1 Understand
Volts, amps, watts: the only maths you need
Four quantities and two formulas cover almost every decision about a small off-grid system.
Think of electricity as water in a pipe. Voltage is the pressure, current is how much flows, resistance is how narrow the pipe is, and power is the work done per second. A battery stores energy: power multiplied by time.
Voltage · V
Electrical pressure. Car and small solar systems: 12 V. Bigger systems: 24 V or 48 V. House sockets in Europe: 230 V AC, lethal.
Current · A
Flow. Current is what heats wires and starts fires. Wire thickness and fuses are chosen by amps.
Power · W
W = V × A. A 60 W load draws 5 A at 12 V but only 0.26 A at 230 V.
Energy · Wh
Wh = W × hours. A 10 W lamp for 5 h uses 50 Wh. Battery: Wh = Ah × V, so 100 Ah at 12 V holds 1200 Wh.
Resistance · Ω
Ohm's law V = I × R. Thin or long wire has more resistance, so it wastes voltage as heat.
AC and DC
Batteries and solar panels give DC (one direction). The grid and inverters give AC (alternating 50 times a second). Most electronics run on DC inside.
What things actually use
Write down every device, its watts and hours per day, then add up the Wh. This number sizes your battery and panels (calculators).
| Device | Power | Typical use per day | Energy |
|---|---|---|---|
| LED bulb | 5–10 W | 5 h | 25–50 Wh |
| Phone charge | 5–20 W | 1 full charge | 10–20 Wh |
| Laptop | 30–65 W | 3 h | 100–200 Wh |
| Radio receiver | 1–5 W | 5 h | 5–25 Wh |
| Wi-Fi router | 6–12 W | 24 h | 150–300 Wh |
| 12 V compressor fridge | 40–60 W running | cycles on and off | 250–500 Wh |
| Efficient house fridge | 60–100 W running | cycles | 300–800 Wh |
| Chest freezer | 60–100 W running | cycles | 400–1000 Wh |
| 12 V water pump | 40–100 W | 30 min | 20–50 Wh |
| Deep well pump (230 V) | 750–1500 W, 3× surge | 30 min | 400–750 Wh |
| CPAP machine | 30–60 W | 8 h | 250–500 Wh |
| Washing machine | 2000 W heating | one cold wash | 200–500 Wh |
| Kettle, heater, oven, hob | 1000–3000 W | – | Too much for small solar. Cook and heat with fire. |
Anything that makes heat electrically is a battery killer. Kettles, heaters, hair dryers, toasters, electric hobs and immersion heaters use in minutes what lights use in a week. Off-grid electricity is for light, communication, pumping, refrigeration of medicine and small tools.
2 Build
Your first 12 V circuit: battery, fuse, switch, load
Every DC system, from a torch to a cabin, is this same loop repeated. Learn it once, safely, at 12 V.
Wire a 12 V lamp
- Start at the battery. Red is positive (+), black is negative (−). Remove rings and watches.
- The fuse goes on the red wire, as close to the + terminal as you can (within 20 cm). It protects the wire, not the lamp.
- The switch also goes on the positive side, after the fuse, so everything past it is dead when it is off.
- Run the red wire to the load, then a black wire from the load back to −. Connect − last.
- Close the switch: current flows round the loop and the lamp lights. Break the loop anywhere and it stops.
Series
Things in one line, one after another. The same current goes through all of them. Fuses and switches are always in series with what they protect.
Parallel
Each load gets its own pair of wires from the supply. They all see the full 12 V and one failing does not switch off the others. House lights and sockets are in parallel.
Short circuit
+ touching − with nothing in between. A car battery can push over 500 A: tools weld, wires glow, batteries explode. That is what the fuse is for.
Good connections
- Crimp with ring or fork terminals and a proper crimping tool, then tug hard. A connection that pulls out will one day arc and burn.
- Terminal blocks or Wago-style lever connectors for low current. Twist-and-tape only as a temporary fix.
- Solder for electronics and thin wire; never rely on solder alone where wire vibrates (vehicles). See soldering.
- Every connection needs strain relief: clip or tie the wire so a pull does not reach the joint.
- A warm connection under load is a bad connection. Check by touch (low voltage only) after 10 minutes of use.
