An electrical setup can be read with four values and a single calculation. Everything else follows from them.
Reading a setup
It all starts with a rating plate. Four values, and nothing else is needed to decide.
- The voltage, in volts. This is the value that destroys. Powering a device at the wrong voltage kills it immediately, sometimes violently.
- The current, in amps. What the device draws.
- The power, in watts. Often written down; with direct current, it's the voltage multiplied by the current.
- The type of current. A straight line for direct current, a wavy line for alternating current. They're not variations of the same thing.
The relationships between voltage, current and resistance, and wiring in series and in parallel, are detailed in Lire un circuit électrique.
One calculation is used every day. Power in watts multiplied by a duration in hours gives energy in watt-hours. It's the only unit that lets you compare a need with a reserve. A capacity in amp-hours says nothing on its own: multiply it by the nominal voltage. That's why two batteries advertised with the same capacity can hold very different amounts of energy.
Once you have that calculation, sizing becomes trivial: add up your watt-hours per day, compare them with what you store, and you know how many days you can last. The same calculation decides whether electric propulsion is realistic, see Boat engines — the complete guide.
Handling rules
- Switch off and check there's no voltage before touching anything. An unplugged device isn't a safe device.
- Read the plate before plugging anything in. Always.
- Respect polarity with direct current. It has a direction. Reversing it destroys electronics instantly, without warning.
- Set aside any deformed lithium cell. Swollen, punctured, crushed: it can't be repaired and it can't be tested. It can go into thermal runawayA self-sustaining chain reaction inside a damaged lithium cell, which needs no outside oxygen..
- Never touch the large capacitors in a switch-mode power supply, a microwave or a camera flash. They hold a charge that can kill for several minutes after being unplugged, sometimes much longer.
Where to find what
You're looking for storage. Standard cylindrical lithium cells are found in a large share of power-tool and laptop battery packs. A dead pack is rarely dead as a whole: it's almost always one or two cells dragging the rest down, while the others are still good.
You have no battery-management electronics. Then prefer lead-acid. Batteries from UPS units and alarm systems keep a residual charge for a long time and can be recharged with basic means. That's their decisive advantage over lithium, which needs precise management or it can catch fire.
You're looking for power generation. Solar panels from road signs, garden lights and street furniture are everywhere, rarely watched, and produce power straight away. Sizing a full setup is covered in Le solaire photovoltaïque, du panneau à la prise.
You're salvaging on the spot. Sorting out what can still be used is in Telling what's still usable, and the search method is in Fouiller un bâtiment abandonné.
You want to go further. Wiring into a system, overcurrent protection, earthing and live work belong in separate sheets. These are subjects where a mistake is deadly, not just expensive: they're covered on their own, never at the end of a sheet.