How to size a solar system for your RV

One of the things that makes living in a home on wheels appealing is the freedom of not being dependent on a fixed energy source, allowing you to travel and stay almost anywhere. A solar system plays a big role in this. However, it can be a confusing topic for many vanlifers who are just getting started. In this post, we look at the key concepts that help you understand and size a solar system for your RV.

Solar system components

A typical campervan solar system is built around the following three main components:

1.Solar panels:

Convert sunlight into DC electricity. Two common types are used on campervans are:

• Rigid panels: Mounted on brackets above the roof, they are durable and offer better airflow than flexible panels, making them more efficient and longer-lasting. Explore our BlueSolar solar panel range.
• Flexible panels:Thin panels that can follow slight roof curves. They sit closer to the roof and are lighter, which can be useful if you want a discreet installation.

2. Solar charge controller:

Regulates the power coming from the panels and charges the battery safely. It prevents overcharging and helps maintain battery health. There are two main solar charge controller types:

• PWM (Pulse width modulation):
  A simple and affordable option, but not very efficient. PWM controllers are typically used in smaller systems where panel voltage closely matches battery voltage.
• MPPT (Maximum Power Point Tracking) solar charger:
  This is the preferred solar charge controller option due to its higher efficiency compared to PWM charge controllers.
  
  Within the Victron Energy MPPT range there are BlueSolar and SmartSolar models. SmartSolar MPPT chargers include built-in Bluetooth, allowing you to monitor and adjust the settings via the VictronConnect app.
  
  BlueSolar MPPT chargers do not include built-in Bluetooth, but their data can still be accessed by connecting them to a GX device, such as a Cerbo GX, or by adding a VE.Direct Bluetooth Smart dongle.

Load output

SmartSolar and BlueSolar MPPT solar charge controllers smaller than 100/30 also include a load output, which is not shown in the wiring diagram above. This allows small DC loads, such as lights or USB chargers, to be connected directly to the controller. The controller can automatically disconnect these loads if the battery voltage becomes too low, helping to protect the battery from deep discharge.

3. Battery bank:

And of course, we need a correctly sized battery to store the energy received from the panels and other charging sources.

Additional Components

Alongside the main components, a safe and reliable PV system also requires:

• PV isolator (photovoltaic isolator): A DC-rated safety switch that allows you to disconnect the solar panels from the charge controller for maintenance or emergency shutdown.
• DC distribution: Such as a Lynx Distributor or a busbar. This provides a central point to connect batteries, chargers, inverters and loads, and helps organise the DC wiring.
• Cables and protection components: Correctly sized solar cables, cable lugs, DC fuses and fuse holders. These protect the system against overcurrent, short circuits and overheating.
• Installation hardware: Such as solar panel brackets (for rigid panels), cable entry housings and other mounting components.

Optional Accessories:

• MPPT WireBox: An additional, optional cover that can be purchased separately to protect the cable connections of your MPPT solar charge controller. It is available in versions for MC4 models (plug-in solar connectors) and TR models (screw terminals for solar cables).
• SmartSolar Control display: An optional plug-in LCD display for SmartSolar and BlueSolar MPPT charge controllers from 150/45 and larger.

So, how to size these components?

All the components in your electrical system work together and should really be sized as a whole, rather than in isolation. It's of course possible to piece together an electrical system from whatever secondhand components you find online, and it might even run a small fridge, a few lights, and some USB chargers. But let's not leave that to luck. Instead, let's have a look at how to size a solar system properly step-by-step.

1.Estimate your daily energy consumption

This is the starting point before buying any components for your RV electrical system. Grab a piece of paper and picture yourself in your recently converted campervan. You're finally on the road! What would your day look like? Would you press the button on your espresso machine first thing in the morning? Would you still want to use your hairdryer on winter evenings? Once you've pictured your day, calculate your daily consumption in watt-hours (Wh) by multiplying the power of each appliance by the number of hours you expect to use it per day.

Example:

• Laptop charging: 60W × 3 hours = 180Wh
  (We're keeping it simple here and skipping inverter losses that occur while converting 12V DC from the battery to 230V AC. In real life, add about 10-15% extra for any 230V appliances, like the laptop charger.)
• Fridge (let's say a 12V, 70L fridge): 500Wh/day
• Lights: 10W × 4 hours = 40Wh

Total daily consumption = 720Wh

This number gives a good starting point for sizing both the solar array and the battery bank, which need to be suitably sized to match each other.

A 12V system with a 720Wh daily consumption would need around 60 Ah per day (power consumption in Wh ÷ system voltage in V = Ah). If you'd like to be able to stay off-grid without having to charge your batteries, for let's say 3 days (i.e. you're parked under the shade somewhere in nature, and don't want to drive around), you would need 180 Ah (daily Ah × number of days off-grid = total Ah needed).

