An off grid solar system has about ten components: the solar panels, the mount that holds them, a charge controller, a battery bank, a BMS, an inverter, fuses, a disconnect switch, a shunt with a monitor, and the wiring that ties it all together.
Each one has one job. Skip one and the system either doesn't work or breaks fast.
A solar panel on a roof, without the other parts, can't run the appliances in your house. You can't plug your fridge into it. So below is every part, what it does in a sentence or two, and a link to the page that tells its full story.
What Is An Off Grid Solar Power System?
An off grid solar power system is a little power station that belongs only to you. It has no access to the power lines on the street, so there is no utility company to fall back on when the sun goes down. That's why it's the usual choice for cabins, boats and farms far from the nearest pole.
That changes the whole design. A grid home can send extra power out and pull power in at night, while an off grid home has to catch, store and use every bit of its own energy.
Engineers have a name for every part that isn't a panel. They call it the "balance of system". That's a fancy way of saying all the other equipment: the wiring, switches, mount, inverter, batteries and charger that make solar power useful for your lights and appliances.
Want to see how this compares with a home that stays on the grid? We wrote a whole page on off grid vs grid tied solar systems.
How Does An Off Grid Solar System Work?
Think of it like a relay race. The electricity gets passed from one runner to the next, and each runner has one job before handing it on.
Here's the path it takes, from sky to socket:
- Sunlight hits the panel and knocks electrons loose
- DC power flows out of the panel
- Through a fuse and a disconnect switch
- Into the charge controller
- Into the battery bank
- Out of the battery, into the inverter
- The inverter changes DC to AC
- AC flows into your house panel
- From there to your lights, your TV, your fridge
The rest of this page walks along that path, one component at a time, and each section links to the page that covers it in full. If you want the bigger story first, start with how sunlight becomes electricity.
Solar Panels And What Holds Them Up
Everything begins with the solar panels. They catch sunlight and produce direct current, or DC. That's electricity that flows one way only, like water running downhill.
The solar panels need something to hold them, and that's the mount. Fixed racks keep them at one angle on a roof or on the ground, while trackers slowly turn to follow the sun across the sky.
How many solar panels do you need? That depends on how much energy you use and how much sun is available at your location. Our page on how many solar panels you need to go off grid works it out step by step, and the solar panel calculator does the maths for you.
The Charge Controller: Guarding The Battery
A solar panel on a bright, cool day can push more power than a battery should take. Something has to stand in between, and that something is the charge controller.
It steps the power down to what the battery wants, and when it's full, it stops the flow.
Think of it like the tap on a water bottle. The river is the panel, the bottle is the battery, and the tap stops you from overfilling.
There are two kinds. A PWM controller is a simple switch, while an MPPT controller is smarter (and more expensive) and squeezes more output from the same panel. That difference in efficiency is easy to see in numbers. In one test by Victron, a 100 W panel charging a 12 V battery on a cool day gave about 81 W through PWM, and close to the full 100 W through MPPT.
The Battery Bank: Energy For Later
Solar panels only work when the sun shines. If you want power at night, you need energy storage.
That storage is a battery bank: one battery, or a stack of them wired together. The bank charges up during the day and feeds your house at night. Most banks run at 12, 24 or 48 volts.
The type of battery matters a lot. Older lead-acid batteries are cheap, but they typically last only 500 to 1,000 charge cycles and wear out fast if you drain them past half.
Lithium-ion batteries are more expensive but go much further. Victron rates its lithium iron phosphate batteries for 2,500 cycles when you use 80% of what they hold each time. At one cycle a day, that's close to seven years before they drop below 80% of what they held when new.
Want to know what's going on inside? Here's how home batteries work.
The BMS: The Battery's Bodyguard
A lithium battery needs a guard, and that guard is the BMS, short for battery management system. It is a tiny computer that watches over the cells.
It reads the voltage, the current and the temperature. If anything gets too full, too empty, too hot or too cold, the BMS cuts the power before trouble starts.
Most lithium packs have one built in, but not all of them do. Our guide on choosing the right BMS for your battery explains what to look for.
The Power Inverter: Turning DC Into AC
Solar panels and batteries give DC power. Your house wants AC power, which flips direction back and forth many times a second. They are two different flavours of electricity, and your fridge will not run on the wrong one.
An off grid inverter is the translator box. It takes the DC from the battery bank and converts it into the AC your wall sockets understand.
Inverters come in sizes. A small one can run a laptop and some lights, while a big one can run a washing machine or an air conditioner. Each one lists a normal rating and a higher peak rating, and the inverter size calculator helps you pick.
Curious why we ended up with two kinds of electricity at all? Read about the war of the currents.
The DC Fuses: The Last Line Of Defence
Fuses protect the wires. If too much current ever flows through a wire (a short circuit or a fault), the fuse blows and breaks the connection before the wire overheats and starts a fire.
DC fuses are not the same as the AC fuses in a normal house. AC current drops to zero many times a second, which helps a spark die out by itself. DC never drops to zero, so a DC fuse has to force the current down on its own, and fuses for solar are designed and tested for exactly that.
Where do they go? One goes between the solar panels and the charge controller, and every device on the battery gets its own. Lithium can push huge currents in a short circuit, so the fuse must be able to handle that too. Our page on what fuse you need and the fuse size calculator take you through it.
The Disconnect Switch: One Off Switch For Everything
A battery disconnect switch cuts the bank off from everything it feeds and everything that charges it.
You want this for two reasons. When you work on the installation, you need to be sure nothing is live. And if something goes wrong, you can shut it all off in one move.
On some switches you can even pull the knob out, so nobody can switch the power back on while you're working.
The Shunt And Monitor: Your Solar System's Dashboard
How full is your battery right now? With lithium, the voltage barely moves until it's almost empty, so a voltmeter can't tell you much.
