When shopping for solar panels, many people choose them randomly without following any sizing guidelines. In many cases, this often leads to undersized solar setups that generate insufficient power.
Most campervan owners also focus only on sunlight hours. However, the 1.2β1.3 multiplier is critical for accounting for external inefficiencies β and it's the step most sizing guides skip entirely.
For a reliable, self-sufficient power setup during off-grid adventures, you must properly size your solar panels. This ensures enough power for charging your leisure battery bank and operating appliances, not only in summer with long sunshine hours but also during the cloudiest winter days. In this blog, we will cover everything you need to properly size caravan solar panels and their supporting components, including performing accurate calculations while factoring in sunlight hours and the external inefficiencies that reduce output power.
What Goes into Sizing a Caravan Solar Setup?
Properly sizing a solar panel setup ensures all devices and appliances in your campervan, caravan or motorhome have sufficient power while off-grid. It also promotes safe and optimal system performance, as it involves auditing not just the solar panels but also other interconnected components.
After reviewing other online sources, we noticed they focus primarily on sizing solar panels, many without even factoring in external inefficiencies. However, properly sizing the solar charge controller, cables, fuses and battery bank is equally important for optimal results. Below, we cover all five parts of a complete caravan solar system.
Part 1: How to Size the Battery Bank for Your Solar Setup
A battery bank is essential for storing solar energy to power devices and appliances when sunlight is unavailable. The leisure battery bank you choose for your campervan, caravan or motorhome depends on your energy requirements.
Many sources claim the best battery bank size depends on your load (devices and appliances) and off-grid duration. While partially true, the proper way to size a leisure battery bank involves considering all your devices, their daily power consumption, and preferred days of autonomy. The type of battery you prefer and its depth of discharge (DoD) are also very crucial.
For example, in our guide on calculating the perfect battery bank size for your campervan, we referenced a cumulative daily power usage of 473.57Ah (or 5460.64Wh). For a deep-cycle lithium battery bank with 80% DoD and two days of autonomy, the ideal battery bank size would be:
2 x 473.57Ah Γ· 0.8 = 1183.925Ah, equivalent to approximately six 200Ah lithium batteries. It is a nice setup for full-time off-grid living that supports basic DC accessories (238.31Ah), recharge gadgets (73.87Ah), and run essential AC appliances (161.39Ah).
If we used the same cumulative daily power usage with 100% DoD lithium batteries, we'd need 947.14Ah (2 x 473.57 Γ· 1), or five 200Ah lithium batteries.
What Battery Size Do You Need for Weekend-Only Trips?
For occasional boondockers, daily power consumption is typically lower, and two days of autonomy may not be necessary. For example, a weekend trip from Friday evening to Sunday might require about 202Ah or less. The 200-ish amp-hour is sufficient for essentials like lights, a Starlink Mini, an air fan, a 12V fridge, and charging phones, tablets or laptops.
Furthermore, a single 200Ah lithium battery with 100% DoD is sufficient for such daily power consumption. But with an 80% DoD, two batteries are more ideal, paired with solar panels.
Note: Both AGM and lithium batteries are deep-cycle types that can withstand repeated discharges and recharges. However, most AGM batteries have a 50% DoD for optimal performance and durability. Newer AGM batteries may reach 80% DoD but still have a limited lifespan of 300β500 cycles.
Part 2: How Many Solar Panels Does Your Caravan Need?
As mentioned at the beginning, many users choose solar panels randomly for off-grid (and even household) use. Properly sizing your solar panels ensures you have sufficient power for your needs.
One of the best ways to size solar panels for your camper is based on the leisure battery bank calculated above. The battery sizing calculation accounts for the daily power requirements of your devices and appliances, which is also crucial in sizing solar. Generally, the three critical factors for sizing solar panels are:
- Cumulative daily energy consumption: Calculate this in watt-hours (Wh), as solar panel output is rated in watts.
- Average sun hours per day: This varies by location, time of year and weather conditions.
- System loss factor: This accounts for external inefficiencies that often reduce actual energy output compared to the panel's rated capacity.
The system loss factor is critical yet often overlooked by caravanners, RVers and boondockers. It accounts for power losses due to external factors, such as shading, wiring, temperature effects, dirt (dust and debris), and charge controller efficiency.
Typically, these external inefficiencies cause a 20β30% loss. As such, your solar panel setup should be 120β130% efficient to deliver your required power. 120β130% efficiency is the same as a 1.2β1.3 multiplier, which you should use for either:
- 1.2 multiplier: For clean, well-maintained setups in sunny regions.
