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This is about solar photovoltaic (PV) panels, ie the sort that use sunlight to give you free electricity. Free that is once you’ve bought them, they are expensive and not as efficient as you might like them to be. Expect to pay between £4 to £5 per Watt (more in the smaller sizes). Panels come in three flavours monocrystalline, polycrystalline and amorphous. Of these the amorphous is the less common needing far more space for the equivalent wattage they are cheaper though and very flexible so can be stuck directly to the curving roof of a motorhome or boat. They also produce a good output in dull conditions.
If you see a panel marked as 100 Watts what does it actually mean? Well it means that if irradiated with sunlight at or above 1,000 Watts per square meter you will get more or less 100 Watts of power.
So this 1,000 Watts per square meter how much sunlight is that? Its about as bright as it gets at the earths surface. You don’t really have to worry about that though. What the Met Office does is take actual measurements averaged out over ten years or so and quote a number of sun hours per day. Each sun hour is equivalent to an hour at 1,000 Watts per square meter. So for argument’s sake if it was sunny for 8 hours but only at 500W per square meter they would say 4 sun hours. I can only quote for UK but for example on the South coast in June and July if your panel is lying horizontally you get 5 sun hours per day on average.
Just to put some figures to that then if you had a 100W panel then it would produce 500 Watt hours in a day in June on the Isle of Wight. Dividing by 12 Volts gives 42 Ampere hours of charge That sounds quite good doesn’t it? Except that well its not really 12V is it? Your battery on charge needs abut 14V so divide the 500 by 14 instead and you get 36 Ampere hours. The next recalculation has to take account of what voltage the panel is working at when it delivers its maximum 100W, unfortunately its not 14V but about 17V. How much less than maximum power it is at 14V depends on design.
There is another way to extract more power, this is by using a regulator. When solar power was first experimented with it was thought that you didn’t need a regulator at all (and that is still true if you have a big battery and a small panel). This is because although the panel 'wants' to deliver its power at 17V the battery whilst it is charging will hold the voltage down, unfortunately as the battery charges this effect becomes less and the panel voltage rises to a point where the battery will gas profusely leading to loss of electrolyte. The 'surface effect' in the battery chemistry plays its part too. It takes time for the charge to move from the surface of the battery plates. So the battery voltage on charge does not accurately reflect the amount of charge the battery has received. Then regulators were produced that avoided damaging batteries but wasted energy by just disconnecting the panel as the voltage went too high. The problem with disconnecting the panel at this point is that although you avoid producing a lot of gas the battery is not fully charged. Pulse Width Modulated (PWM) regulators deliver more power to the battery at the top of the charge (say above 80% charged) by effectively matching the incoming power to the correct charging voltage, ie just keeping the battery at the gas producing threshold. Maximum Power Point Tracking (MPPT) regulators go a stage further by allowing the panel to always work at its maximum power point whilst giving three or four stage charging to the battery. Both PWM and MPPT regulators have temperature compensation, as the final charging voltage at 25°C might be 14.4V whereas at 0°C its over 15V. A good PWM regulator might cost £70 a MPPT regulator twice as much.
I hope that has lowered your expectation of what a panel can produce. We can do better than that though. For a start 5 sun hours was with the panel lying horizontally stuck to your motorhome roof. I happen to know that the sun hour figure rises if you arrange to point it at the sun as the sun tracks across the sky. This effect is marginal in summer but much better in the other three seasons. However automatic tracking systems are more expensive still perhaps £20 per Watt and beware of results that show a 5 times improvement that is only valid at the extremes of the day, likewise percentage improvements quoted for February are only good in February. That said the further North you travel and the more you venture out in late Autumn, Winter or early Spring the more benefit they will bring. They nearly always include MPPT regulators. It is up to you and the space you have whether you go for the efficiency of a tracking system or just add more panels.
There is one more component of your solar system, that is the blocking diode. This diode, which is often included within the regulator, prevents your battery discharging at night back through the solar panel. It is in series with the cable from the panel and acts to only allow electricity to flow one way, from panel to battery. There can be some confusion with the bypass diode which is often fitted into the terminal box of the panel for a completely different purpose. The specification of the panel and regulator will need to be consulted if you are unsure if a blocking diode has been supplied. Note some pundits say that the blocking diode is unnecessary* and some regulators achieve the requirements of the blocking diode without actually using a diode. (* argument is beyond the scope of this FAQ)
So there you have it, free energy ... at a price. Even if they are not able to replace all the power you used last night they will extend your stay, postponing the moment when you have to hook-up, connect a generator or drive. They are also very very very quiet.
There is a MHF detailed guide to solar systems here >click here for part 1< Note that it does not yet cover some more modern aspects such as MPPT regulators.
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