However, batteries can't be fully discharged, so depending on the battery chemistry, you'll need to consider a buffer. Lithium batteries can typically be safely discharged to about 90% of their rated capacity, while lead-acid batteries usually only allow around 50% usable capacity. So for this system, you'd need roughly a 360Ah battery bank for lead-acid (180 ÷ 0.5), or about 200Ah for lithium (180 ÷ 0.9).

2. Calculate your solar array size

The watt rating of a solar panel shows the maximum power it can produce under ideal conditions, i.e. without partial shading or dirty panels.

But as you can guess, solar panels can't supply this power all day. Instead, solar calculations use Peak Sun Hours (PSH), which represent the equivalent number of hours per day when sunlight intensity averages 1000 W/m².

Average PSH values for different locations and seasons can be found online.

Peak sun hours vary depending on where you travel and during which months. An Alpine village in summer, for example, may get around 5 peak sun hours per day. So, an RV designed for summer trips in the Alps, with a daily energy consumption of 720Wh, would roughly need a 144W solar array to fully replenish that energy each day (720Wh ÷ 5 hours = 144W).

However, there's one more thing to consider. In the example above, we sized the battery bank to supply energy for 3 days without charging (optional, but we wanted more freedom), and concluded that a 200Ah lithium or 360Ah lead-acid battery bank would be a better fit for this system.

Now, let's assume, for a moment, that solar is the only charging source for this system (disregarding alternator and shore power charging). In this case, it would take roughly 18 peak sun hours to fully recharge the 360Ah lead-acid battery bank, or 15 peak sun hours to fully recharge the 200Ah lithium battery bank, using only the 144W array.

For an RV that doesn't drive around much and stays off-grid most of the time (i.e. relying mainly on solar to recharge), this isn't ideal, and a bigger solar array is recommended. Being able to fully recharge your batteries in 4-5 peak sun hours, using solar alone, is a much more practical target, since it means your batteries can realistically recover within a single sunny day, rather than needing 3-4 days of consecutive sunshine.

Based on this, you'd need to roughly triple the array size, to around 450-500W, to bring full recharge time down to about 5 peak sun hours. This is why solar arrays are often sized well beyond the daily-consumption minimum, especially for RVs that spend extended periods off-grid.

3. Calculate the size of your solar charge controller

Before selecting a solar charge controller, let's look at the key specs of solar panels, the wiring options, and how each affects the system, and how that connects to controller selection.

Solar panel specs

When comparing solar panels, three values are especially important: wattage (W), open circuit voltage (Voc), and short circuit current (Isc).

• Wattage: Indicates the maximum power the panel can produce under ideal conditions.
• Open circuit voltage (Voc): This is the maximum voltage the panel can produce in full sunlight. Higher voltage gives more margin for charging in cloudy conditions or winter. When multiple panels are wired in series, the voltage increases while the current stays the same.
• Short circuit current (Isc): This indicates how much current the panel can produce. Higher current allows batteries to charge faster, provided the charge controller supports it.

How to wire solar panels

Solar panels can be wired in series, in parallel, or in a series-parallel combination, and each affects the system differently.

• Series wiring: the voltage increases while the current stays the same. This works well with MPPT charge controllers, which are designed to efficiently convert higher voltage down to your battery voltage, and can help charging start earlier and continue later in the day, since higher-voltage strings stay above the controller's minimum start-up voltage even in low-light morning or evening conditions. The downside is that if one panel is shaded, it can reduce the output of the whole string.
• Parallel wiring: the voltage stays the same while the current increases. This handles shading better, which can be useful on campervans where roof vents or gear may cast shadows, but it requires thicker cables because of the higher current.

Choosing the right Victron Energy MPPT charge controller

Now that we know the solar panel specs and how wiring affects the system, we're ready for the next step: figuring out what size solar charge controller you need.

If you'd like a quick answer without digging into the details, our MPPT sizing calculator can do the work for you. Otherwise, read on for a deeper understanding of how it all fits together.

Victron Energy MPPT chargers are named directly after their two key limits: maximum PV input voltage and maximum charge current. Take the SmartSolar MPPT 100/30, for example:

This means the charger can accept up to 100V from the solar array and deliver up to 30A of charging current to the battery.

So, selecting the correct solar charge controller size depends on:

• The combined Voc of your solar panels (especially relevant when wired in series)
• The combined Isc of your solar array (especially relevant when wired in parallel)
• Your battery voltage (12V, 24V, or 48V)

Final Thoughts

Sizing a solar system for your RV starts with understanding your energy use and choosing components that work well together. A well-designed system not only considers your power requirements, but also how you'd like to use your van. For example, do you usually drive from one spot to the next after a few days? Or do you prefer staying longer at one spot? Do you usually go to campsites and want shore hookup to help with solar, or do you want complete freedom? We hope this post helps you build a system that fits your travel style.

And for those of you looking for a better explanation of how MPPT charging works, have a look at this video:
How does a Victron Energy MPPT Solar Charge Controller work?

If you need professional help with system sizing or installation, you can use our dealer overview to find the nearest Victron Energy installer. They can help ensure your system is safe, reliable, and suited to the way you travel.