That's where a battery shunt comes in. It's a small metal bar that every bit of current passes through. A monitor measures that current and the voltage, then works out how full the bank is and how long it will last.
A monitoring system takes it one step further and puts it all in an app: the solar production, what the bank holds and what your house uses. With an extra gateway box, some systems allow you to check it all from far away, and some can send a push notification to your phone, for example when the charge runs low.
It's an option, not one of the requirements. However, most people who get one end up checking it every day.
The Wiring That Connects The Components
Wires tie all the components together. But not all wires are equal.
PV wire runs from the solar panels. It has a thick plastic jacket and is listed as sunlight resistant, so it survives years of sun, rain and snow.
Battery and inverter cables carry the biggest currents in the system, so they're the thickest wires you'll see.
AC wiring runs from the inverter to your house electrical panel. This is the same kind of cable your electrician already knows.
A grounding wire ties the metal parts of the system to the earth. It carries no current in normal use, but if a fault happens it gives the current a safe path, so the fuse or breaker trips fast.
Pick a wire that's too thin and it gets warm, which wastes energy as heat and lowers the efficiency of the whole system. Wire size is given in AWG, and the smaller the number, the thicker the wire. Our AWG conversion chart and the cable size calculator help you get it right.
How To Build An Off Grid Solar System That Fits Together
Here's the part people often get wrong: the components have to match each other. A huge inverter on a tiny bank, or a charge controller too small for the solar panels, makes the whole solar power system struggle. However good each box is on its own, it only works as well as the weakest one.
So size them in order. First work out your daily energy requirements, then the panels, then the batteries, then the charge controller and inverter, and last the fuses and wires. Each step depends on the one before. And if you're not sure about the wiring, get an electrician to check the installation before you switch it on.
Our calculators follow that same order: battery size, charge controller size, inverter size, fuse size and cable size. Before you buy anything, read what to think about before going off grid with solar too.
Sources
- Wikipedia: Balance of system
- U.S. Department of Energy: Overview of trade and policy measures for U.S. solar manufacturing (modules, inverters, mounting)
- U.S. Department of Energy: Solar integration, inverters and grid services basics
- Victron Energy: Which solar charge controller, PWM or MPPT?
- Victron Energy: Lithium Battery Smart datasheet
- Sandia National Laboratories: Energy storage overview
- Battery University: BU-908 Battery management system
- Mersen: Photovoltaic market, over-current protection
- Victron Energy: Wiring Unlimited, DC wiring
- Victron Energy: Battery Switch 275 A manual
- Victron Energy: SmartShunt manual, introduction
- Wevolver: LiFePO4 voltage chart
- Victron Energy: VRM remote monitoring
- Portlandia Electric Supply: Solar wire and cable guide
- Wikipedia: American wire gauge
Hard Words
| Term | What it means |
|---|---|
| Balance of system | All the parts in a solar system that are not panels. The boxes, wires, switches and fuses that make the electricity useful. |
| Mount | The rack that holds the panels on a roof or on the ground. Some turn to follow the sun. |
| Inverter | The box that converts DC power into AC power for your house. Without one, your fridge cannot use solar. |
| Charge controller | The box between the panels and the battery that keeps the battery from being overcharged on sunny days. |
| MPPT | The smarter kind of charge controller. It gets more power from the same solar panel than the simpler PWM kind. |
| Battery bank | One or more batteries wired together to store solar power for night use. |
| BMS (battery management system) | A tiny computer that watches the battery's voltage, current and temperature. Stops trouble before it starts. |
| Disconnect switch | An off switch that cuts the battery off from the rest of the system. Used for repairs and emergencies. |
| DC fuse | A fuse built and tested for direct current. Blows if too much current flows, so the wire does not overheat. Different from the AC fuses in your house panel. |
| Shunt | A small metal bar that all the battery current flows through, so a monitor can count what goes in and out. |
| Monitoring system | An app or display that shows what the panels make, what the battery holds, and what your house uses, all in real time. |
| PV wire | Special sun-resistant wire with a thick jacket that runs from the panels. Built to survive years of sun, rain, and snow. |
| Grounding wire | A wire that ties the metal parts of the system to the earth. Gives fault current a safe path so the fuse trips. |
| AC wiring | The standard house wiring that runs from the inverter to your electrical panel. |
Common Questions
Why can't I just plug things straight into a panel?
Because the panel makes DC power, not the AC power your fridge and TV expect. You need at least an inverter in between, and in an off grid home batteries and a charge controller too.
What does a charge controller actually do?
It stops the solar panels from overcharging the battery. It steps the flow down and shuts it off when the battery is full.
Do I need batteries at all?
Only if you want power at night. Grid-tied homes can skip them because the utility is the backup. Fully off grid setups need them. Hybrid setups use a smaller battery just for blackouts.
What is an on-grid solar setup?
An on-grid (grid-tied) setup is connected to the power lines. It sends extra solar power out during the day and pulls power in at night, so it usually needs no battery. Our page on off grid vs grid tied solar systems compares the two.
Can AC run on off grid solar power?
Yes. An air conditioner or any other AC appliance runs fine as long as the inverter converts the DC into AC and is big enough for it, and the batteries and panels can keep up with how much energy it uses.
What's the BMS for?
It watches the battery's voltage, current and temperature. If anything gets too hot, too cold, too full, or too empty, the BMS cuts the circuit before anything goes wrong.
Why are DC fuses different from the fuses in my house?
AC current drops to zero many times a second, which helps a spark die out. DC never does, so a DC fuse has to stop the current on its own. Use fuses designed and tested for DC.
Is a monitoring system worth it?
For most people, yes. You see what your roof makes, how full the bank is and what the house uses, and some systems send an alert when the charge runs low.