Solar System Basics

Solar panels convert sunlight into DC electricity through the photovoltaic effect. When photons hit the silicon cells, they knock electrons free, creating a flow of current. Panel output varies with sun angle, shading, and temperature — hotter panels actually produce less voltage.

Key specifications to understand: Wattage is the rated output under standard test conditions. Voc (open circuit voltage) is the highest voltage a panel produces with no load connected. Isc (short circuit current) is the maximum current it can deliver. Vmp and Imp are the voltage and current at the maximum power point, where the panel operates most efficiently.

Connecting panels in series adds their voltages while current stays the same — useful for reaching higher system voltages. Connecting in parallel adds their currents while voltage stays the same — useful for maintaining output when one panel is shaded.

Your system voltage is one of the most important design decisions. It affects wire sizing, component selection, and overall efficiency. Higher voltages mean lower currents for the same power, which means thinner, cheaper wires and less voltage drop.

Batteries store the energy your panels produce so you have power when the sun is not shining. Key specs include capacity (measured in amp-hours, Ah), voltage, chemistry type, depth of discharge (DoD — how much of the capacity you can safely use), and cycle life (how many charge-discharge cycles before significant degradation).

Series connections add voltage: two 12 V batteries in series give you 24 V at the same capacity. Parallel connections add capacity: two 100 Ah batteries in parallel give you 200 Ah at the same voltage. Never mix different battery types or ages in the same bank.

The charge controller sits between your solar panels and your battery bank. It regulates the charging process to prevent overcharging, which can damage batteries or cause fires. There are two main types.

PWM (Pulse Width Modulation) controllers are cheaper but less efficient. The panel voltage must closely match the battery voltage — a 12 V panel for a 12 V battery. They work by rapidly connecting and disconnecting the panel to control the charge rate.

MPPT (Maximum Power Point Tracking) controllers are 15-30% more efficient. They convert excess panel voltage into additional charging current, so you can use higher-voltage panels with a lower-voltage battery bank. For example, a 30 V panel can charge a 12 V battery efficiently.

To size a controller: divide total panel watts by battery voltage to get approximate charge current. For 400 W of panels on a 12 V system: 400 / 12 = 33 A, so you need at least a 40 A controller. Also check the maximum PV input voltage — your panel Voc must not exceed this, especially in cold weather when Voc increases.

Inverters convert the DC power stored in your batteries into AC power (230 V or 120 V depending on your region) that household appliances use. If you only run DC devices directly from the battery, you may not need an inverter at all.

Modified sine wave inverters are cheaper but produce a rough approximation of AC power. Some electronics can be damaged by this, and motors tend to run hotter and less efficiently. Pure sine wave inverters produce clean power identical to what comes from the grid — they are safe for all electronics and are strongly recommended.

Key specs: Continuous watts is what the inverter can sustain indefinitely. Surge watts (typically 2x continuous) handles short bursts for motor startup. Input voltage must match your battery bank voltage. Efficiency is typically 90-95%. To size an inverter, add up the wattage of all AC loads that might run simultaneously, then add 20-25% headroom.

Every wire in your system needs overcurrent protection. Without it, a short circuit can melt wires and start a fire. Fuses are one-time devices — cheap and precise, but must be replaced after blowing. Circuit breakers are resettable and more convenient, though more expensive.

Busbars are heavy metal bars that serve as central distribution points. Instead of connecting multiple wires directly to a battery terminal (which is messy and unreliable), you run one thick wire from the battery to the busbar, then branch out from there with individual fused circuits.

Where fuses or breakers are needed: between the battery and everything it connects to (this is the most critical), between the charge controller and the busbar, and on every branch circuit leaving the busbar. Fuse sizing should be between the wire's maximum amp rating and 125-150% of the expected continuous load.

The energy flow in an off-grid solar system follows this path: Sun hits the panels, which generate DC electricity. The charge controller regulates this power and charges the battery bank. The battery stores energy for later use. From the battery, you can either run DC loads directly or send power through an inverter to run AC appliances.

Every connection between components needs properly sized wire. Undersized wire causes voltage drop (wasted energy), overheating, and potential fire risk. This is why wire sizing based on current, distance, and system voltage is so important — and why Solar Planner calculates it automatically.

Solar Planner lets you drag real solar components onto a canvas, connect them with wires, and get automatic wire sizing and safety validation. Start with a pre-made template for your system type, use the load calculator to determine your energy needs, then customize the design with your actual components and distances.