Problems with stand-alone solar – much applies also to RVs

by Collyn Rivers

Q. Solar on the roof – or free-standing on racks?

My roof faces north-east and slopes at about 45 degrees. It has a clear view of the sky. The land faces north but it is shaded in winter until 8-9 am. How should I locate the solar modules for optimum winter input?

A. This is one of the most common problems with stand-alone solar. Use the north-facing land. You will not have much solar input in winter from 8-9 am regardless of location. For optimum winter input (at a minor loss in summer) angle the modules at your latitude angle plus 15-20 degrees.

Q. Must solar modules be at an exact angle?

I’d like to have the solar modules of a 1.5 kW system directly on the roof of a north-facing shed. They will be about five degrees steeper than recommended. The proposed installer insists they must be at exactly the same angle as my latitude and is asking over $1000 to do so. Does this really matter?

A. No. Even 10 degrees will make only a few per cent difference. Five degrees, either way, make next to none. Suggest you talk to another installer – or insist this one does what you ask.

Q. Do I need a solar tracking system in Australia?

A. Not unless you are in Hobart – and even then it’s marginal. Tracking increases solar input in high latitudes, but is costly, complex and requires ongoing maintenance. It was marginally worthwhile until 2010. Solar capacity costs have since dropped by 75%-80%. It is now far cheaper to accept the loss and add solar capacity to compensate.

Q. Do stand-alone solar systems produce what installers claim?

Many of my (outback) neighbours have big stand-alone solar systems. All say they produce only 70% of that expected. What are they doing wrong?

A. That experienced by your neighbours is typical for stand-alone systems. In typical usage, no stand-alone (or grid-connect) solar system produces its promoted output. That seemingly claimed can only be achieved under controlled laboratory conditions. These conditions do not replicate typical usage. The lab data, however, is misleadingly used for the promotionally claimed output.

The most typical output is less than seemingly claimed. This is shown in solar module makers’ technical literature. It is often shown also on a data panel on the back of solar modules (as below). There, the most probable output, as shown in the third column, of a nominally 120-watt solar module is 87 watts. This output, (known to the industry as NOCT – Nominal Operating Cell Temperature) is that most likely in typical usage. The ‘explanation’ for this apparent deception is largely historical. It is long since the time it ceased.

The third column of this data panel (of a typical high quality nominally 120 watts solar module)shows the most probable output as 87 watts. Pic: Author.

There are losses in the cabling, regulator and inverter, and yet more in the process of battery charging. A typically promoted 1.5 kW stand-alone system is unlikely to produce under 1.2 kW most of the time.

Q. Do some solar modules (of the same area) produce more than others?

There are two main types of solar modules: monocrystalline and polycrystalline. Monocrystalline modules have silicon formed into bars and then cut into single crystal cells. That single-crystal structure allows electrons that generate a less impeded flow of electricity. As a result, monocrystalline panels are more efficient than their polycrystalline counterparts.

Polycrystalline solar panels

Polycrystalline solar panels generally have lower efficiencies than monocrystalline options, but their advantage is a lower price point. In addition, polycrystalline solar panels tend to have a blue hue instead of the black hue of monocrystalline panels.

Polycrystalline solar panels too are made from silicon. However, instead of using a single crystal of silicon, manufacturers melt many fragments of silicon together to form wafers for the panel. Polycrystalline solar panels are also referred to as “multi-crystalline,” or many-crystal silicon. Because there are many crystals in each cell, there is less freedom for electrons to move. As a result, polycrystalline solar panels have lower efficiency ratings than monocrystalline panels.

A. As of 2020, the most efficient commercially available solar panels are monochromatic. They cost more (per watt) than polychromatic panels but are 18%-20.5% efficient. They produce a bit more in low light.

Polychromatic solar panels are 13%-16% efficient, and thus larger per watt, but cost less per watt. The now rare amorphous solar panels are barely affected by heat, can be made in a thin flexible form, but are only 10-12% efficient.

 Solar technologies (from left to right): polychromatic, monochromatic and amorphous.

Q. Why do some people use the term solar module – and other people use solar panel. Which is correct?

A. The industry convention is that a single unit is a module. A number of interconnected modules on a single rack is a solar panel. Interconnected panels are an ‘array’. Correctly expressed, the photo below of our (previously owned) self-designed and built system below is an array of six panels each of six modules. 

Our previously owned property north of Broome (Western Australia). The solar system shed and main house were designed and built personally by myself and my wife. It and its design and construction are described in our article all solar house.

