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
Reminder - Don't forget when
the vessels engine is running your fridge and equipment are
powered by the engine alternator not the battery(s).
|Wall lights x 4
|Fridge (12 v)
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
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!
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.
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.
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
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
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
the live experiment page
Make sure you read the solar
wiring page before starting an installation.
a panel on your boat
Other pages to help you
with the installation are:-
To communicate with us over
technical issues please use the Solar
Chat Forum, also take a look at the Solar
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.
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.
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