Battery and solar power 101

Collyn Rivers — 29 April 2015

Northern auto-electricians see a constant flow of caravans with flat batteries, fridges full of decaying food and beer too warm to contemplate. The main causes are that there is less solar input up north than most people expect, and that it’s hot all night for most of the year.

Down south, solar input varies from 2-3 Peak Sun Hours/day (PSH/day) in winter to 6-7PSH/day in summer. Across much of the Top End, once north of Townsville, Qld/Geraldton, WA, typical solar input is 5PSH/day in winter, and a tad over 6PSH/day for the rest of the year.


Sunlight is measured by the solar industry as if it were rainfall: much like capturing sunlight in ‘standard-sized’ buckets. Once ‘full’, that bucket represents 1PSH. For the technically minded, 1PSH is the irradiation of 1000W/sqm per hour. The input during a Melbourne mid-winter day is typically 2PSH, while a Brisbane summer may be 6-7PSH.

The available energy for campers is limited by solar panel efficiencies of 14-20 per cent, and reduced by ambient heat. All but the rarely-used amorphous solar panels lose input when hot. Vendors may suggest this is above 25°C, but that is not the ambient temperature, it’s the temperature of black solar cells under hot glass.

At 25°C ambient, those cells are at 46-48°C. They have already lost 10 per cent. At a not uncommon 40°C ambient, they are down by about 17.5 per cent. (This is revealed in the specifications – under Nominal Cell Operating Temperature).

Heat also adversely affects fridges. Apart from May/June, northern nights are hotter and longer than summer down south, particularly on the coast.

Systems with marginal energy capacity cannot cope with the extra load, let alone less solar input and few caravanners realise that inadequate generating capacity is causing their woes.


Before you leave, you can simulate up-north usage to see how your existing system will cope by loading the fridge with bottles of warm water twice a day and heating the caravan to about 35°C all day and 30°C all night. If, in such conditions, the solar system does not bring the batteries to full charge by midday on most summer days, it has zero hope of coping up north, except, perhaps, in May/June.

If this is the case, there are two main fixes and, ideally, you’d do both before departure: reduce your energy use and increase your energy input.


The three main energy loads in a typical caravan are (in order): fridge/freezer, microwave oven, incandescent (including halogen) lighting and older fluorescent tubes.


Fridges will not cool as intended (or will draw excess energy) unless optimally installed. Few are, because fridge operation is often misunderstood. A fridge does not ‘make cold’; it pumps heat from where it’s unwanted to where it does not matter (outside your caravan) by dissipating it via cool air drawn over fins at its rear or, in a few, from their sides. There must be a cold air inlet at its base. Hot air must readily flow to the outside.

Unless installed correctly, three-way fridges barely cool in hot areas. Electric fridges may cool correctly but will draw excess energy.

Since 2000 or so, three-way fridges have classifications that specify the ambient temperature over which they must deliver their rated performance: N (sub normal) 14-32°C, N (normal) 18-32°C, ST (sub-tropical) 18-36°C, and T (tropical) 18-43°C. The T version is the only one that works satisfactorily up north.

Electric fridges must have their intended working voltage. This is a particular issue for caravanners running the fridge and charging the caravan battery while driving.


An 800W microwave oven produces 800W of heat, but draws about 1200W. If it’s inverter-powered, the total draw is 1300-1350W (about 110A at 12V). Deep-cycle batteries, however, only deliver their rated capacity for loads no higher than 5 per cent of that capacity. A 200Ah battery bank has only that capacity if the discharge is limited to no more than 10A. So the microwave’s 110A draw causes energy to be lost internally as heat. The loss depends on battery type and size but may deplete the battery by twice or thrice the draw.

Ideally, you should use a microwave oven only where mains (or generator) 230V is available.


Many older caravans have energy-gobbling incandescent globes (including halogen). Tube fluorescents are not as bad, but it’s still better to replace them.

Existing 12V 10-20W halogen globes usually have pins 4mm apart. The 35-50W versions have pins 5.1mm apart. A high quality 7W LED provides the equivalent light of a 35W halogen globe. High quality LEDs are not cheap, but should last 10-20 years if used every night. There are also LED replacements for fluorescent tube fittings.


High quality, locally-made TVs now draw a fraction of the energy of the older versions. The 12V units (mostly from India) draw a lot of current. You’ll save $100 by buying one but spend $500 more for extra solar and battery capacity to run it.

Laptop computers draw relatively low current, but desktop units far more. Draw is a function of the computer’s processing power and screen size and brightness. TVs and computers are usually adjustable to reduce energy draw.


If you’re going to be driving for a few hours each day or two, you can achieve a major increase in charging input by installing a DC-DC alternator charger in the caravan. These units convert whatever voltage is available to that required for optimum charging voltage. They are programmable for battery size and type. Some also double as solar regulators.

These units are essential with post 2000 tow vehicles that have alternator voltage too low for deep and reasonably fast charging. They can be self-installed by those used to 12V equipment. If not, have an auto-electrician do it for you. The fridge to caravan battery cable should be at least six square mm. Ideally, upgrade the cabling from the alternator to the caravan. Take the feed via an Anderson plug and socket.


If the energy shortage is 20-35 per cent of that available, the simplest approach is to add that amount more solar. You can now buy solar panels for about $1 a watt.

A further (and overall) 10-15 per cent is usually obtainable by replacing an existing Pulse Width Modulation (PWM) solar regulator by a high quality Multiple Power Point Tracking (MPPT) unit. This does not so much gain more input, but saves that otherwise lost in non-MMPT units. Beware of eBay look-alikes. A good MPPT regulator costs about $300 or more.

By all means, replace faulty batteries but, otherwise, put your main effort into reducing energy usage and increasing energy input. As a rough guide, you ideally need about 200W of solar per 100Ah of (12V) battery bank. Many systems will actually work better with less battery capacity. It is impossible to have too much solar – and, as there is almost always some solar input, it is often feasible to actually reduce battery capacity.


If energy shortage is serious and not realistically fixable via an alternator or solar charging, consider using the 230V output of a Honda/Yamaha inverter generator to charge the batteries during the day via a high current 12V multi-stage battery charger. You ideally need a charger that has an output of 15-20 per cent of the amp hour capacity of your battery bank. For example, a 200Ah battery bank really needs a 30-40A charger. Charging is likely to take about 60-90 minutes a day. This way, fuel cost is minimised and running the generator only during the day is less likely to annoy other campers.

Do not attempt to charge from the 12V outlets on these generators. Even if marked ‘battery charger’, the output is a mere 8A and at a voltage too low for effective charging. That outlet is intended to drive small 12V appliances directly and is typically limited to 13.65V and 8A.

It is totally feasible to have reliable solar up north, even in mid-summer; I had a big, self-designed and solar-only property system and two solar equipped RVs for when I lived in Broome from 2000-2010.

The full feature appeared in Caravan World #536 April 2015. 


Battery and solar power solar input generators