Q. Can a stand-alone solar system produce sudden peaks?

I read in one of your books that solar input can suddenly peak. What causes it? Can it damage the solar regulator?

A. It can happen briefly where nearby surfaces are reflective (e.g. near an expanse of water or light sand). Sunlight is received directly by the solar modules, but some is reflected upward from such surfaces. If there is some light white scattered cloud, that energy may then be reflected down again an received by the solar modules. There is usually little if any nett gain as the scattered cloud also blocks the sun from time to time. Solar modules self-limit (at their claimed output) anyway. Almost all solar regulators current limit in the event of excess – so there is no need for concern.

We experience it now (50 metres from Pittwater – above) and previously with our 3.8 kW stand-alone system north of Broome (WA). The Broome array is close to sand dunes, a tidal lagoon and the Indian ocean.

Q. I have no more solar power when it’s very sunny – why?

I self-built my own system following the excellent advice in your book Solar Success. It achieves what your book says almost all the time. There is, however, only a little more input when there’s sun all day. What’s wrong?

A. My advised approaches are conservative. They are scaled to allow for periods of low sun. When solar input is more than normal the batteries thus charge sooner. The solar regulator cuts back the charge accordingly. This protects your batteries from overcharging.

Where applicable, excess such energy can be used to pump bore water to a high-located tank, increase irrigation, or be used to do that long-put off welding etc. Some solar regulators have an auxiliary terminal that can be programmed for such use.

Q. Do solar arrays need regular cleaning?

I’m planning to have a big (10 kW) solar array in an outback area. Do solar modules need regular cleaning? If not, will they lose much power? It only rains here during a month or two each year.

A. Dry dust is usually blown off by wind gusts. Heavy rain cleans them adequately but minor rain (if the modules are dusty) tends to turn that dust into mud (that may then dry on them). High humidity can do the same. Apart from that, there is usually no need or cleaning (as long as the modules slope sufficiently to stop animals using them as toilets!). The loss through dust alone rarely exceeds 5%.

Q. Charging voltage too high?

My solar regulator allows my deep-cycle batteries to go to almost 14.7 volts before dropping back to 14.2 volts. I’ve heard that 14.7 volts is way too high.

A. For those batteries, that brief peak voltage is only too high if they exceed 40º C. Most solar regulators and chargers are programmable for various types of batteries. Some have a (usually optional) temperature battery probe that adjusts charging voltage to suit the ambient temperature. Gel cell and AGM batteries charge at a lower voltage. Be guided by the battery maker.

Q. Do MPPT regulators increase input by ‘up to 30%’ as many vendor’s claim?

A. ‘Up to’ statements have next to no meaning. A Multiple Power Point Tracking (MPPT) regulator does not ‘increase input’ as such. It assists recover energy otherwise lost. That ‘30% more’ is likely to be for an hour or so very early in the morning and late afternoon. It is ‘up to 30%’ of very little. They do work but do not expect to recover more than 10%-15% a day. Be aware that many cheap ones marketed as MPPT (typically on eBay) have no MPPT function. Buy only well-known brands.

Q.  Solar brings my deep cycle batteries to 14.5 volts during the day but they drop to 12.8 volts shortly after the sun goes down. They also drop to only 12 volts when the microwave oven is in use.

A. Your battery bank is just fine! That drop from 14.5 volts to 12.8 volts is normal. The battery’s voltage begins to fall when charging ceases. It typically falls to 12.75-12.80 volts offload. The drop to 12 volts with the microwave oven is likewise normal.

Instantaneous voltage under load gives little indication of a battery’s state of charge. That charge (in effect) is held in the electrolyte (the water/acid mix). All you ‘measure’ when the battery is charging, or supplying a heavy load, is the voltage on the plates’ surface at that moment. The internal reactions are so slow it may take days for a totally off-load voltage measurement to reflect the true state of charge.

Because of this a close-to fully charged battery can seem almost flat for some time after a heavy load (like a microwave oven). Many perfectly good batteries are replaced by not understanding this.

Further information

Designing, specifying and installing a stand-alone solar system is readily possible for anyone handy with tools and ideally with a basic knowledge of electrics. My book Solar That Really Works! tells all you need to know (and more) for stand-alone solar in boats, cabins, camper trailers, caravans and motorhomes. My associated book, Solar Success does likewise for homes and property systems.

For full information relating to RV electrics and solar, my book Caravan & Motorhome Electrics, covers the topic in depth. My other books are the all-new Caravan & Motorhome Book, and the Camper Trailer Book. For information about the author please Click on Bio.