Solar Panels & Batteries

Panel wattage tells you how much power a panel produces under ideal conditions (1,000W/m² of sunlight, 25°C cell temperature). In real-world Australian conditions, expect 70–80% of the rated output on a clear day.

A rough starting point: divide your daily energy use in watt-hours by the average peak sun hours for your location (typically 4–5 hours across most of Australia). A 200Ah 12V battery has a nominal capacity of approximately 2,400Wh (200Ah × 12V). At 50% usable depth of discharge, approximately 1,200Wh is usable would need around 500–600W of panels to recharge fully in a day.

More panels can compensate for cloudy days, partial shading and efficiency losses in hot weather — however, panel array voltage and current must remain within your controller's limits and battery charging requirements. If roof space allows, err on the side of more wattage.

Battery capacity is measured in amp-hours (Ah) at a given voltage, or watt-hours (Wh). A 200Ah 12V battery stores 2,400Wh of energy. How much of that you can actually use depends on the battery chemistry and its depth of discharge (DoD).

Calculate your overnight load: add up the wattage of every appliance you run after sunset and multiply by the hours of use. Add a 20–30% buffer for efficiency losses and unexpected loads. That figure is your minimum usable battery capacity.

Lithium Iron Phosphate (LiFePO4) batteries are the modern standard for off-grid solar. They offer 80–100% usable capacity, 2,000–5,000+ charge cycles, faster charging, lighter weight and no maintenance. They cost more upfront but deliver a significantly lower cost per cycle over their lifespan.

AGM (Absorbent Glass Mat) lead-acid batteries are cheaper to buy but only offer 50% usable capacity, 300–500 cycles and are sensitive to being left in a partially discharged state. They are heavier and require more careful management. For a permanent caravan or off-grid setup, lithium is almost always the better long-term investment.

Gel batteries sit between AGM and lithium — slightly more cycle-stable than AGM but still limited to around 50% DoD. They are less common in modern off-grid builds.

12V systems are the most common for camping and caravans. They are compatible with most 12V appliances, easy to wire and straightforward to expand. Suitable for systems up to around 400–600W of solar and 200–400Ah of battery storage.

24V systems halve the current for the same power, which means thinner cable runs and less voltage drop over long distances. Well suited to larger caravans, camper trailers and small off-grid cabins with 600W–2,000W of solar.

48V systems are used in larger off-grid homes and tiny homes where high power loads (inverter air conditioners, induction cooktops, power tools) are common. They require a 48V battery bank and a 48V-compatible inverter-charger. Not typically needed for camping or caravanning.

A charge controller sits between your solar panels and battery bank, regulating the charge to protect the batteries from overcharging. There are two main types: PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking).

MPPT controllers are generally more efficient than PWM — the actual advantage varies with panel voltage, temperature and operating conditions, especially in cold weather, partial shade or when the panel voltage is much higher than the battery voltage. For any system larger than a basic weekend setup, MPPT is the right choice.

PWM controllers are cheaper and work adequately for small, simple systems where the panel voltage closely matches the battery voltage. They are rarely the best choice for a permanent caravan or off-grid installation.

Every charge controller has a maximum solar input in watts and a maximum open-circuit voltage (Voc). Exceeding either will damage the controller. Always check the Voc of your panel array — especially in cold weather, when Voc can be 10–15% higher than the rated figure.

Size your charge controller to handle at least 25% more than your current panel wattage to allow for future expansion. A 40A MPPT controller on a 12V system can handle around 500W of panels — enough for most caravan setups.

A common rule of thumb is to size your solar array so it can fully recharge your battery bank in one good day of sun. For a 200Ah lithium battery (2,400Wh usable), that means roughly 500–600W of panels in most Australian locations.

Oversizing your panels relative to your battery is generally fine — the charge controller will simply stop charging once the battery is full. Undersizing means you may not fully recharge on shorter winter days or in overcast conditions.

For extended off-grid travel, solar sizing should be based on your daily Wh consumption, system losses and available peak-sun hours — a simple panel-to-battery ratio is not a reliable guide. Adjust for high-draw appliances like a compressor fridge, CPAP or induction cooktop.