Putting together a do-it-yourself solar system means understanding how each part fits into the bigger picture. Whether you are planning a small off-grid setup, a backup power supply, or a portable solar kit, every component has a specific job. The main electricity-producing modules get most of the attention, but a whole collection of supporting parts, called the balance of system, is just as critical for safe and reliable operation. The Electrical and Mechanical Services Department of Hong Kong (EMSD) explains that the balance of system includes combiner boxes, charge controllers, storage batteries, inverters, mounting structures, wiring, switchgear, fuses, surge arrestors and earth-fault protection devices. The EMSD notes that for small systems, these parts can account for up to half of the total capital cost and most of the ongoing maintenance.
Solar modules and the balance of system
A photovoltaic panel is where sunlight gets turned into direct current electricity. The Pennsylvania Public Utility Commission’s solar basics fact sheet describes how solar cells inside a panel generate electricity when exposed to light, and how the output drops under clouds, shade or at night. But a panel by itself cannot power standard household equipment. All the extra hardware that forms a complete photovoltaic system is collectively called balance of system (BOS), a term the EMSD uses to cover everything from wiring and fuses to mounting frames and charge controllers.
Solar panels and their cables
Panels usually come with special waterproof connectors, the most common being MC4 connectors. Victron Energy’s Wiring Unlimited guide explains that a male MC4 connector attaches to the positive cable and a female connector to the negative cable. Extension cables are often pre-assembled with one male and one female end, which makes extending the run simple. Solar cable itself is designed for outdoor use: it is dust, age and UV resistant and has tinned copper wire strands to resist corrosion. For small arrays in vehicles or boats, a dual-core solar cable with the same UV rating and tinned copper is common.
One critical safety point from Victron’s guide is that solar panels must never be connected directly to a battery. A solar charger must sit between the two, converting the higher panel voltage down to a level suitable for battery charging. If a panel is hooked straight to a battery, the battery will get damaged. The EMSD report reinforces that standalone systems need a charge controller for exactly this purpose.
Depending on local rules, a fuse, circuit breaker or residual current device may also be required in the wiring between the array and the solar charger. The right cable thickness is determined by the array size and voltage, as thicker cables are needed for higher currents. Victron advises checking the product manual for the correct cable size and, for a quick estimate on runs shorter than 5 meters (about 16 feet), dividing the current in amps by 3 to get a cross-section in square millimeters.
Charge controllers: protecting the battery
A charge controller regulates the flow of current from the array to the battery. The EMSD describes how, when sunlight is strong and the battery reaches a “full” state, the controller disconnects the battery from the array. A maximum power point (MPP) charge controller goes a step further by incorporating a DC-to-DC converter, which lets the array operate at the voltage that delivers the most power under the prevailing irradiance. Victron’s Wiring Unlimited similarly states that the solar charger converts the higher panel voltage to a voltage appropriate for battery charging and warns against skipping this component.
When designing an array, you must keep the array’s open-circuit voltage (Voc) below the voltage rating of the MPPT controller. Victron suggests using its MPPT sizing calculator to match the solar array to the right solar charger. For more guidance, see our articles on charge controllers and solar electricity basics.
Batteries: storage and configuration
Storage batteries make it possible to use solar energy after sunset or during cloudy stretches. The EMSD states that lead-acid deep-cycle batteries are the most popular choice for photovoltaic systems because they are built to handle frequent, deep discharges, with typical lifespans ranging from 3 to 8 years. Battery capacity is usually expressed in ampere-hours (Ah), and the kWh capacity equals the Ah rating times the voltage divided by 1000. The Pennsylvania PUC fact sheet notes that batteries are optional and can be configured for needs such as backup during power outages.
When multiple batteries are wired together into a bank, the rules are straightforward. Victron’s guide explains that connecting batteries in series raises the voltage, while connecting them in parallel raises the capacity. For lead-acid banks, Victron recommends limiting paralleled strings to around three or four, because wiring variations and slight differences in internal resistance make it tricky to keep a large bank balanced. If a bigger bank is needed, using large single-cell 2V lead-acid units in series or switching to lithium batteries with built-in battery management systems (BMS) is a better approach. Lithium batteries, as described in the same guide, incorporate internal cell balancing and external BMS communication, and they can often be paralleled in larger numbers because each battery monitors itself and shares state-of-charge data with a monitoring device.
Cable layout matters just as much as the batteries themselves. Victron shows that an improperly wired parallel bank will force the bottom battery to work harder, charge and discharge unevenly, and fail prematurely. The solution is to make the total cable path for each battery equal, using busbars, equal-length cables from posts, or halfway or diagonal connections. More details on battery selection and wiring can be found in our batteries section and battery safety guide.
