Understanding volts, amps, watts, and Ohm's law is essential before designing any solar power system.
Every electrical circuit has three measurable properties. Think of electricity like water flowing through a pipe:
Unit: Volts (V)
Voltage is electrical pressure — the force that pushes electrons through a wire. Like water pressure in a pipe.
Higher voltage = electrons pushed harder. Off-grid systems typically run at 12V, 24V, or 48V DC.
Unit: Amperes / Amps (A)
Current is the flow rate of electrons — how many are passing through per second. Like the volume of water flowing through a pipe.
Wire thickness must match current — too thin and wires overheat.
Unit: Watts (W)
Power is the rate of energy use — how much work the electricity is doing right now. Voltage × Current.
P = V × I
A 12V system drawing 10A uses 120W.
The most important equation in electricity. It relates voltage, current, and resistance:
V = Voltage (Volts)
I = Current (Amps)
R = Resistance (Ohms, Ω)
Rearranged: I = V / R (find current) • R = V / I (find resistance)
Combined with Ohm's law, power can be calculated multiple ways:
| Formula | When to Use | Example |
|---|---|---|
| P = V × I | You know voltage and current | 12V × 10A = 120W |
| P = I² × R | You know current and resistance | 10A² × 1.2Ω = 120W |
| P = V² / R | You know voltage and resistance | 12V² / 1.2Ω = 120W |
Power (watts) tells you the rate of energy use. Energy tells you the total amount consumed or stored over time.
Wh = Watts × Hours
A 60W light running for 5 hours uses 300Wh. This is the most useful unit for sizing a solar system — your electric bill is in kWh (1 kWh = 1,000 Wh).
A 100Ah 12V LiFePO4 battery stores 1,280Wh.
Ah = Amps × Hours
A battery rated 100Ah can deliver 1A for 100 hours, or 10A for 10 hours (simplified). Ah doesn't account for voltage, so always convert to Wh for comparisons.
Wh = Ah × V
100Ah × 12.8V = 1,280Wh
Electrons flow in one direction, like a river. The voltage stays constant (e.g., a steady 12V).
Solar panels produce DC. Batteries store DC. Your entire off-grid core is DC. Many 12V appliances (lights, fans, water pumps, fridge) can run directly on DC without an inverter.
Electrons oscillate back and forth, changing direction 60 times per second (60Hz in the US, 50Hz in Europe). Voltage swings between +170V and -170V (averaging 120V RMS).
AC can be stepped up to very high voltages (100,000V+) for efficient long-distance transmission, then stepped back down for homes. DC loses too much energy over long distances at low voltages.
How you connect batteries or solar panels determines the system voltage and current capacity.
| Series (+ to -) | Parallel (+ to +, - to -) | |
|---|---|---|
| Voltage | Adds up (12V + 12V = 24V) | Stays the same (12V) |
| Capacity (Ah) | Stays the same (100Ah) | Adds up (100Ah + 100Ah = 200Ah) |
| Total energy (Wh) | Doubles (24V × 100Ah = 2,400Wh) | Doubles (12V × 200Ah = 2,400Wh) |
| Use case | Increase system voltage (12V → 24V → 48V) | Increase capacity at same voltage |
For the same power (watts), a higher voltage system draws less current:
| System Voltage | Current for 2,400W | Wire Size Needed | Best For |
|---|---|---|---|
| 12V | 200A | 4/0 AWG (very thick) | Small systems under 2,000Wh |
| 24V | 100A | 2 AWG | Medium systems 2,000–8,000Wh |
| 48V | 50A | 6 AWG (thinner, cheaper) | Large systems 8,000Wh+ |
Lower current means thinner (cheaper) wires, smaller fuses, and less energy lost as heat in the wiring. This is why large off-grid homes use 48V systems.
Undersized wires cause voltage drop (energy wasted as heat) and are a fire hazard. The two factors that determine wire size:
The wire must handle the maximum current without overheating. Every wire gauge (AWG) has an ampacity rating — the maximum safe continuous current.
Longer runs need thicker wire to keep voltage drop under 3% (ideal) or 5% (acceptable). A 20-foot run at 12V/30A needs much thicker wire than a 3-foot run.
| AWG | Diameter (mm) | Max Amps (chassis) | Common Use |
|---|---|---|---|
| 14 | 1.6 | 15A | Light circuits, small loads |
| 12 | 2.1 | 20A | Branch circuits, 12V accessories |
| 10 | 2.6 | 30A | Charge controller to battery |
| 8 | 3.3 | 50A | Inverter to battery (small) |
| 6 | 4.1 | 65A | Sub-panels, large inverters |
| 4 | 5.2 | 85A | Battery bank interconnects |
| 2 | 6.5 | 115A | Large inverter cables |
| 1/0 | 8.3 | 150A | High-current battery to inverter |
| 4/0 | 11.7 | 230A | 12V systems with 3,000W+ inverters |
Every circuit in a solar system needs overcurrent protection. If a wire is rated for 30A and a short circuit draws 200A, the fuse blows before the wire melts.
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