How to Size a Complete 12V Caravan Off-Grid System

A reliable 12V caravan or off-grid system starts with the loads, not with a guess at panel or battery size. The practical sequence is: estimate daily energy use, size usable battery storage, calculate the solar array, check the charge controller, then select an inverter and cables that can safely carry the current.

This guide explains that process for Australian caravan, camper-trailer and small off-grid systems. It is general planning guidance only. Final cable sizes, fusing, isolation, earthing and installation must suit the equipment manufacturer's instructions and applicable Australian requirements. Use a suitably qualified installer where required.

Step 1: list every electrical load

Record each appliance's power in watts and how many hours it is likely to run per day. Daily energy is:

Daily energy (Wh) = power (W) × operating time (hours)

For cycling loads such as compressor fridges, use measured daily consumption or a realistic manufacturer figure rather than multiplying the maximum wattage by 24 hours. Hot weather, ventilation, thermostat setting and how often the fridge is opened can change consumption substantially.

Example load Planning allowance Daily energy
12V compressor fridge Measured or estimated daily use 500Wh
LED lighting 30W for 3 hours 90Wh
Phones and small devices 40W for 3 hours 120Wh
Laptop 65W for 3 hours 195Wh
Water pump and fans Combined allowance 120Wh

In this example the daily total is about 1,025Wh. Adding a sensible allowance for conversion losses and unexpected use gives a planning target around 1,200Wh per day.

Step 2: size the battery by usable energy

A battery's amp-hour rating is not the same as its usable energy. A simple estimate is:

Battery energy (Wh) = nominal voltage × amp-hours

Usable capacity depends on battery chemistry, discharge limits, temperature, age and the battery management system. For an illustrative 1,200Wh daily load:

  • LiFePO4 example: 1,200Wh ÷ (12.8V × 0.80 usable fraction) ≈ 117Ah before adding reserve.
  • AGM example: 1,200Wh ÷ (12V × 0.50 usable fraction) = 200Ah before adding reserve.

Those figures are starting points, not universal limits. Confirm the battery maker's recommended depth of discharge, continuous current, peak current, charge settings and low-temperature restrictions. If you want more than one day without charging, multiply the daily requirement by the desired autonomy before allowing for usable capacity.

Compare suitable lithium batteries and AGM deep-cycle batteries, or read our lithium vs AGM guide before choosing.

Step 3: calculate the solar array

A useful planning formula is:

Solar array (W) = daily energy (Wh) ÷ (peak-sun hours × system factor)

The system factor allows for panel temperature, cable and controller losses, imperfect angle and other real-world conditions. For 1,200Wh per day, four peak-sun hours and a 0.75 system factor:

1,200 ÷ (4 × 0.75) = 400W of solar

Peak-sun hours are not the same as hours of daylight. They vary with location, season, weather, shading and panel angle. Use a conservative seasonal figure for the places and months in which you actually travel. If shade or winter touring is common, allow additional array capacity or another charging source.

Roof space, weight and portability also matter. Browse fixed solar panels, flexible solar panels and portable solar blankets, or compare pre-matched options in our off-grid solar kits.

Step 4: match the solar charge controller

The controller must suit both the battery bank and the panel array. An approximate output-current check is:

Controller output current ≈ array watts ÷ battery charging voltage

For a 400W array charging near 14.4V, the theoretical output is about 27.8A. A controller needs appropriate headroom, but current rating alone is not enough. Check:

  • the array's maximum open-circuit voltage (Voc), including cold-condition rise;
  • maximum PV input current and power;
  • battery voltage and supported chemistry;
  • the battery manufacturer's charge-voltage and current limits;
  • whether panels are wired in series, parallel or a suitable combination.

MPPT solar charge controllers are often preferred for larger arrays and changing conditions, but the correct model still depends on the full electrical specification. Do not connect an array that can exceed the controller's PV input limits.

Step 5: choose the inverter from peak and continuous loads

Add the simultaneous 240V loads and identify appliances with high start-up demand. Choose a quality pure sine wave inverter where the appliance manufacturer requires it, and verify both continuous and surge capability.

The battery-side current can be much higher than many owners expect:

DC current ≈ AC watts ÷ (battery voltage × inverter efficiency)

A 1,000W load at 12.8V and 90% efficiency draws about 87A. A 2,000W load can approach twice that current. The battery, battery management system, interconnects, fuse, isolation and cable length must all support the demand. Higher-power systems may be better designed around 24V, but every compatible component must match that system voltage.

Explore pure sine wave power inverters and use the appliance's real running and surge requirements—not just a marketing wattage—to make the final selection.

Step 6: account for alternator charging

For a caravan or touring vehicle, solar is only one charging source. A correctly selected DC-DC battery charger can manage alternator input to an auxiliary battery, including installations with many smart alternators. The charger must suit the alternator, battery chemistry, cable run and required charge current. Some models also include a solar input; check how the unit prioritises or combines sources.

Step 7: design the protection and cabling

High-current 12V systems need careful protection. Voltage drop, cable temperature and fault current all matter. As a minimum, the design should address:

  • correct cable cross-section for current and round-trip length;
  • over-current protection close to each energy source;
  • appropriately rated switches, busbars, lugs and enclosures;
  • secure mounting and protection from water, heat, abrasion and vibration;
  • ventilation where required by the battery or equipment manufacturer;
  • clear isolation for service and emergency use.

Never select a fuse simply to stop nuisance blowing. The protective device must protect the cable and remain within the equipment manufacturer's limits.

A worked 12V system example

For a touring setup using about 1,200Wh per day, a reasonable design conversation might begin with a 400W solar array, an appropriately rated MPPT controller, and battery storage selected for the chosen chemistry and desired reserve. A 1,000W inverter may be enough for light 240V use, but only if its surge rating, the battery discharge capability and the DC cabling all suit the actual appliances.

This is not a universal shopping list. A shaded winter campsite, induction cooking, coffee machine, medical device or extended stationary use can change the design materially. For a deeper look at array sizing, see how much solar a caravan or 4WD needs. Vehicle-based systems should also use our 12V 4WD touring setup guide.

Complete 12V system checklist

  • Calculate daily energy in watt-hours.
  • Choose battery chemistry and usable capacity.
  • Allow for the number of days you need without charging.
  • Size solar using conservative local peak-sun hours.
  • Verify controller voltage, current and PV input limits.
  • Check inverter continuous and surge requirements.
  • Confirm battery and BMS discharge capability.
  • Design cable, fusing, isolation and mounting as one system.
  • Recheck compatibility against every manufacturer's data sheet.

Frequently asked questions

Is a 200Ah battery twice as useful as a 100Ah battery?

It stores roughly twice the nominal amp-hours at the same voltage, but usable energy still depends on chemistry, allowable depth of discharge, discharge rate, temperature and the battery management system.

Can a solar controller be too large?

A controller with additional output capacity can be acceptable if the manufacturer permits the connected array and its charge settings suit the battery. The PV input voltage and current limits must still be respected.

Do I need 24V instead of 12V?

Many caravan loads are designed for 12V. A 24V system can reduce current for higher-power loads, but it adds compatibility and conversion considerations. It should be a deliberate whole-system design choice.

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