Solar That Really Works! – sample chapter
On clear days around noon, up to 1000 watts of solar energy (enough to boil a kettle in about five minutes), is theoretically available on each square metre of much of the Earth’s surface. Commercially available solar modules (in 2020) convert only 20% or so of that energy into electricity. By using appropriate and efficient appliances, however, such solar can free recreational vehicles and cabins substantially or totally from mains, alternator or generator power.
Whether an RV, a cabin or even a big property system, the differences are mostly of scale. As apparently similar lights and appliances may use hugely different amounts to achieve much the results, a good starting point is to know approximately how much energy different things require.
If installed properly (and Chapter 21 shows how) today’s up-market RV fridges draw only a third or so of the energy of many an eBay special. Microwave ovens draw far more current (i.e. electrical energy) than many people suspect. Done properly, pumping water needs little energy, done badly it needs a lot.
Where can solar energy be used?
It is light, not heat, that solar modules convert to electrical energy. The amount of energy they produce depends on how much light falls on them and for how long. They lose output when hot, so work best in cool places under a bright sun. All solar modules need at least some sunlight to operate. None work in total shade.
How is available solar measured?
A solar module’s output is measured in so-called ‘Peak Sun-Hours’ (PSH). The PSH concept (conceived by the solar industry) is like using a rain gauge to measure a day’s ‘downfall. It is in general solar industry use but is not recognised or generally used academically.
Figure 1.1. Our previously owned OKA in Kakadu National Park (1998). Two 80 watt Solarex modules provided ample power for all needs - including a 71-litre Autofridge. During our ten-year ownership, the batteries did not once run out of power. The spade marks where the damper was cooking. Pic: rvbooks.com.au.
The solar industry defines 1 PSH as an intensity of sunlight equivalent to that ‘falling’ on 1000 watts per horizontal square metre. That intensity varies with the sun’s position in the sky and atmospheric conditions (such as haze and clouds). The peak input is usually at noon.
For the purposes of this book, each PSH can be seen is like a ‘standardised drum full of sunlight’. That drum may ‘fill’ in only an hour or so in Australia’s Cairns or Broome during most of the year, but may take all day during a Melbourne mid-winter. Each full drum can thus be seen as holding of 1 PSH.
Figure 2.1. Peak Sun-Hours (mid-January). Multiplying the data shown by a solar module’s true wattage gives the total average output for that day in watt hours/day. This map, plus that for mid-July, is reproduced at larger scale as Figure 1.13 in Chapter 13.
This book shows output in PSH. Multiplying the solar modules’ true energy output (typically 70% of that seemingly claimed) by the PSH shown is the amount of the energy you can, on average, expect each day. This energy is measured (as we do at home) in watts.
The average amount of sunlight (irradiation) varies more or less linearly from mid-summer to mid.winter. Figure 2.1 shows PSH for a typical Australian January (mid-summer). As can be seen, in many places and times there will be at least 3 PSH each day; in some there will be 7 PSH or more. Full-size versions of these maps for Australian summer and winter are reproduced in Chapter 13 - as Figure 1.13. New Zealand’s North Island, and the eastern part of the country’s South Island have a fairly uniform 4.2-6.5 PSH between September and June, and 2-4 PSH in between.
Meteorological offices worldwide have solar irradiation maps for almost anywhere, but may need you to juggle scientific units.
Solar anomalies and limitations
Peak Sun-Hour maps allow for average seasonal cloud cover, but there are day to day variations. Output is usually high on sunny days that have light haze. It may increase yet further if sunlight is reflected from water or light sand and back to the haze layer, from where it is reflected down again.
Input typically halves during heavy cloud. Bush fire smoke may reduce it by two-thirds but it is rare to have no solar input.
Shortfalls resulting from long periods of cloud cover and night-time usage are covered by drawing on energy stored in battery banks. For most RVs the amount of energy storable is limited (due to battery size and weight), so larger systems, and particularly those with electric-only fridges, are likely to need generator or fuel-cell back-up (these issues are in Chapter 7).
The solar industry has an ‘unusual’ way of quoting this output that can cause buyers to expect 25%-30% more than they thought they had paid for. This particularly catches out those who understand electrics and/or physics - but are unaware of this ‘marketing anomaly’. Chapter 3 explains all, but in essence you need to design the system assuming no more than 70% of the promoted solar module output. If ample space permits, 50% is safer.
Subject to the above, solar can be used successfully with RVs in most temperate areas between about 30 degrees north and south. By and large whatever works well in all parts of Australia (except Melbourne in mid-winter) is valid in most places where you are likely to use an RV. Differences in its scale and implementation, however, depend on the various needs, and (for most RVs) space and weight carrying ability. Solar capacity, however, is now so cheap that cost is rarely an issue.
Space and weight limitations
The load carrying capacity of a vehicle’s axles, wheels and tyres is legislated - and also directly related to cost. Most caravan builders provide a so-called ‘personal allowance’ that rarely exceeds 250 kg (550 lb) and 350 kg (770 lb) respectively for single and twin-axle units. Included in that personal allowance’ are gas, water, food and personal possessions: in essence everything placed in that caravan after it leaves the factory. ‘Optional extras’ (even if specified in the original contract) are usually installed by the dealer and thus likely to further reduce available allowance.