- 1.3 multiplier: For setups with higher inefficiencies due to variable weather, heavy shading, and incorrect wire sizing.
Solar Panel Sizing Calculator
Many online solar panel sizing calculators are available, but their accuracy varies. The best formula to calculate solar panel size for your needs is:
Ideal solar panel size (Watts) = Cumulative daily energy usage (Wh) Γ· Average daily sun hours x System loss factor
Example: For a full-time off-grid campervan with 5460.64Wh daily energy usage, 4β10 sun hours daily (winter to summer), and a 1.3 loss factor:
- Winter: 5460.64 Γ· 4 x 1.3 = 1774.708W solar power required
- Summer: 5460.64 Γ· 10 x 1.3 = 709.8832W solar power required
Therefore, for the best scenario, in summer, but with worst-case inefficiencies, you need 709.8832W of solar energy, which equates to:
| Panel Wattage | Ideal Solar Panel Size Calculations | Number of Solar Panels Required |
|---|---|---|
| 500W Solar Panel | 709.8832W Γ· 500W = 1.4198 | 2x 500W Solar Panels |
| 430W Solar Panel | 709.8832W Γ· 430W = 1.6509 | 2x 430W Solar Panels |
| 215W Solar Panel | 709.8832W Γ· 215W = 3.3018 | 4x 215W Solar Panels |
| 175W Solar Panel | 709.8832W Γ· 175W = 4.0565 | 4x 175W Solar Panels |
| 100W Solar Panel | 709.8832W Γ· 100W = 7.0988 | 7x 100W Solar Panels |
Meanwhile, for the worst scenario, in winter, and the most extreme inefficiencies, you need 1774.708W solar energy, which equates to:
| Panel Wattage | Ideal Solar Panel Size Calculations | Number of Solar Panels Required |
|---|---|---|
| 500W Solar Panel | 1774.708W Γ· 500W = 3.5494 | 4x 500W Solar Panels |
| 430W Solar Panel | 1774.708W Γ· 430W = 4.1272 | 5x 430W Solar Panels |
| 215W Solar Panel | 1774.708W Γ· 215W = 8.2545 | 9x 215W Solar Panels |
| 175W Solar Panel | 1774.708W Γ· 175W = 10.1412 | 11x 175W Solar Panels |
| 100W Solar Panel | 1774.708W Γ· 100W = 17.7471 | 18x 100W Solar Panels |
Why Many Online Solar Calculators Fall Short
The reason we said many online solar sizing calculators are inaccurate is that one we used suggested 1606.07W solar energy required during winter and 642.43W for summer. Compared to our calculations above, this underestimates the solar energy you need by 168.64W and 67.45W, respectively.
For the best off-grid experience, we recommend sizing your solar panels based on the winter scenario to ensure sufficient energy year-round. If your battery bank has multiple days of autonomy, you can choose a size between the best and worst scenarios or use a lower loss factor multiplier, provided it covers the worst case.
Part 3: How to Size the Wiring for Your Solar Setup
Properly sized wiring ensures safety, performance and efficiency in your solar panel setup. Undersized or overly long wires increase resistance, causing voltage loss and heat generation. Meanwhile, appropriately sized cables minimise power loss, delivering DC electricity from panels to the battery bank efficiently.
Manufacturers usually provide cables with predetermined power ratings, so you only need to calculate the current flowing through your circuits and select cables with matching ampacity.
To size solar panel wiring, use the panel's rated maximum current (short-circuit current, ISC) and a 1.56 safety factor multiplier, per the National Electrical Code (NEC). The 1.56 multiplier comes from multiplying the 125% (1.25) Irradiance Enhancement Factor by the 125% (1.25) Continuous Load Factor.
- Irradiance Enhancement Factor: Accounts for situations where solar irradiance exceeds the standard 1000 W/mΒ², e.g., on clear days with high sunlight intensity.
- Continuous Load Factor: Protects wiring from overheating during prolonged operation, as solar panels can operate at maximum current for 3+ hours.
Therefore, the formula for ideal solar cable size (ampacity) = Short-circuit current (ISC) x 1.56.
Example: For a 500W solar panel with a 12.82A ISC, the ideal cable ampacity is 19.999A (12.82 x 1.56), requiring at least an 8.36mmΒ² cable. However, the specific cable size may vary by manufacturer, provided the minimum ampacity is 20A.
