It’s been over a month since our “big yellow umbilical chord” has been out of it’s locker and used to attach us to shore power. Instead, we now rely on our new solar panels to keep our house batteries topped up. The difference it’s made to our cruising life has been substantial: no more searching for a working power socket when we arrive in a harbour, and no more running the engine (or generator) in order to enjoy a cool beer on a sunny afternoon.
Quite apart from the convenience of it, we’re now saving money by not paying to plug in to shore power, or for diesel to run the generator.
This is how our panels look on a particularly good day at anchor:
Although there’s nothing new about using solar power on a boat, the use of flexible panels on a bimini seems to be less well documented, so I thought I’d write about the choices we made and the approach we took in case it is of use to others.
Assessing our power requirements
Typically, the advice for estimating your power requirements involves creating a spreadsheet which lists all devices on board, their power consumption, and the time they are used for in a 24hr period. From this you can determine a theoretical number of watts (or amp hours) that you will need to generate to meet their needs.
To me, this advice sounds flawed. Can you reliably tell me how many minutes your 7 amp fridge runs in a 24hr period for example? Does your autopilot consume the same power when sailing in rough rather than calm seas? Clearly not.
Instead of adopting the suggested approach, I simply spoke with owners of similar boats which already had solar panels and determined how satisfied they were with them. The general line was “more is better”, but once over 200W most of the people I spoke with felt that they wouldn’t want to be without it. Given these numbers, we measured the space available and decided to use as much of it as possible. This resulted in us buying 4 x 100W panels (400W in total).
Siting the panels
We don’t have an arch at the back of our boat, and didn’t want to incur the cost (both financially and aesthetically) of having one built and fitted. As we always sail with our bimini up, a bimini mounted solution seemed like a more sensible option for us. A downside of mounting the panels on the bimini rather than an arch would be the increased shading from the ship’s rigging.
Mitigating power loss from shading
Solar panels don’t like shade. Shading a single cell can cause the current running through the whole panel (and hence that panel’s power output) to drop significantly. Furthermore, this current ‘bottleneck’ will limit the current running through any other panels connected in series, affecting the output of the entire array.
To avoid the losses described above, the panels can be wired in parallel. The downside of this approach is that the current increases as more panels are added resulting in power lost as heat, and subsequently more bulky cables are required.
There are many resources on the net explaining the advantages between wiring panels in series or parallel, and to save regurgitating it all, I’ve summarised the key pros and cons:
- Series, output current remains low and voltage increases as more panels are added:
- CON: Shading a single panel reduces power produced by all panels.
- PRO: As current remains low, smaller cable diameters can be used.
- PRO: Modern MPPT controllers can convert the high voltage back to a usable charge voltage and current.
- Parallel, voltage remains the same, but current increases as more panels are added:
- PRO: Shading a single panel will not affect the output of other panels in the array.
- CON: As currents are high, thicker cables are required to carry same power.
Rather than opting for either of the above schemes when wiring our panels, I came up with a hybrid design that aimed to strike a balance between the efficiency of a series installation, without incurring big losses when the panels would invariably suffer from shading.
My reasoning was that any significant shade from the boom or the mast would fall on either the port or starboard side of the boat, but not both sides at the same time, and in such cases it was important to retain the maximum available output from the unshaded panels.
In the diagram you can see that the port and starboard side pairs of panels are wired in series, and that these each have their own MPPT charge regulator. The regulators (aka controllers) are wired separately (i.e. in parallel) to the house batteries. Note: if you are planing a similar installation, that you must find controllers that can work together in parallel.
This design seems to work well and I’m pleased with the results. When one side of the boat is in shade, the opposite side generates the bulk of the power. Then, as the sun moves, and the shadows go from one side to the other, the previously shaded array becomes more active.
Attaching the panels to the bimini
We used the services of Massimo, the local ‘canvas guy’, to attach the panels to the bimini and he did a first class job. The long sides of the panels are zipped and Velcro’d in place and the short sides are attached with Velcro only. The highest wind we’ve been in so far was just over 30kts and these fixings worked perfectly.
He also stitched cable bags between the panels that we can coil the cables into to keep things tidy.
You can see from the above shot that the boom is fairly close to the bimini. This means that it’s shadow is only cast over the panels when the sun is fairly low. For most of the day, we charge well from both the port and starboard arrays.
What did it cost?
Our choice of equipment has been influenced by us not wanting to install an arch. Flexible panels are more expensive than rigid ones but the gap is closing. If money’s no object and you elect to purchase a well marketed flexible ‘marine’ panel (like Solbian for example) then be prepared to shell out about £700 for a 100W panel. I sourced our panels from a reputable UK dealer for a little over £200 each. The controllers were about £150 each. With shipping to Italy from the UK, the total cost of materials was around £1200. The canvas guy charged us about €400 for the modifications to the bimini.
If you have any questions or relevant experiences then please get in touch in the comments section below.