Campervans and motorhomes makers’ major buyers are RV rental companies that seek to provide the maximum possible living space in vehicles still light enough to be driven by holders of a car licence. This has resulted in a caravan-like situation: ample loading space may be available but weight restrictions limit its use. This is less of an issue with larger specialist-built motorhomes: their load-carrying capacity is negotiable, and their length allows more space for solar modules.
Until recently, weight issues restricted battery capacity, but the much lighter lithium batteries now available substantially ease this. This is covered in Chapter 5.
Most fifth-wheel caravans (i.e. those that have their tow hitch above the tow vehicle’s rear axle/s) have greater payload capacity (Figure 3.1). It is also usually feasible to house some part of the battery bank behind the cab of the towing vehicle, or in that vehicle’s under-floor lockers.
Converter electrical systems
Almost all US made and now many locally-made RVs have 12 volt systems of which the battery back-up is intended only for occasional single overnight use away from 230 volt power. Unless extensively modified, these systems are close to useless for extended overnight camping. Chapter 25 addresses this.
Cabins have fewer restrictions. There is usually ample space for solar modules and batteries. Theft was initially an issue but far less so since solar module cost dropped dramatically (post 2010).
Figure 3.1. This 11.3 metre fifth-wheeler built by Glenn Portch is exceptional in weighing only 3200 kg (7050 lb). It has a payload of an extraordinary 1300 kg! Pic: Glenn Portch.
For cabins used irregularly, sealed lead-acid deep-cycle can safely be left permanently on charge as long as the necessarily high-quality solar regulator is programmed for the specific battery type.
The once popular 12 volt gel cell batteries are still made. Their main plus is that they are claimed to safely discharged to 10.8 volts, but cannot deliver high current.
Providing they are fully charged beforehand, AGM batteries may be left for 12 months or so before dropping below about a (non-damaging) 60% remaining charge at ambient temperatures below 25℃. It is generally best to avoid ‘trickle charging’ as the amount lost is so tiny that any form of long-term charging is liable to damage them (whilst otherwise rugged AGMs will not withstand overcharging).
Lithium iron batteries withstand even years of non-use without apparent damage, but the industry nevertheless suggests to initially leave them at about half charge for long term non-use.
Until recently, solar capacity cost far more than battery capacity. Owners accordingly skimped on solar capacity: resulting in inadequately charged batteries that had their life limited accordingly. Furthermore, if the battery bank is overly-large relative to the charging source, that source may not be able to recharge it fully, let alone quickly. Adding more batteries alone is thus like opening more bank accounts for the same money deposited. It retains the same input as before -but increases your overhead losses.
Economise on batteries but never on solar modules. As a rough guide (for RV use) you need at least 200 watts of solar for every 100 amp hours of a 12 volt battery. Ideally have as much as the available space allows. There is no risk of overcharging as the associated solar regulator prevents this.
See also Chapter 29 re ‘split systems’ - where both tow vehicle and trailer each have their own self-contained (but interconnectable) systems. This is really worthwhile considering.
If you (improbably) carry energy-hungry arc welders and/or big angle grinders used only occasionally, scale the system for ‘normal’ loads and supply the rarely-used excess by a generator. This also applies if planning to spend only an odd winter month in places with short hours of sunlight (despite lower fridge energy usage in winter).
Cooking and heating
As roof space for solar modules is limited, solar generated electricity (alone) is not really practicable in RVs shorter than about 7 metres for anything that, as its main purpose, generates heat. Electric ovens, fryers, and water heaters are thus best avoided. Hair dryers are borderline. Electric irons are best used only where there is 230 volt mains power. For all but the largest RVs, use LP gas for cooking and heating water.
For cabins and the rare RVs that have ample space for solar modules and battery storage, it is feasible to use solar energy for cook tops (but less so for ovens). Use LP gas/solar water heaters for water heating generally.
Coffee grinders, blenders and other small appliances vary in efficiency but, if used only occasionally, their energy use is rarely of concern. All microwave ovens, however, use more energy than many users suspect. Their wattage rating refers to the work they do (i.e. ‘cooking power’) not the energy used when doing so. Most ‘800 watt’ rated microwave ovens consume about 1350 watts, or 1500 watts via an inverter. Ten minutes use may draw a day’s output from a 100 watt solar module. That oven may thus cost only $195 or so, but running it from solar can add many times that for the extra solar capacity and battery capacity needed to drive it. It can still only be used when there’s enough power. Excepting for big rigs with ample solar capacity, or a generator, consider running a microwave oven only when you have 230 volt mains access.
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 (Chapter 11). Mains-voltage pumps are available but they use far more energy for pumping the same amount of water.
Where there is a washing machine or dishwasher, and also in large cabins with flush toilets, a ‘pressure accumulator’ (Chapter 11) overcomes the otherwise high energy draw of pumping water. It also results in a system that does not fluctuate in pressure, is silent most of the time and saves electrical energy.