3 Protect
Wire size and fuses: where fires start
At 12 V the currents are large. A wire that is fine for a 230 V lamp can melt at 12 V carrying the same power.
| Copper cross-section | Max continuous current (short runs, in air) | Typical use at 12 V |
|---|---|---|
| 0.75 mm² | 6 A | LED strips, sensors, radios |
| 1.5 mm² | 10–15 A | Lights, USB sockets, small fans |
| 2.5 mm² | 20 A | Pump, fridge, socket circuit |
| 4 mm² | 30 A | Solar panel to controller, small fuse box |
| 6 mm² | 40 A | Controller to battery |
| 10 mm² | 60 A | Main feed to fuse box |
| 16 mm² | 80 A | 500 W inverter |
| 25–35 mm² | 120–150 A | 1000–1500 W inverter, battery links |
| 50 mm² | 200 A | 2000 W inverter, starter cable |
Voltage drop: the hidden loss
Long thin wire eats voltage before it reaches the load. Keep the drop under 3 % (0.36 V at 12 V). Copper:
drop (V) = 2 × length (m) × current (A) × 0.0175 ÷ area (mm²)
The 2 is because current goes out and back. Example: a pump 5 m from the battery drawing 10 A. With 2.5 mm² the drop is 0.7 V (6 %, too much). With 6 mm² it is 0.29 V (2.4 %, good). Put the battery close to the loads, or move up to 24 V.
Fuses: size them to the wire
Rated below what the wire can carry
The fuse must blow before the wire overheats. 1.5 mm² wire: 10 A fuse at most.
Rated above the normal load
About 1.25 × the continuous current, so it does not blow in normal use. A 6 A load gets an 8 or 10 A fuse.
At the source end
Within 20 cm of the battery, on the positive wire. Every wire that leaves the battery gets its own fuse. A fuse at the far end protects nothing.
A main fuse on the battery
A big fuse (ANL, MEGA or MRBF) right on the + terminal, sized to the biggest cable. Lithium and big lead-acid banks can deliver thousands of amps into a short.
Never "fix" a blowing fuse with a bigger one
Find the fault. Never bridge a fuse with wire or foil. That is how vehicles and cabins burn.
Car blade fuses (ATO/ATC, mini) and their holders are perfect for 12 V systems. Every car has 20–60 of them; strip the fuse box and inline holders (vehicle salvage). Blade colours: 5 A tan, 7.5 A brown, 10 A red, 15 A blue, 20 A yellow, 25 A clear/white, 30 A green.
4 Store
Batteries: treat them well and they last years
The battery is the most expensive and most easily ruined part. Its type decides how deep you may discharge it and how to charge it.
| Type | Use down to | Life (cycles) | Notes |
|---|---|---|---|
| Car starter (lead-acid) | 80 % full | 30–150 deep | Built for short bursts. Dies quickly if drained. Fine as a stopgap. |
| Leisure / deep cycle flooded | 50 % | 300–1000 | Top up with distilled water. Gives off hydrogen when charging: ventilate. |
| AGM / gel | 50 % | 400–1200 | Sealed, no watering, less gas. Hates overcharge. |
| LiFePO4 (lithium iron phosphate) | 10–20 % | 3000–6000 | Best choice. Light, efficient, safe chemistry. Built-in BMS. Do not charge below 0 °C. |
| Li-ion (18650, laptop, e-bike, power tool) | 10–20 % | 300–1000 | High energy, real fire risk when damaged or overcharged. Needs a BMS. |
| NiMH AA/AAA | empty | 500–1000 | Torches, radios. Low self-discharge types (Eneloop) hold charge a year. |
How full is it? Read the voltage
Measure after the battery has rested an hour with nothing charging or drawing. Lead-acid reads well by voltage; lithium is so flat that only a battery monitor (shunt) is accurate.
| Charge | 12 V lead-acid (at rest) | 12 V LiFePO4 (at rest) |
|---|---|---|
| 100 % | 12.7 V | 13.4 V |
| 75 % | 12.4 V | 13.2 V |
| 50 % | 12.2 V · stop here | 13.1 V |
| 25 % | 12.0 V · damage | 13.0 V |
| 10 % | 11.9 V | 12.8 V · stop here |
| Empty | 11.8 V | below 12 V, BMS cuts off |
Charging voltages
Flooded lead-acid
Absorption 14.4–14.8 V, float 13.5 V. Equalise monthly at 15 V if the maker allows. Recharge to 100 % often; left half-empty, it sulphates and dies.
AGM / gel
Absorption 14.4 V (gel 14.1 V), float 13.6 V. Never equalise gel.
LiFePO4
Charge to 14.2–14.4 V, no float or 13.5 V. Happy at 30–80 %, no need to fill it daily. No charging below 0 °C.
Series and parallel banks
- Hydrogen: lead-acid batteries release explosive gas while charging. Ventilate, no sparks nearby, connect the last clamp away from the battery.
