Electricity

Current through the body is what kills, and voltage is what drives it. The heart and breathing muscles run on tiny electrical signals, and a current across the chest overrides them.

The body's resistance, hand to foot, is roughly 1,000–2,000 ohms at mains voltage, and lower with wet skin. So 230 V drives something like 100–230 mA through a person — well into the lethal range. Around 50 V AC is where shock becomes dangerous in wet conditions, which is why that is the usual limit for extra-low voltage.

The rules, in the order they matter:

1. Isolate, lock, prove dead. Switch off, lock the switch so nobody can turn it back on, and test the circuit is dead with a tester you have just proven on a live source. Only then touch it.
2. A 30 mA residual current device (RCD) on every socket circuit, and on anything outdoors, wet or portable.
3. Earth every exposed metal part, so a fault blows the fuse or trips the RCD instead of waiting in the casing for a hand.
4. Fuse every circuit, sized below what its cable can carry — see the table in the next section.
5. One hand when working near anything that might be live, and no work alone.
6. Capacitors stay charged after power is off; discharge them with an insulated tool first.

• Voltage (V) — the push.
• Current (A, amps) — how much flows.
• Resistance (Ω, ohms) — how hard the path resists it. Ohm's law: volts = amps × ohms.
• Power (W, watts) = volts × amps. Energy = power × time, counted in watt-hours (Wh) and kilowatt-hours (kWh).

The practical consequence: the same power at a lower voltage needs more current, and current is what heats wires. A 60 W load draws 0.26 A at 230 V but 5 A at 12 V — twenty times the current, and so a much thicker cable for the same run.

Measuring: a voltmeter goes across two points; an ammeter goes in series, in the path of the current — or use a clamp meter round one conductor. Never put a meter set to amps across a battery or a socket: its resistance is almost zero, and that is a short circuit through the meter and your hands.

Direct current (DC) flows one way at a steady voltage: batteries, solar panels, electronics.

Alternating current (AC) reverses direction many times a second, following a sine wave — 50 Hz in most of the world, 60 Hz in the Americas and parts of Asia. The number on the label is the RMS value, the DC voltage that would heat a resistor equally: 230 V RMS peaks at 325 V; 120 V RMS peaks at 170 V.

Why grids are AC: only AC can be transformed — stepped up to high voltage to travel, and down again to use (see Moving it). Three-phase AC — three sine waves a third of a cycle apart — carries more power per kilogram of copper and makes motors self-starting; in a 230 V system the voltage between phases is 400 V.

Where DC wins: anything stored in batteries, most electronics, and very long links — high-voltage DC beats AC for overhead lines beyond roughly 500–800 km and for undersea or underground cables beyond about 50–100 km.

Converting: a rectifier — diodes, often four in a bridge — turns AC into DC. An inverter turns DC into AC; pure sine-wave inverters run motors and sensitive electronics properly, while cheap modified sine ones make motors run hot and noisy.

Almost every source spins a generator; they differ in what does the spinning.

• Heat engines — coal, gas, oil, wood and other biomass, nuclear fission, geothermal heat and concentrated sunlight all boil water into steam that drives a turbine, or burn fuel in an engine. A steam plant turns roughly a third to two-fifths of its fuel's energy into electricity; the rest leaves as heat through the cooling towers.
• Moving water and air — hydro, wind, tide and waves turn a turbine directly, with no fuel and no heat to reject.
• Direct conversion — photovoltaic panels turn light straight into DC with no moving parts, and fuel cells turn hydrogen into DC.

Capacity factor — the share of the year a plant's full output is actually delivered — decides how much generation you really need, and dispatchability — whether it runs when you want — decides how much storage.

Each source in more depth, with its figures and its hazards: energy sources.

A changing magnetic field through a coil makes a voltage (Faraday's law). The voltage rises with how fast the field changes, how strong the magnets are, and how many turns the coil has — so a generator gives more volts the faster it turns.

• Alternator — produces AC. Most generators are alternators; car alternators and wind turbines rectify the output to DC.
• Dynamo — a commutator on the shaft flips the connections each half-turn to give DC.
• Frequency = number of magnetic poles × revolutions per minute ÷ 120. Four poles at 1,500 rev/min gives 50 Hz.

