Understanding solar electricity starts with a few simple concepts: voltage, current, power and energy. These ideas explain how a photovoltaic panel turns sunlight into usable electricity, how to wire panels together, and why system voltage matters for efficiency. This article walks through the fundamentals using plain language and practical examples, so you can plan a small off-grid, backup or portable solar system with confidence.
How a Solar Cell Generates Direct Current
A photovoltaic (PV) cell, often called a solar cell, is a nonmechanical device that converts sunlight directly into electricity, according to the U.S. Energy Information Administration (EIA). Sunlight is made of photons, and when enough photons strike the semiconductor material inside a PV cell, electrons are dislodged from the material’s atoms. Special treatment during manufacturing makes the front surface of the cell more receptive to these free electrons, so they naturally migrate there. This movement creates an imbalance of electrical charge between the front and back of the cell, producing a voltage potential similar to the terminals of a battery. When conductors are connected to an external load, such as a battery, direct current (DC) electricity flows through the circuit.
The EIA notes that a single PV cell can only produce 1 or 2 watts, enough for small devices like calculators. To get useful power, cells are packaged into weather-tight PV panels (modules), and panels can be connected in groups to form a PV array. The number of panels in an array determines how much electricity the array can generate. All of this electricity starts as DC, which is why understanding DC fundamentals is the first step in solar electricity basics.
Voltage, Current, Resistance and Ohm’s Law
Victron Energy’s Wiring Unlimited book explains that electricity is the movement of electrons in a conductor. The force that makes electrons flow is called voltage (measured in volts, symbol V), and the flow itself is current (measured in amperes or amps, symbol A). When current passes through a material, it meets resistance (measured in ohms, symbol Ω).
The relationship among these three quantities is described by Ohm’s law: voltage equals current multiplied by resistance (V = I × R). Victron’s guide puts it simply: when resistance is low, many electrons move and current is high; when resistance is higher, fewer electrons move and current is lower. This law is the foundation for almost all electrical calculations in a solar system, from choosing cable thickness to sizing fuses. For a deeper look at how these pieces fit together, see our article on DIY solar system basics.
Power: The Rate of Doing Work
Power expresses how much work an electric current can do, and it is measured in watts (symbol P). Victron’s Wiring Unlimited derives power from Ohm’s law with the formula P = V × I. This means that if you know the voltage and the current, you can calculate the power. One especially useful form is I = P ÷ V, which tells you how much current flows for a given power and voltage.
Victron provides a worked example: a 12-volt battery connected to a 2,400-watt load draws 200 amps (2,400 W ÷ 12 V = 200 A). The book also highlights a key advantage of thinking in terms of power: power is independent of voltage. In a system with a DC battery, an AC load, and a solar panel at a different DC voltage, the power remains the same across those different voltages (ignoring inverter inefficiencies). For instance, a 2,400-watt AC load running through an inverter from a 12-volt battery still draws 2,400 watts from the battery. This concept is especially helpful when you start sizing your energy needs.
Energy: Watt-Hours and Kilowatt-Hours
While power is a snapshot, energy measures how much electricity is used over time. The EIA explains that electricity use over time is measured in watthours (Wh). One watthour is the energy of one watt used for one hour. For larger amounts, the kilowatthour (kWh) is used: one kWh equals 1,000 watthours. As an example, the EIA notes that a 40-watt light bulb left on for five hours uses 200 watthours, or 0.2 kilowatthours.
This distinction is critical for solar system planning. A solar panel’s power rating (in watts) tells you its instantaneous output under standard conditions, but the energy it produces over a day (in watthours or kilowatthours) depends on how many hours of useful sunlight it receives. When you calculate how much battery capacity you need, you will work in watthours. Our DIY Solar Fundamentals section covers these planning steps in more detail.
Series vs. Parallel: How Panel Wiring Changes Voltage and Current
When one solar panel is not enough, you create a solar array by connecting multiple panels. Victron’s Wiring Unlimited solar section states the rule clearly: if you connect solar panels in series, the voltage increases; if you connect them in parallel, the current increases. The total power of the array is simply the sum of the power of each module, regardless of whether they are wired in series or parallel.