Most front-loading washing machines use less energy and water than top-loaders. The more efficient units run readily from a medium-sized RV solar system and inverter. They wash well using only cold water as long as cold water washing powder is used. These machines are fine also for cabins. Many current models draw only 200 watts or so when run from cold water.
Dishwashers need a hot water supply. It is not feasible to supply this (for RV use) via solar electricity, but a number of owners build their own thermal solar water heaters from coiled copper tubing or black polypipe.
If doing so, to avoid scalding (especially of children) it is essential to include a ‘tempering valve’ (from plumbing suppliers) to ensure the water does not exceed 50℃. In some jurisdictions that valve is legally required.
Figure 4.1. This Sony Bravia 32 inch TV draws 55 watts. Pic: Sony
Unless left on day-long, there are no major energy problems with recently-made TVs with 36 inch (92 cm) LED screens. Most draw 50-60 watts. Older ones may draw over 150 watts. Avoid any 12 volt TV made for sale in underdeveloped countries. They are ultra-cheap to buy but have energy-gobbling technology.
The larger laptop computers double as TVs, but as the screens are responsible for most of the draw their is no energy advantage. LED screens draw the least. See also Chapter 12.
Allow for the energy draw of charging iPads and also that drawn by communication modems. Be wary of specialised game-playing computers, they use far more energy, and tend to be used for longer.
Incandescent (230 volt) globes are no longer legally sold in many countries. Halogen globes use about half the energy for the same amount of light but were an interim technology now mostly replaced by light emitting diodes (LEDs) - Chapter 10. Halogen globes were banned from sale in Australia in 2020.
Fluorescent globes and tubes and compact fluorescents use only a quarter of the energy of incandescent globes but the latest white and warm white LEDs use even less. Chapter 10 refers.
Given at least 750 watts of solar modules for this alone, solar-powered air conditioning is feasible for daytime use, but unless backed up by mains electricity, or from a generator, having air conditioning all night is not practicable in any but the very largest RVs. Solar modules and air conditioners are, however becoming increasingly efficient. Later editions of this book may have a different view of feasibility.
Evaporative coolers use much the same energy as big cooling fans but lose effectiveness above 25% humidity. Their vendors often claim they work in up to 40% humidity. But vendors sell them - not necessarily use them.
Twelve and (the now rare) twenty-four volt systems are cheap and simple. Their wiring is relatively easy and for RVs and cabins in Australia and New Zealand) still legal to self-install. There is negligible risk from electric shock.
One drawback (for 12 volts) is that surprisingly heavy cable has to be used to reduce energy losses (Chapter 16). There is a wide range of 12 volt appliances, but (apart from fridges), very few for 24 volts.
Some coaches and a few motorhomes have 24 volt alternators and batteries. To run 12 volt lights and appliances, there are two ways of doing so.
Figure 5.1. Roof-mounted air conditioner. Pic: original source unknown.
Companies such as Redarc and GSL offer 24-12 volt charge equalising units. These draw the required 12 volts from one of the two series-connected 12 volt batteries used in most 24 volt systems, whilst constantly equalising the voltage across both batteries. A more efficient approach for lighter loads is to use a 24-12 volt dc-dc converter.
Mains-voltage via an inverter
An inverter provides mains-like electricity. Many seemingly identical units cost far less but may only be able to supply their rated maximum output for a second or two. They only seem identical. This is less of an issue with those over 1000 watts or so because they are made for a more electrically sophisticated market.
It is legal to self-install an inverter (made for this purpose) in an RV but only those into which appliances plug in directly (or via multi-outlet power board).
Those intended for connecting into a cabin or RVs fixed 230 volt wiring are of a different kind. They have no external socket outlets. They require specialised installation that (in Australia and New Zealand) must only be done by a licensed electrician. Chapter 17 explains more.
The cost of an RV or cabin’s solar system is to some extent dictated by the fridge’s system size and cost. With large ones, to safeguard against large-scale spoilage, it is advisable to have a back-up generator.
A microwave oven may cost only $195 but the solar capacity and battery capacity to drive it may add $1000. By all means have one, but (for use with small solar systems) run it only from a generator, or when you have access to mains power.
Solar is increasingly becoming cheaper, but battery capacity is not. Having adequate solar capacity alters the role of the battery. Sufficient battery capacity is still needed overnight and for dull days but, given sufficient solar, battery capacity can be reduced because solar modules charge to some extent even on overcast days.
Solar module price plummeted after 2010. Battery prices soared, but still differ considerably from vendor to vendor. It pays to compare prices but, because most batteries are so heavy, transport costs can wipe out otherwise seemingly bargain prices.
Small scale fuel-cell technology slowly continues. The initially promising Truma’s VeGA LP gas.fuelled product (at 12,000 Euros) sadly proved far too costly. It was withdrawn from sale in 2014. The EFOY methanol-fuelled product is still on sale and rival LP gas-fuelled fuel cells are just (late 2019) appearing on the market. See Chapter 7.
Avoid cheap products
In the RV area particularly, unless you really know what you are doing, it is better to spend more and buy high-quality products from well-established companies rather than seeking bargain-priced products of unknown provenance and often negligible technical support. There are the odd bargains on eBay, but much is close to junk.