Basic Electrical for Solar
The electrical basics every solar designer and installer needs — explained simply, with formulas, worked examples, a practice calculator and a quiz. Free study material; join the live course for hands-on practice.
The basic electrical quantities
Think of electricity like water in a pipe: voltage is the pressure, current is the flow, and resistance is how narrow the pipe is.
| Quantity | Symbol | Unit | What it means |
|---|---|---|---|
| Voltage (potential difference) | V | volt (V) | The "push" that drives current. Home supply in India: 230 V AC. |
| Current | I | ampere (A) | The flow of charge through a conductor. |
| Resistance | R | ohm (Ω) | Opposition to current. Longer or thinner wires have more resistance. |
| Power | P | watt (W), kW | The rate of using or producing energy. |
| Energy | E | kWh ("unit") | Power × time. 1 unit on your bill = 1 kWh. |
| Frequency | f | hertz (Hz) | How many times AC changes direction per second. India: 50 Hz. |
| Charge / battery capacity | Q | ampere-hour (Ah) | Battery storage. Energy (Wh) = Ah × battery voltage. |
Conductors
Let current flow easily: copper, aluminium, silver. Copper is used for most solar DC and house wiring; aluminium for large LT/HT cables.
Insulators
Block current: PVC, XLPE, rubber, glass, porcelain. Cable insulation and the PV module back-sheet are insulators.
Ohm's law and electrical power
Ohm's law links voltage, current and resistance. Combine it with the power formula and you can find any value from two others.
Ohm's law
V = I × R I = V ÷ R R = V ÷ IExample: a 12 V battery across a 4 Ω heater → I = 12 ÷ 4 = 3 A.
Power
P = V × I P = I² × R P = V² ÷ RExample: a 230 V, 2 kW geyser draws I = 2000 ÷ 230 ≈ 8.7 A.
Energy
E (kWh) = P (kW) × hoursExample: a 1.5 kW AC running 8 h/day uses 12 kWh/day ≈ 360 units/month.
Losses in cables
Power loss = I² × RDoubling the current makes cable losses four times bigger. That is why solar strings use high voltage and low current.
Series and parallel circuits
The same rules apply to resistors, solar panels and batteries.
Series (end to end)
- Same current through every part
- Voltages add up
- Rtotal = R₁ + R₂ + R₃ …
- Solar: panels in a string — 6 × 41.6 V Vmp = 250 V
- Batteries: 4 × 12 V in series = 48 V, same Ah
Parallel (side by side)
- Same voltage across every branch
- Currents add up
- 1/Rtotal = 1/R₁ + 1/R₂ + …
- Solar: 2 identical strings in parallel = double current, same voltage
- Batteries: 2 × 150 Ah, 12 V in parallel = 300 Ah at 12 V
AC, DC, single-phase and three-phase
DC — direct current
Flows in one direction at a steady voltage. Produced by solar panels and batteries. DC arcs do not self-extinguish, so DC switches and fuses must be DC-rated.
AC — alternating current
Reverses direction 50 times a second in India. Supplied by the grid and by the inverter, which converts the panels' DC into AC.
| Supply | Typical voltage (India) | Power formula | Used for |
|---|---|---|---|
| DC | 12 / 24 / 48 V battery; 300–1000 V PV string | P = V × I | Panels, batteries, charge controllers |
| Single-phase AC | 230 V phase-to-neutral | P = V × I × PF | Homes, small shops, inverters up to ~6 kW |
| Three-phase AC | 400–415 V line-to-line | P = √3 × VL × IL × PF | Large homes, industry, pumps, inverters above ~6 kW |
Power factor (PF)
Ratio of real power (kW) to apparent power (kVA): kW = kVA × PF. Heaters and bulbs have PF ≈ 1; motors and ACs are around 0.8. Inverters and generators are rated in kVA.
RMS value
The "230 V" we quote is the RMS (effective) value. The peak of the AC wave is about 1.414 × 230 ≈ 325 V.
Cables, wire sizes and colour codes
Choose a cable by its current rating first, then check the voltage drop.
| Copper cable size | Approx. current (A) | Typical home use |
|---|---|---|
| 1.5 mm² | 10–14 | Lighting and fan circuits |
| 2.5 mm² | 16–20 | Socket circuits |
| 4 mm² | 25–30 | Air conditioner, geyser, solar DC strings |
| 6 mm² | 32–40 | Main supply to the DB, larger solar strings |
| 10 mm² | 45–55 | Main incomer, inverter output of larger systems |
Ratings are approximate for PVC-insulated copper in conduit. Always apply the manufacturer's derating for temperature, grouping and installation method.