Part 4: How to Size the Fuses for Your Solar Setup
Fuses are also critical for protecting your solar setup against excessive current, especially in parallel arrays where multiple panels increase output current. Each solar panel has a maximum series fuse rating, indicating the highest current its internal components can handle in series or parallel.
In a parallel array, the total ISC may exceed the maximum series fuse rating. For example, a 500W solar panel with a 12.82A ISC can have a 30A maximum fuse rating. When connected in parallel with two others, this yields a total ISC of 38.46A (12.82 x 3), exceeding the 30A rating. Hence, each panel's circuit requires a 30A fuse for safe operation.
If the maximum series fuse the manufacturer recommends is unavailable, you can calculate the minimum fuse size by multiplying the ISC by 1.56. For example, our 500W panel will require a minimum 20A fuse (12.82 x 1.56).
However, your solar fuse must also protect the circuit cable, so you should use the cable's current rating for your calculations. Our example requires a 20A cable, so the minimum fuse should be 31.2A (20 x 1.56). But since the manufacturer's 30A fuse is sufficient, you can proceed using it.
Important note: A master fuse or circuit breaker is required for the circuit to the solar charge controller. Calculate this by multiplying the total ISC by 1.56. For example, our three 500W panels have a total ISC of 38.46A, so the master fuse/breaker size will be 60A (38.46 x 1.56).
Part 5: How to Size the Solar Charge Controller
An MPPT solar charge controller (or PWM charge controller) is the component that regulates and stabilises DC electricity from the panels, ensuring safe use by batteries or appliances. Its proper sizing is critical for safe operation and optimal power delivery.
Regardless, no calculations are needed here. You only need to ensure the charge controller's electrical ratings β maximum voltage, current and power output β are compatible with your solar setup:
- Open-Circuit Voltage (VOC): The charge controller must withstand the maximum voltage a solar panel produces without a connected load.
- Maximum ISC: The charge controller must handle the total short-circuit current of your connected panels to avoid damage.
- Output Voltage: The controller's output must match the battery bank or appliance (e.g., 12V-rated for 12V batteries and appliances).
Does Solar Panel Efficiency Affect Sizing?
Solar panels are the best solution for reliable, self-sustaining power in campervans, caravans, RVs and motorhomes. However, you must choose the right size to meet your off-grid needs. At this point, you will notice we haven't used the solar panel efficiency values anywhere in our calculations.
Solar panel efficiency (like 18β24%) typically refers to the percentage of sunlight converted to usable DC electricity. This efficiency is built into the panel's design and doesn't directly affect output calculations, but it impacts physical size. Higher efficiency (e.g., monocrystalline panels, 18β24%) allows smaller panels to generate more power compared to polycrystalline panels (15β20%) of the same wattage.
A perfect example for comparison is Victron BlueSolar Monocrystalline and Polycrystalline panels, where the largest model in both categories is a 1980 x 1002 x 40mm series. Nonetheless, the Monocrystalline version offers 360W nominal power, 47.4V VoC and 10.24A short-circuit current, whereas the Polycrystalline version is 330W, 44.72V VoC and 9.57A short-circuit current.
Frequently Asked Questions
How much solar power do I need for full-time off-grid living?
Using the example of 5460.64Wh daily energy usage with a 1.3 loss factor, you'd need 1774.708W of solar in winter (4 sun hours) or 709.8832W in summer (10 sun hours). We recommend sizing for the winter scenario β for instance, 4x 500W panels β to ensure sufficient energy year-round.
What is the system loss factor and why does it matter?
External inefficiencies such as shading, wiring, temperature effects, dirt and charge controller efficiency typically cause a 20β30% loss. To compensate, multiply your calculated solar requirement by 1.2 (clean setups in sunny regions) or 1.3 (setups with higher inefficiencies).
What size cable do I need for a 500W solar panel?
Multiply the panel's short-circuit current by 1.56. A 500W panel with a 12.82A ISC needs a cable ampacity of 19.999A, which means at least an 8.36mmΒ² cable with a minimum 20A rating.
Should I size my solar panels for winter or summer?
Size for winter to guarantee enough power year-round. If your battery bank has multiple days of autonomy, you can choose a size between the winter and summer scenarios or use a lower loss factor multiplier, provided it covers the worst case.
Ready to build a solar setup that never leaves you flat? Browse our full range of caravan and campervan solar panels at Campervan Builders and size your system with confidence.