Small hydro: power in watts ≈ 9.81 × flow (litres per second) × head (metres) × efficiency, with 50–70% overall for a small plant. 20 L/s falling 10 m gives about 1 kW — 24 kWh a day, every day. Use a Pelton wheel for high head, a crossflow turbine for medium, and a propeller turbine for low head. An electronic load controller keeps the frequency steady by diverting unused power into a heater.

Small wind: power ≈ 0.6 × swept area (m²) × wind speed (m/s)³ × efficiency. No rotor can exceed 59% (the Betz limit), and small turbines reach 25–35%. A 3 m rotor in a 5 m/s wind gives only about 160 W. Mount it at least 9 m above anything within 150 m, and never let it run without a load.

Improvised generators: an ordinary induction motor becomes a generator driven slightly faster than its rated speed with capacitors across its terminals — a standard micro-hydro technique. Car alternators need a field current and around 1,000+ rev/min at the alternator shaft before they charge.

Wire has resistance, and the power lost in it rises with the square of the current. Double the voltage and, for the same power, the current halves and the loss falls to a quarter. That single fact is why power is transmitted at high voltage.

Worked example: send 1 kW 100 m over 2.5 mm² copper — 200 m of wire out and back, about 1.4 Ω.

• At 230 V: 4.3 A, a drop of 6 V (2.6%), and 26 W lost. Fine.
• At 12 V: 83 A, and the calculated drop is 115 V — more than the supply. It simply cannot be done. Even over 10 m, holding the drop to 3% at 12 V needs a conductor of about 80 mm², as thick as a finger.

Transformers make high voltage possible: the voltage ratio equals the turns ratio, and they work only on AC. A typical grid generates at 10–25 kV, steps up to 110–765 kV for transmission, steps down to 11–33 kV for distribution, and down again to 230/400 V or 120/240 V at the street. Insulators get longer as voltage rises, and clearances grow with them.

For a small settlement: keep the generator close to the loads, choose a higher system voltage when distances are long, and size every cable by voltage drop as well as by current.

Capacity in watt-hours = amp-hours × volts. A 12 V, 100 Ah battery holds 1.2 kWh — but you cannot use all of it without destroying it.

Lead-acid — the makeable one, see electrochemistry. Keep discharge to about 50% for a long life. State of charge from the resting voltage of a 12 V battery, after at least four hours without charge or load: 12.7 V full, about 12.4 V at 75%, 12.2 V at 50%, about 12.0 V at 25%, 11.9 V or less flat. Flooded batteries charge at about 14.4–14.8 V and float at 13.5–13.8 V, and give off hydrogen while charging. Never leave one flat — it sulphates and is ruined.

Lithium iron phosphate (LiFePO4) — 3.2 V per cell, so four in series make a 12.8 V battery. It tolerates 80–90% discharge for thousands of cycles, and is the safest common lithium chemistry. It needs a battery management system watching every cell, and must never be charged below 0 °C.

Beyond batteries: pumped water — 1,000 tonnes lifted 100 m stores about 270 kWh; hot water — 1.16 kWh per cubic metre per degree of temperature rise; and simply doing energy-hungry work while the sun shines or the wind blows.

Design from the loads backwards:

1. List every load, its watts and its hours per day, and add up the watt-hours per day — see the table.
2. Pick the system voltage by power: 12 V up to about 1 kW, 24 V up to about 2–3 kW, 48 V above. Higher voltage means less current and thinner cable.
3. Size the storage — see Storing it.
4. Size the generation. For solar: daily watt-hours ÷ peak sun hours ÷ 0.75. 1,000 Wh a day with 4 peak sun hours needs about 330 W of panels. For hydro or wind, use the formulas in Generating it.
5. Add a charge controller. For solar, an MPPT controller gets roughly 10–30% more from the panels than a simple PWM one. For wind and hydro, the controller must divert surplus to a dump load, never simply disconnect.
6. Size the inverter above your largest combined load. Motors draw 3–7 times their running current for a second or two when starting, so check the inverter's surge rating.
7. Fuse at the battery, as close to the terminal as possible, then a fuse or breaker on every circuit. Use fuses and breakers rated for DC: a DC arc does not go out by itself the way AC's does at every zero crossing, and AC-only breakers can fail to break it.

Salvaged panels: measure open-circuit voltage and short-circuit current in full sun. Series adds volts, parallel adds amps, and panels in series must match. A panel is live whenever light reaches it: cover it with something opaque before working on the wiring.