On clear days around noon, about 1000 watts of solar energy (enough to boil a kettle) is theoretically
available on each square metre of much of the Earth’s surface. Commercially available solar modules
convert only 10%-14% of that energy into electricity but, by using appropriate, energy-efficient
appliances, the resultant 100-140 watts/square metre can free recreational vehicles and cabins
substantially or totally from mains or generator power.
Whether an RV of any type, or a cabin, the fundamentals are similar: differences are mostly a
matter of scale. A good starting point is to know roughly how much energy different things require.
Apparently similar lights and appliances may use hugely different amounts of energy to achieve the
same ends. Some fridges use two to three times the energy of others; microwave ovens use more
energy than many people think. Water pumping may require vast amounts of energy - or very little.
Where can solar energy be used?
It is light, not heat, that solar modules turn into electricity, in fact most solar modules dislike heat.
They lose output as they become warm and work best in cold places under a bright sun. The amount
of energy they put out depends on how much light falls on them, and for how long, but they all need
at least some sunlight to operate. None work in total shade.
A solar module’s output is measured in handy (photovoltaic) units called ‘Peak Sun Hours’. Each
is like a standardised drum full of sunlight. Think of sunlight as rainfall and you’ve got the general
idea. That drum may fill in only an hour or so in Broome most year-round, but may take all day
during a Hobart winter. Each full drum holds the equivalent of 1 Peak Sun Hour and you’ll get more
full drums each day in some places and times of year than others. The concept usefully averages the
measurement of the sun’s daily energy.
The map below shows the Peak Sun Hours for a typical Australian January (midsummer).
This is the most irradiation that is likely. Irradiation varies more or less linearly from
mid-summer to mid-winter. In many places and times, there will be at least three Peak Sun Hours
each day. In some there will be six or more. Full-size versions of these maps for Australian summer
and winter are reproduced on page 35.
 |
|---|
Our previously-owned OKA in Kakadu National Park (in 1998). Two 80-watt Solarex modules provided ample
power for all needs - including a 71-litre Autofridge. During our entire ownership, from mid-1996 until August
2006, the batteries had not once run out of power. The spade marks where the damper was cooking. |
On clear days around noon, about 1000 watts of solar energy (enough to boil a kettle) is theoretically
available on each square metre of much of the Earth’s surface. Commercially available solar modules
convert only 10%-14% of that energy into electricity but, by using appropriate, energy-efficient
appliances, the resultant 100-140 watts/square metre can free recreational vehicles and cabins substantially
or totally from mains or generator power.
Whether an RV of any type, or a cabin, the fundamentals are similar: differences are mostly a
matter of scale. A good starting point is to know roughly how much energy different things require.
Apparently similar lights and appliances may use hugely different amounts of energy to achieve the
same ends. Some fridges use two to three times the energy of others; microwave ovens use more
energy than many people think. Water pumping may require vast amounts of energy - or very little.
Where can solar energy be used?
It is light, not heat, that solar modules turn into electricity, in fact most solar modules dislike heat.
They lose output as they become warm and work best in cold places under a bright sun. The amount
of energy they put out depends on how much light falls on them, and for how long, but they all need
at least some sunlight to operate. None work in total shade.
A solar module’s output is measured in handy (photovoltaic) units called ‘Peak Sun Hours’. Each
is like a standardised drum full of sunlight. Think of sunlight as rainfall and you’ve got the general
idea. That drum may fill in only an hour or so in Broome most year-round, but may take all day
during a Hobart winter. Each full drum holds the equivalent of 1 Peak Sun Hour and you’ll get more
full drums each day in some places and times of year than others. The concept usefully averages the
measurement of the sun’s daily energy.
 |
|---|
The map on the facing page shows the Peak Sun Hours for a typical Australian January (midsummer).
This is the most irradiation that is likely. Irradiation varies more or less linearly from
mid-summer to mid-winter. In many places and times, there will be at least three Peak Sun Hours
each day. In some there will be six or more. Full-size versions of these maps for Australian summer
and winter are reproduced on page 35.
New Zealand’s North Island, and the eastern part of the country’s South Island have a fairly
uniform 4.2-6.5 Peak Sun Hours between September and June, and 2.0-4.0 in between.
Peak Sun Hours (mid-January). Multiplying the data shown for any area by the true output of a solar module (in
watts) gives the total average output for that day in watt-hours/day (Wh/day). This map, plus a mid-July map, are
printed full-size on page 35.
Battery capacity
With battery capacity, ‘more’ does not necessarily equate with ‘better’. Batteries are damaged if
left discharged for more than a few days or so and, if the battery bank is overly-large relative to the
charging source, insufficient energy is available to recharge quickly. Economise on batteries but not
modules. Adding more batteries alone is like opening a second bank account for the same money.
If you plan to use occasional power-hungry equipment (e.g., arc welders, big angle grinders,
clothes dryers), but only from time to time, consider scaling the system for ‘normal’ loads and
supplying the excess via a generator. Much the same applies to those spending only the odd midwinter
month or so in cold places with short hours of sunlight (although fridge energy usage is lower
in winter).
Cooking & heating
Solar generated electricity is not practicable for anything that, as its main purpose, generates heat.