- Acid: sulphuric acid blinds. Eye protection when topping up; rinse skin and eyes with lots of water for 15 minutes.
- Lithium fire: a swollen, hot, hissing or damaged lithium battery can burst into a fire that water barely controls. Move it outdoors onto soil or stone if it is safe to do so, and keep away. Never charge lithium unattended at night near sleeping people.
- Spanners across terminals weld and burn. Wrap tools in tape, disconnect − first and reconnect it last.
5 Generate
Solar: the power source that keeps going
No fuel, no moving parts, 25+ years of life. A panel, a charge controller and a battery run lights, radios, phones and a pump indefinitely.
Charge controllers: PWM or MPPT
PWM cheap
Works only with "12 V" panels (36 cells, about 18 V working, 22 V open circuit). Simple and robust. Wastes 20–30 % of a panel's power.
MPPT better
Converts high panel voltage down to battery voltage and harvests 15–30 % more, most in cold and cloudy weather. Can use large house-roof panels (60/72/108/120 cells, 30–50 V).
Panel open-circuit voltage (Voc, on the label) rises in the cold, about +12 % at −10 °C. Panels in series add their Voc. Two 40 V panels = 80 V → 90 V on a frosty morning: fine for a 100 V controller. Three would be 134 V and destroy it.
Connect a solar kit in the right order
- Fit the battery fuse first and leave it out while wiring.
- Connect the controller to the battery first, then insert the fuse. The controller detects 12 or 24 V from the battery.
- Only now connect the panel. Cover it with a blanket or connect in the dark: panels are live whenever light hits them.
- Connect loads last. To disconnect, reverse the order: loads, panel, then battery.
Sizing a small system
Panel: daily Wh ÷ sun hours ÷ 0.75 (losses). Sun hours in central Europe: about 4–5 in summer, 0.5–1.5 in December and January. Size for the worst month you need power in.
Battery: daily Wh × days of cloudy reserve (2–3) ÷ usable fraction (0.5 lead-acid, 0.8 LiFePO4) ÷ 12 V = Ah.
| Worked example | Summer | Winter |
|---|---|---|
| Lights 4 × 5 W × 5 h, phones 4 × 15 Wh, radio, laptop 2 h | 300 Wh | 300 Wh |
| 12 V fridge (winter: cold box outdoors instead) | 400 Wh | 0 |
| Water pump 30 min | 50 Wh | 50 Wh |
| Total per day | 750 Wh | 350 Wh |
| Panel needed | 750 ÷ 4.5 ÷ 0.75 ≈ 220 W | 350 ÷ 1 ÷ 0.75 ≈ 470 W |
| Battery, 2 days, LiFePO4 | 750 × 2 ÷ 0.8 ÷ 12 ≈ 150 Ah (lead-acid: 250 Ah) | |
Placing panels
- Face due south (in the northern hemisphere). Any shade on part of a panel cuts its whole output, so avoid trees, chimneys and snow.
- Tilt from horizontal ≈ your latitude for all year, latitude + 15° for winter (steeper also sheds snow), latitude − 15° for summer.
- Wipe dust, pollen and bird droppings off with water. Brush off snow.
- Mount firmly: a panel is a sail. Keep the cable short from panel to controller and the controller close to the battery.
- Salvaged house panels are still fine at 20+ years old, losing roughly 0.5 % a year. Cracked glass lets water in; seal with clear silicone and use it anyway.
6 Convert
Inverters: 230 V from a battery, at a price
An inverter makes household AC from 12 V DC. Useful, but it wastes energy and pulls huge currents from the battery.
Pure sine wave
Clean AC like the grid. Needed for motors, fridges, pumps, power tool chargers, medical equipment and anything with a clock or dimmer. Buy or salvage this kind.
Modified sine
Cheaper, choppy output. Fine for lamps and laptop bricks. Motors run hot and noisy; some chargers and devices die.
Idle draw
A big inverter burns 10–40 W doing nothing, 240–960 Wh a day. Switch it off when not in use. Often it uses more than the loads.
Surge
Motors take 3–7 × their running power to start. A 150 W fridge may need a 1000 W inverter to start reliably.
Current from the battery ≈ watts ÷ 10 at 12 V. A 1000 W load pulls about 100 A: 25–35 mm² cable, under 1.5 m long, with a 125–150 A fuse at the battery. At this size, 24 V halves the current.
Better: run things on 12 V directly. Phones and USB from 12 V USB sockets, 12 V LED lights, 12 V fans and pumps, laptop 12 V car adapters. Every conversion loses 10–20 %.
7 Backup
Generators and cars: fuel turned into power
Useful for bursts: pumping a well, charging a battery bank in winter, running a power tool. Fuel runs out; plan to depend on it less every month.