Inverters: DC to AC power
Solar panels and batteries produce direct current, but most household appliances run on alternating current. The U.S. Department of Energy (DOE) explains that an inverter handles this conversion. Systems may use one central inverter for the entire array, or microinverters attached to individual panels, which can be helpful when some modules are shaded. The DOE also notes that advanced “smart inverters” can communicate with the utility to help balance supply and demand, and that an inverter will likely need to be replaced at least once over a 25-year array lifetime.
The EMSD adds that conversion efficiency for inverters of different designs typically falls between 85% and 95%. When choosing an inverter, it pays to consider the quality of the AC output, such as harmonic content and voltage stability, and the presence of protection features like short-circuit and overload protection. Standalone inverters also include deep discharge monitoring to protect the battery. For more on matching an inverter to your system, see inverters and inverter sizing.
Wiring, fuses and safety components
All connections in a solar system must be properly sized and protected. Victron’s Wiring Unlimited is clear that too-thin cable reduces system performance and can be a safety hazard. To avoid very thick cables, the guide advises increasing the system voltage when possible, for instance moving from 12V to 24V or 48V for systems with a large inverter. If a single cable of sufficient thickness cannot be found, doubling up two cables per connection is acceptable as long as the combined cross-sectional area matches the recommendation in the product manual.
DC fuses and circuit breakers are the primary defense against overcurrent and short circuits. A fuse contains a wire or metal strip that melts when current exceeds its rating, breaking the circuit; a circuit breaker uses either a bi-metal strip that bends when heated or an electromagnet that trips on high current. Victron states that every consumer connected to a battery must have its own fuse, placed in the positive cable, because batteries can deliver very high currents that could start a fire. A DC system also typically includes a main battery fuse and, for lithium batteries, a fuse with an interrupt rating high enough to handle the battery’s potentially enormous short-circuit current.
A DC isolation switch lets you disconnect the battery from the rest of the circuit for maintenance or when the system is not in use. The Wiring Unlimited guide notes that removing the main fuse often achieves the same result, and that if an isolator is installed, only the positive battery cable needs to be switched. The Pennsylvania PUC fact sheet mentions that ground-fault protection, surge protection and disconnect devices are common additional equipment in a residential system. For detailed advice, visit cables and connectors and electrical safety.
Mounting and positioning the array
The DOE describes mounting structures as stable, durable frameworks that must hold the array against wind, rain, hail and corrosion for decades. To capture the most energy over a year, panels in the northern hemisphere should face due south and be tilted at an angle roughly equal to the local latitude. Rack mounting is the most common method because it is robust and easy to install. Ground-mounted systems can also use one-axis or two-axis tracking mechanisms that follow the sun, but these add upfront cost and require more maintenance. The DOE mentions that building-integrated photovoltaics, where panels become part of roofing or façades, are a promising alternative, though technical and commercial barriers still exist. More on racking and placement is available in mounting equipment and system design.
Key components at a glance
The table below summarizes each major part, its role, and what to check before you buy.
| Component | Role | What to check before buying |
|---|---|---|
| Solar panel | Produces direct current from sunlight | Open-circuit voltage, connector type (MC4), UV and weather resistance of included cables |
| Charge controller | Regulates charging and prevents battery overcharge | MPPT or PWM type, maximum PV input voltage, battery voltage compatibility |
| Battery | Stores energy for use when panels aren’t producing | Chemistry (lead-acid or lithium), capacity in Ah, internal BMS for lithium, allowed number of parallel strings |
| Inverter | Converts DC to AC for household loads | Continuous power rating, waveform type, conversion efficiency, protection features |
| Cables and connectors | Carry current between components with minimal loss | Solar-rated, tinned copper strands, correct cross-section per product manual, MC4 gender and extension needs |
| Fuses and breakers | Protect wires and equipment from overcurrent and short circuits | Current rating matched to cable or load, DC voltage rating, interrupt rating for lithium systems |
| Mounting structure | Positions and secures the array at the correct tilt | Material durability, load capacity for local wind and snow, adjustability for latitude-based angle |
Frequently asked questions
Can I connect a solar panel directly to a battery?
No. Victron Energy’s wiring guide explicitly states that a solar charger must sit between the panel and the battery, otherwise the battery will be damaged. The charge controller converts the panel’s higher voltage to a safe charging voltage and stops charging when the battery is full.
How many batteries can I wire in parallel?
The answer depends on the chemistry. Victron recommends limiting lead-acid banks to around three or four parallel strings to avoid imbalance. Lithium batteries with their own battery management systems can often be paralleled in larger numbers because each unit manages its own state of charge and communicates with the system monitor.
How do I know what cable thickness to use?
The simplest approach is to consult the manual of the charge controller, inverter or battery you are using; manufacturers list recommended cable and fuse sizes. Victron also offers a free toolkit app that calculates voltage drop based on voltage, current, cable length and cross-section, aiming to keep the drop below 2.5%.
For a deeper dive into each part and how to choose them, browse our solar system components category page, where you’ll find detailed guides on panels, controllers, batteries, inverters, wiring and mounting.
