Sailing and boating people tend to be good practical DIY types, so the job of fitting a solar panel to the deck and running a bit of wiring here and there will not be too daunting an exercise. However, what does get the grey matter stirring is the OHMS Law and getting the specification right. So here is where I will try to lay the method out simply for all to see and hopefully with a bit of help from your crew you will be able to have a good solar system up and running in no time.

One thing worth saying is that the technology has moved on a pace with plenty of protection for your battery(s) in place, plus it is now very affordable and I am sure will get even cheaper as time goes by. It is also worth saying that the best solution for some boats is a mixture of sun and wind power, but for now we are concentrating on the solar solution.

First of all we need to know what the boats energy needs are when the boat is at anchor or moored with no shore power. It is of course perfectly feasable to have a system to make shore power a thing of the past by adding an inverter into the system for your 240 volts AC needs.

So here is the working out - we will need to detail the boats equipment and the current ratings, NO don't start running away just yet, generally this will be detailed on the product label or in the user guide and will be stated in Amps, if not then read of the watts and divide by 12 for a twelve volt system (Watts ÷ 12 volts = Amps).

If the information we need is not on the product label then give the manufacturer a ring and ask for it, they will be only to pleased to discuss this with you. So lets start with an example of the energy needs of a typical vessel (although appliance Current/Amps figures displayed below are entirely fictitious):-

Appliance	Current/Amps	Hours On	Amp Hours Used
Wall lights x 4	4	3	12
Fridge (12 v)	5	9	45
Radio/Stereo	1	3	3
Nav lights	2	7	14
All pumps	4	1	4
			______________
			Total 78 Ah

Reminder - Don't forget when the vessels engine is running your fridge and equipment are powered by the engine alternator not the battery(s).

Solar Panel manufacturers are starting to state their solar panels in 'Watt Hours Per Day' written as Wh/d, you can divide this figure by the panel nominal voltage of around 16 - 19 volts to get the very approximate Amp figure for the day which is the most useful. Knowing how many Amp hours (Amph) you can expect from a solar panel is what we want to know! These manufacturers figures are normally based upon a mid summers day as regards the number of sunlight hours the panel received. Getting all the figures into Amps makes the most sense as batteries are at the heart of your system and are all rated and stated in Amp hours as is wiring and fuses, plus, this is the bank where all your free power is deposited!


Tried and tested marine solar panels in a variety of sizes - these panels are very thin and can be walked on.

The fictitious example figure of 78 Ah in the table above is a very high figure for the average vessel and you would be best trying to reduce it if at all possible. Try fitting led lighting fittings where possible or by limiting the number of hours appliances are turned on for. This can all help with reducing the Amp hours and will help with the cost and size of the solar panel(s) you will need to put back the 78 Ah into the battery bank each day.

Lets look now at the solar panel size required for the summer period to put back the 78 Ah into the battery bank, there are a number of scenarios here to be considered:-

First of all let me say - that I don't like running my batteries anywhere near to the bottom of their operational range, so this will be reflected in my working out below, and depending where your boat is moored in the UK the amount of sunshine hours will differ a lot so you will have to make allowances for that. Remember: what would be sufficient solar panels in the summer will not be sufficient in the winter!

Scenario 1
Obviously if the engine is started and you are motoring for a number of hours each day then the batteries will be charged and may well fully charge your battery bank in that time. So in this scenario you will be best to fit a smallish solar panel as a topping up exercise, normally a 20 - 30 watt panel would be sufficient. Of course the added bonus will be that during your time away from your vessel your batteries will be kept topped up ready for your next trip.

Scenario 2
On the other hand you may be moving off every 2 - 3 days and need to keep the batteries topped up while you are at anchor or berthed without shore power. In this scenario you will have already stored in your battery bank around 30% of your leisure battery usage if you are fully charged, lets say for example you have a battery bank of 200 Ah then you will have at least 60 Ah of very safe use before the next charge from starting the engine. So if we divide the 60 Ah over the three days stoppage reducing the 78Ah daily power requirement by 20 Ah then we need to only replace 58 Ah per day by solar power before the next charge. In this instance a solar panel(s) with an output of 150 - 175 watts would be a good choice. The solar panels could be made up in a combination of different ways, you could have some flexible panels mounted on the deck and have a portable panel or two which you could plug in when your vessel is at rest. This is a good option for smaller boats with little option for mounting permanent solar panels. It is also worth mentioning here that flexible panels do not have as good a power output as the rigid panels, so one or two portable rigid panels are worth having.

Scenario 3
In this scenario you would be staying for indefinite periods of time on board and need to cover your energy requirement completely. To get an output of 78 Ah from a solar panel you would need panels to the value of 250 watts baring in mind this will be fine for the summer period when the light is good. For a mid winter period you would need to double this panel wattage output.

If panel mounting space is at a premium then you can consider using wind power as part of the supply. These wind power generators can be noisy with a little vibration and of course you have to keep away from the blades when spinning which can be a nuisance.

Whatever scenario fits your particular situation you will need to fit a regulator to stop the power from reversing back into the panel overnight and to stop overcharging the battery bank when fully charged. These regulators nowadays are very inexpensive and come either in a very basic model or with lots of facilities for monitoring the whole battery management side of things. They can be purchased for as little as £10 and go up to the 70's or more.


Solar panel regulator - stops the power from reversing back into the panel overnight and stops overcharging of the battery bank. This model costs around the £20 mark.

Putting an ammeter and voltage meter in the system means you can keep an eye on the charge and discharge rates and then make any adjustments to your system that are needed. Perhaps you might need an extra solar panel or an additional/larger leisure battery. Nothing can beat having the system up and running to see what you are actually achieving on an average day. See the live experiment page

Make sure you read the solar wiring page before starting an installation.


Other pages to help you with the installation are:-

	How to wire up a basic solar panel system Is your battery bank correctly wired up?
	All About Inverters in a solar system
	The Big Live Solar Panel Experiment See how our 27w solar panel performs under live conditions
	Battery care
	Looking after a leisure battery
	Solar circuits and solar power circuitry
	Choosing and using your wire in your solar system

To communicate with us over technical issues please use the Solar Chat Forum, also take a look at the Solar Q&A page.


Note: Solar power - Is not an exact science working out solar panel sizes (array) is very much based firstly on a calculation as in the table and scenarios above, but there are other factors to consider. Wiring runs, air temperature, and of course the intensity of sunlight which none of us can forecast, these all affect the performance of our solar power system overall. Therefore solar panel sizes used in our examples above are a guide only. Generally it is prudent to go a bit bigger on the solar panel sizes as a good solar regulator holds back any excess.

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Global Warming - it's why we are making changes to our way of life!

David Bellamy has a very interesting article on global warming where he gives another point of view, Read the article