Electric ovens, fryers, and water heaters are out. Hair dryers are borderline. Electric irons are best
used where there is mains power, but what’s so special about flat clothes whilst camping?
For RVs, use gas for cooking and for heating water. For cabins, use gas for cooking, and solar
water heaters for water heating generally, not least because gas-fuelled water heaters fail frequently
if used only from time to time.
Energy-efficient appliances
Coffee grinders, blenders, and other small appliances vary in efficiency but, if used normally, their
daily energy use is not of major concern. Microwave ovens however use more energy than most
people suspect. Their nominal wattage rating refers to the work they do (i.e., ‘cooking power’) not
the energy used in doing so. Most ‘800-watt’ ovens consume close to 1350-watts. Or 1500 watts if
driven via an inverter. Ten minutes use can draw a day’s output from an 80-watt module. That oven
may cost only $275, but running it from solar can add at least $1000 for the extra solar capacity and
batteries needed to drive it. And it can still only be used when there’s enough power. If you must
have a microwave consider using it only when you have mains power.
Water pumping
Apart from hand or foot operated pumps (both are still available) the only practicable pumps for RVs
are those that run from 12 or 24-volts. Mains-voltage pumps are available but they use several times
as much energy for moving the same amount of water.
A ‘pressure accumulator’ (page 29) overcomes the otherwise high energy draw of pumping water
where there is a washing machine or dishwasher, and also in large cabins with flush toilets. It also
results in a system that is silent most of the time.
Washing machines/dishwashers
Most front loading washing machines use less energy and water than top-loaders. The more efficient
readily run from a medium-sized RV solar system and inverter. These machines are fine also for
cabins. Where hot water is available, this may be connected via a thermostat set to whatever the
washing machine maker suggests (typically 50 degrees C). Much the same applies to dishwashers.
Television
Unless watched day-long, there are no energy problems with recently-made 25-35 cm (10-14 inch)
TVs. Most use 20-40 watts (whether 12 or 240-volt). Older ones however may use up to 80 watts.
The 66 cm (26 inch) TVs typically use up to 300-350 watts, but those with the new LCD screens
consume about half of that.
Computers
The most realistic choice for RVs and cabins is a laptop: desktop units use too much energy. As with
TVs, conventional screens are responsible for about half of the TV’s draw - and by the same amount.
LED screens draw less. Specifying a laptop will inevitably result in teenager tantrums: laptop computers
are distinctly ‘uncool’. But, as teenagers are likely to sit on a computer for hours on end, solar
realism is a laptop or nothing. They (laptops) also travel better. See also page 30.
Lighting
Incandescents use by far the most energy of all. Halogen globes use about half the energy for the
same amount of light. They are available in 12- and 24-volt versions. Fluorescent globes and tubes,
and compact fluorescents, use only a quarter of the energy of incandescents. The very latest white
LEDs use even less. See page 27.
Air conditioning
Unless run from mains electricity, or directly (and expensively) from a generator, air conditioning is
not practicable in RVs and cabins. Running from batteries needs at least 10-kWh (about 835-Ah at
12-volts) weighing about 400 kg, and a solar array that would cover the roof of a large coach.
Evaporative coolers use much the same energy as does a big fan but they lose effectiveness once
humidity exceeds 25% - 30%. Some vendors claim they work up to 40% humidity. But they sell
them rather than necessarily use them.
What voltage?
Twelve and twenty-four volt systems are cheap and simple. Their wiring is relatively easy (and legal)
to install yourself. There is negligible risk from electric shock. Some computers and printers can be
adapted to run from 12 or 24-volts. Most 12-volt power tool batteries can be charged directly.
The main drawback (for 12-volts particularly) is that surprisingly heavy cable has to be used for
long runs in order to reduce energy losses. There is a fair range of 12-volt domestic appliances available
in Australia, but very few 24-volt ones. The alternative is to use an inverter to supply 240-volts
for lighting and most appliances.
Mains voltage via an inverter
An inverter provides mains-like electricity. With the higher quality units, turning on any light or
appliance instantly turns on the inverter - and it more or less goes to sleep when you do.
Good inverters are costly. Many seemingly identical units cost much less but they only seem
identical. This is less of an issue with the larger ones (over 1000 watts) as these are made for a more
sophisticated market. More on inverters - pages 21-24.
Costs
Two main items substantially determine the cost of a solar system.
1. Any electric-only fridge more than doubles the cost - and that is without the strongly-recommended
back-up generator.
2. A 240-volt microwave oven may cost a mere $175 but the solar capacity and bigger batteries
needed to drive it can add $1500.
A good system for an RV or cabin that has a 220-litre (or larger) electric fridge and 800-watt
microwave will cost close to $7,500. With a gas/electric fridge and no microwave, the cost will be
around $3000. That difference is a high price for marginally colder beer in Darwin, and/or the ability
to heat up a Chinese take-away on a cold night in Christchurch! (Gas cost is likely to be only a little
higher than periodic battery replacements).
Shop around
Solar equipment prices vary. It pays to shop around - even to the extent of buying inter-state. The
same solar modules can cost $495-$775. The same regulators may be priced from $275-$395. Batteries
are more competitive. Prices still differ from vendor to vendor but their high transport costs can
wipe out what may otherwise seem bargain prices.