Off-Grid Solar Sizing Guide for Homes, Cabins, Caravans & Boats

Sizing an off-grid solar system means matching your daily energy use with the right solar panel output, battery storage, inverter size, and electrical protection. If one part is too small, the whole system can struggle, especially during cloudy weather or short winter days.

This is important for cabins, rural homes, garden rooms, workshops, caravans, motorhomes, boats, and off-grid properties across Europe. Solar conditions can vary widely between southern Spain, northern Scotland, alpine regions, coastal areas, and shaded woodland sites. A system that works well in summer may not be enough for winter use.

The good news is that the sizing process is straightforward when you break it into steps. Start with your energy use, estimate solar generation, calculate battery capacity, choose the inverter, and make sure the wiring and protection are suitable.

Step 1: Calculate Your Daily Energy Use

Before choosing panels or batteries, work out how much electricity you need each day. List every appliance or device the system will power. Include both large and small loads, because even low-power devices can use a lot of energy if they run for many hours.

For each load, find the wattage and estimate the number of hours it will be used per day. Multiply watts by hours to get watt-hours.

Formula:

Daily energy use (Wh) = Power rating (W) × Hours used per day

In this example, the total daily energy use is 1,710Wh, or 1.71kWh.

For real-world sizing, add a margin of around 20% to 30%. Solar systems lose some energy through wiring, charging, inverter conversion, and battery storage. So a 1.71kWh daily load should be sized closer to 2.1kWh to 2.3kWh per day.

Step 2: Estimate Solar Panel Requirements

Solar panels are sized based on how much energy you need and how many peak sun hours your location receives. Peak sun hours are not the same as daylight hours. They describe the strength of usable sunlight over the day.

In southern Europe, summer solar production can be strong. In northern Europe, winter solar production may be much lower because of shorter days, low sun angle, cloud cover, and shading. For year-round off-grid use, always size the system around the weaker season.

Basic formula:

Solar array size (W) = Daily energy target (Wh) ÷ Peak sun hours

Then adjust for real-world losses by dividing by 0.75 to 0.80.

Example:

• Daily energy target: 2,200Wh
• Average peak sun hours: 4 hours
• Solar wattage before losses: 2,200Wh ÷ 4 = 550W
• Adjusted for losses: 550W ÷ 0.8 = about 690W

In this example, a solar array of about 700W would be a sensible starting point. That could be built with two larger residential panels, several smaller panels, or a mix that suits the available roof, ground mount, caravan roof, or boat deck space.

Step 3: Calculate Battery Storage

The battery bank stores energy for nighttime and low-sun periods. In an off-grid system, the battery is what keeps everything running when the panels are not producing enough power.

Start by choosing your days of autonomy. This means how long the system should run without useful solar charging. For a small caravan or weekend cabin, 1 day may be enough. For a remote cabin, rural home, boat, or critical backup system, 2 to 3 days is more realistic.

Formula:

Battery storage needed (Wh) = Daily energy target × Days of autonomy

Example:

• Daily energy target: 2,200Wh
• Days of autonomy: 2
• Usable battery storage needed: 2,200Wh × 2 = 4,400Wh

Next, account for usable capacity. Different battery types can safely use different amounts of their rated capacity.

If you need 4,400Wh of usable storage and use LiFePO4 batteries with around 90% usable capacity, you need roughly 4,900Wh of rated battery capacity. With lead-acid batteries at around 50% usable capacity, you may need closer to 8,800Wh of rated capacity.

To convert Wh to Ah:

Battery capacity (Ah) = Battery capacity (Wh) ÷ Battery voltage

For example, 4,900Wh on a 24V system equals about 204Ah. On a 48V system, the same energy equals about 102Ah.

Batteries should be selected to match your system voltage, inverter, charge controller, and expected load profile.

Step 4: Choose 12V, 24V, or 48V

Off-grid systems commonly use 12V, 24V, or 48V battery banks. The right voltage depends on system size and power demand.

For small leisure systems, 12V may be enough. For larger inverters, longer cable runs, and higher daily energy use, 24V or 48V is often more efficient and easier to wire safely.

Step 5: Size the Inverter

The inverter converts battery DC power into AC power. In most European homes and appliances, this means 230V AC. To size the inverter, add up the appliances that may run at the same time and check their startup surge.

Some appliances use far more power when they start than when they run. Fridges, pumps, compressors, power tools, washing machines, and air conditioners can all have high surge demand.

Example:

• Fridge: 150W running with startup surge
• LED lights: 50W
• Laptop: 60W
• Water pump: 500W running with startup surge
• Kettle or small cooking appliance: 1,000W to 2,000W

If you plan to run a fridge, pump, laptop, and lights, a 1,000W to 2,000W inverter may be enough. If you want to run kettles, microwaves, tools, or larger appliances, you may need a 3,000W inverter or larger.

Be careful with high-heat appliances such as electric kettles, heaters, ovens, and induction hobs. They can drain batteries very quickly and may require a much larger inverter and battery bank.

Step 6: Size the Charge Controller

The charge controller manages the power from your solar panels to your battery bank. MPPT charge controllers are usually preferred for off-grid systems because they are more efficient and work better with higher-voltage solar arrays.

Basic formula:

Charge controller current (A) = Solar array watts ÷ Battery voltage

For example, a 700W solar array charging a 24V battery bank may produce around 29A before design margin. A 40A MPPT charge controller may be a better choice than a controller that is too close to the limit.

Always check both the current rating and the maximum PV input voltage. Also make sure the controller supports your battery chemistry, especially if you are using LiFePO4 batteries.

Step 7: Plan Wiring and Safety Protection

Safe wiring is critical in any off-grid solar system. Low-voltage DC systems can still carry very high current, and poor wiring can cause overheating, voltage drop, or fire risk.

• Use the correct cable size: Cable size must match current, distance, and acceptable voltage drop.
• Install fuses and breakers: Protect the battery, solar panels, charge controller, inverter, and circuits.
• Use isolation switches: Battery, solar, and inverter disconnects make servicing safer.
• Protect against moisture: Boats, caravans, outdoor sheds, and cabins need suitable enclosures and cable protection.
• Follow local electrical rules: For fixed buildings, grid-interactive systems, or high-power AC wiring, use a qualified installer or electrician.

Quick Off-Grid Solar Sizing Example

Final Thoughts

A good off-grid solar system starts with honest load calculations. Once you know how much energy you use each day, you can size the panels, batteries, inverter, and charge controller with far more confidence.

For caravans, boats, and small cabins, the system may be simple. For off-grid homes, high-power appliances, winter use, or long cable runs, the design needs more careful planning.

The best system is not always the largest one. It is the one that matches your daily energy needs, local sunlight, backup expectations, battery chemistry, and safety requirements.