Never run a generator, car engine or petrol tool in a house, garage, basement, tent or shed, even with the door open. Place it outdoors, at least 6 m from windows and doors, exhaust pointing away. CO has no smell. Headache, dizziness and nausea in several people at once means get everyone into fresh air now.
Plugging a generator into a wall socket with a male-to-male lead energises the whole house wiring and the street cable. It can kill line workers and neighbours, and the plug pins are live in your hand. Use extension leads from the generator to devices, or have an electrician fit a transfer switch or interlock that disconnects the grid first.
Using a generator well
- Run it loaded for a few hours rather than idling all day. Charge batteries, pump water, do laundry and power tool work in the same run.
- Petrol generators use roughly 0.5–1 L per hour at half load. A small inverter generator (1–2 kW) is far more efficient at light loads than a big site generator.
- Earth it as the manual says; keep it dry under an open-sided cover.
- Change oil after the first 5–10 hours, then every 50–100 hours. Stored petrol: see fuel storage.
The car as a power station
- A running car charges its own battery at about 14 V and can charge a second battery through jump leads or a split-charge relay (or a DC-DC charger for lithium).
- Idling burns about 0.6–1 L an hour and charges slowly. Better to charge while you are driving anyway.
- With the engine off, use the 12 V socket for phones and radios only. Stop before the battery drops below 12.2 V or the car will not start.
- An electric car or plug-in hybrid holds 10–80 kWh. Many can power 230 V devices through a V2L adapter or run a 12 V inverter from the 12 V system while "ready". Do not open the orange high-voltage cables.
8 Measure
The multimeter: your eyes for electricity
A cheap meter answers every question: is the battery charged, is the fuse blown, is this wire broken, how much does this draw.
Three measurements
- Voltage: dial to V DC (20 V range), black in COM, red in V. Touch red to +, black to −. The meter goes across (in parallel).
- Continuity: power off, dial to the beeper or Ω. Probes on both ends of a fuse, wire or switch. Beep or near 0 Ω = good. "OL" or 1 = broken.
- Current: move red to the A socket, open the circuit and put the meter in the loop (in series). Never touch the probes across a battery in this mode. Move the red lead back to V afterwards.
Something does not work? Measure voltage at the battery, then at the fuse, the switch and the load, moving outward. Where the 12 V disappears, the fault is between that point and the last good one. A reading that is fine with no load but collapses when switched on means a bad connection.
9 Alternatives
Wind, water and muscle: when the sun is weak
Solar is weakest in winter, exactly when you need light most. These fill the gap in the right places.
Micro-hydro best if you have it
A stream with a drop runs day and night. Power ≈ 5 × flow (L/s) × head (m) watts after losses. 2 L/s falling 10 m through a pipe gives about 100 W, 2.4 kWh a day, more than a large solar array in December.
Small wind
Only worth it on open, exposed sites: hilltops, coasts. Needs a tall mast (clear of obstacles by 10 m), a brake, and maintenance. Power grows with the cube of wind speed, so a sheltered garden gives almost nothing.
Bicycle generator
A fit person makes 50–100 W for an hour. That is 50–100 Wh: phones and lights for a day, not a fridge. A dynamo hub on a bike charges USB while riding.
Hand-crank and solar gadgets
Crank radios, dynamo torches, solar garden lights (salvage the small panel, NiMH cell and LED). Keep several; they need no system at all.
Direct-drive washing machine motors, treadmill DC motors and car alternators can be turned into generators, but alternators need 1500+ rpm and power to excite the field. Motors used as generators need a rectifier, a charge controller and a dump load so the battery is not overcharged. Treat it as a project, not a plan.
! Stay alive
Electrical safety: rules without exceptions
12 V will not shock you but it will burn a building down. 230 V kills through the heart in a fraction of a second.
Before working on anything
Do not touch the person until the power is off. Switch off at the breaker or pull the plug; if impossible, push them free with dry wood or plastic while standing on something dry. Then check breathing and start CPR if needed (CPR). Anyone who had mains current through the body should be watched for hours: heart rhythm problems and internal burns can appear later.
RCD / GFCI
A 30 mA residual-current device cuts power when current leaks through a person. Use one on every generator and inverter circuit that feeds sockets, especially outdoors. Press its test button monthly.
Cables
Unroll extension reels fully under load, or they overheat. No cables under rugs or through doors. Keep plugs off wet ground.
Mains work
If you are not trained, do not wire 230 V. Build your off-grid system at 12 or 24 V with plug-in inverters, where your mistakes are survivable.