For example, two 100-watt panels in series will have a combined voltage equal to the sum of their individual voltages, while the current stays the same as a single panel. The same two panels in parallel will have a combined current equal to the sum of their individual currents, while the voltage stays the same. In both cases, the total power is 200 watts. This flexibility lets you match an array to the input limits of a charge controller. Victron advises that the array’s open-circuit voltage (Voc) must never exceed the voltage rating of the MPPT charge controller. For instance, a 12-volt panel may have a Voc around 22 volts, and a controller rated for 75 volts could accept up to three such panels in series.
Why Higher Voltage Helps Reduce Cable Losses
A major practical benefit of wiring panels in series to raise the array voltage is lower current for the same power. Victron’s 2014 blog post on charge controllers explains this using Ohm’s law: losses due to cable resistance are proportional to the square of the current (Pc = Rc × I²). Because loss rises with the square of the current, the post says that doubling the array voltage lets the cable cross-sectional area be reduced by a factor of four for the same cable loss, which means a thinner, less expensive cable.
The blog post notes that this becomes a compelling reason to use an MPPT charge controller as soon as the array power exceeds a few hundred watts on a 12-volt battery, or several hundred watts on a 24-volt or 48-volt battery. The MPPT controller decouples the array voltage from the battery voltage, allowing you to run the array at a higher voltage and still charge a lower-voltage battery efficiently. For more on how charge controllers work, see our comparison of MPPT vs PWM charge controllers.
AC vs. DC: The Role of the Inverter
Solar panels generate DC electricity, but most household appliances and the electrical grid run on alternating current (AC). The U.S. Department of Energy (DOE) explains that an inverter converts DC to AC by switching the direction of the DC input back and forth very rapidly. In DC, electricity is maintained at a constant voltage in one direction. In AC, electricity flows in both directions as the voltage changes from positive to negative, forming a repeating sine wave that the grid and appliances are designed to use.
Modern inverters do more than conversion. The DOE notes that a household solar system’s inverter can also monitor the system and provide a portal for communication. In a solar-plus-battery storage system, advanced inverters can allow the system to operate without grid support during outages, if designed to do so. However, connecting an inverter to a home’s AC panel or the grid is not a DIY task. That work requires a licensed electrician, permits, and approval from the local utility. For off-grid and portable setups, you can use an inverter to power AC loads directly from a battery bank, as long as the system is properly sized and protected. Learn more about inverters and their ratings in our components section.
Key Electrical Quantities at a Glance
The table below summarizes the main quantities you will encounter when planning a solar system.
| Quantity | Unit | What it tells you |
|---|---|---|
| Voltage (V) | Volt (V) | The electrical pressure pushing current through a circuit |
| Current (I) | Amp (A) | The rate of electron flow |
| Resistance (R) | Ohm (Ω) | How much a material opposes current flow |
| Power (P) | Watt (W) | The rate of doing electrical work (P = V × I) |
| Energy | Watthour (Wh) or kilowatthour (kWh) | Power used over time (energy = power × time) |
Frequently asked questions
What is the difference between power and energy in a solar system?
Power, measured in watts, is the rate at which electricity is generated or used at a given moment. Energy, measured in watthours or kilowatthours, is the total amount of electricity used over a period of time. As a simple arithmetic example, a load drawing 100 watts for 3 hours uses 300 watthours of energy.
Why would I wire panels in series instead of parallel?
Wiring panels in series increases the array voltage while keeping the current the same as a single panel. Higher voltage means lower current for the same total power, which reduces resistive losses in the cables. This can allow you to use thinner, less expensive wiring and is a key advantage when the array is located far from the charge controller. However, the total open-circuit voltage must stay within the charge controller’s maximum input rating.
Do I need an inverter for a small off-grid solar system?
Only if you want to run standard AC appliances. Many small off-grid systems power DC loads directly from the battery, such as 12-volt lights, USB chargers, or DC refrigerators. If you need to run household AC devices, you will need an inverter sized to handle the continuous and surge power of those loads. Any connection to a building’s AC wiring must be done by a licensed electrician.
For a complete overview of the concepts covered here, visit our Solar Electricity Basics category page, or explore the broader DIY Solar Fundamentals section.
Sources
- Victron Energy, Wiring Unlimited (PDF)
- U.S. Energy Information Administration, Photovoltaics and electricity
- U.S. Energy Information Administration, Measuring electricity
- Victron Energy blog, Which solar charge controller: PWM or MPPT?
- U.S. Department of Energy, Solar Integration: Inverters and Grid Services Basics