AC colour code
- Current standard (IS 732:2019 / IEC): L1 brown, L2 black, L3 grey, neutral blue, earth green-yellow
- Older Indian wiring: phases red / yellow / blue, neutral black, earth green
Solar DC cables
- Use UV-resistant solar cable (EN 50618 / IEC 62930), usually 4 or 6 mm²
- Common practice: red = positive, black = negative
- Use matching MC4 connectors from the same brand
Protection devices and earthing
| Device | Protects against | Key points |
|---|---|---|
| Fuse | Overcurrent, short circuit | Melts and must be replaced. Solar uses gPV DC fuses on strings. |
| MCB | Overload, short circuit | Resettable. Curve B trips at 3–5 × rated current (lighting), C at 5–10 × (general, inverters), D at 10–20 × (motors). |
| RCCB / RCD | Earth leakage, electric shock | Trips when current leaks to earth: 30 mA for human safety, 100–300 mA for fire protection. |
| MCCB | Overload, short circuit | Larger currents (typically 100–1600 A), adjustable trip settings. |
| SPD | Lightning and voltage surges | Type 2 SPDs on both the DC and AC sides of a solar system. |
| Isolator | Safe disconnection for maintenance | No automatic tripping. Use a DC-rated isolator before the inverter. |
Earthing
Connects metal frames and equipment to the ground so that fault current flows safely and protection devices can trip. Solar systems need earthing for module frames and structures, the inverter body and the SPDs, plus a lightning arrester where required.
Good earth resistance
Measured with an earth tester. A common target is below 5 Ω, and lower for large plants. Keep the earth pit moist and check it every year.
Measuring instruments
Multimeter
Measures voltage (AC/DC), current and resistance, and checks continuity. Measure voltage in parallel and current in series. Use it to check string Voc before connecting the inverter.
Clamp meter
Measures current without cutting the wire: clamp around one conductor only. A DC clamp meter is needed for solar string current.
Insulation tester (megger)
Applies 500–1000 V to check cable and module insulation. Low readings mean damaged insulation or moisture.
Earth tester and energy meter
The earth tester measures earth-pit resistance. The energy meter records kWh; a net meter records import and export separately.
Electrical safety rules
Isolate, lock and tag
Switch off and lock out the supply, and tag it so no one switches it back on while you work.
Test before you touch
Always confirm "dead" with a tested voltage tester. Solar panels are live whenever there is daylight; you cannot switch them off at the panel.
Wear PPE
Insulated gloves, safety shoes, a helmet, safety glasses, and a harness when working at height.
Never break DC under load
Open the DC isolator only after the inverter has stopped; unplugging MC4 connectors under load can cause an arc.
Know first aid
In case of shock, cut the power first. Never touch the person while they are still in contact with the supply.
Ohm's law, power and energy calculator
Change any value — results and the full working update instantly.
Quick formula sheet
Revise before your exam or site visit. Use "Print / save notes as PDF" at the top to keep a copy.
| Topic | Formula / fact |
|---|---|
| Ohm's law | V = I × R |
| DC power | P = V × I = I²R = V²/R |
| Single-phase AC power | P = V × I × PF |
| Three-phase AC power | P = 1.732 × VL × IL × PF |
| Energy | kWh = kW × hours (1 unit = 1 kWh) |
| kW and kVA | kW = kVA × PF |
| Series resistance | R = R₁ + R₂ + … |
| Parallel resistance | 1/R = 1/R₁ + 1/R₂ + … |
| Battery energy | Wh = Ah × V |
| Voltage drop (2-wire) | Vd = 2 × L × I × ρ ÷ A, ρCu ≈ 0.0175 |
| AC peak voltage | Vpeak = 1.414 × VRMS (230 V → 325 V) |
| India supply | 230 V single-phase, 400–415 V three-phase, 50 Hz |
| RCCB for shock protection | 30 mA |
Key terms
- Short circuit: phase touches neutral or earth, causing a very high current
- Overload: more current than the circuit is designed for
- Earth leakage: current escaping to earth through insulation or a person
- DB (distribution board): the panel holding the MCBs and RCCB
- Voc / Isc: a panel's open-circuit voltage and short-circuit current
- Vmp / Imp: voltage and current at maximum power
- Inverter: converts DC to AC
- Net meter: a bidirectional meter recording import and export
Basic Electrical topics checklist
Basic Electrical quiz
1. A 24 V supply is connected across a 6 Ω resistor. What is the current?
2. A 1 kW heater runs for 5 hours. How many units does it use?
3. Six 40 V panels are connected in series. What is the string voltage?
4. Which device protects a person from electric shock caused by earth leakage?
5. What is the supply frequency in India?
6. Two identical 12 V, 100 Ah batteries are connected in parallel. The result is…
7. A 10 kVA inverter supplies a load with a power factor of 0.8. What real power can it deliver?
8. Why are solar panels dangerous even when the inverter is switched off?