s.
However, most rotational speeds are given in rpm (n).
Therefore, it is necessary to convert:
3. Neglecting efficiencies and real losses
In an energy chain (battery → converter → motor), each link has an efficiency below 100%:
converter: ~90–95%
motor: ~85–95%
mechanical transmission: ~80–90%
Common mistake: calculating power or autonomy as if everything were perfect.
Example: a 1000 W motor powered by a 100 Wh battery will not run for 0.1 h, but rather 0.07–0.08 h after accounting for losses.
Similarly, current spikes at startup can cause voltage drops, or even trigger protections if the system is undersized.
4. Undersizing cables and ignoring mechanical constraints
Choosing the cable section is essential: too high a current in a too thin conductor causes:
dangerous heating;
Joule effect losses (I²R);
a voltage drop at the arrival.
Common mistake: choosing a section “by guess” without consulting tables or ignoring the cable length.
Moreover, poor tightening of electrical terminals is another frequent source of incidents: insufficient tightening torque creates a contact resistance, which heats up and can lead to electrical arcs.
In summary
| Type of error | Description | Consequence |
| Confusion W / Wh / Ah | Misinterpretation of capacity or autonomy | Incorrect calculations |
| Forgetting conversion n → ω | Incorrect mechanical power | Over/underestimation of performance |
| Neglected efficiencies | Overly optimistic energy chain | Reduced real autonomy or power |
| Undersized cable / poor tightening | Heating, losses, fire risk | Electrical danger |
In electricity, quantities may look similar but should not be confused. Taking the time to check units, conversions, and real usage conditions helps avoid many errors, sometimes costly or dangerous.
*: The technical information presented in this article is provided for guidance only. It does not replace the official manuals of manufacturers. Before any installation, handling, or use, please consult the product documentation and follow the safety instructions. The Torque.works site cannot be held responsible for inappropriate use or incorrect interpretation of the information provided.