Sizing my solar panel

[FairweatherDave]

Very likely I will install solar as I am over-wintering on a swinging mooring. I have done some reading around but still not got "the answer" as to the size. As far as I understand it if all you are doing is keeping your batteries topped up with a small panel then your recharge time is significant if you have a good weekend away on the boat. The impossible thing to factor in is how much you might use the engine, which might in fact leave the batteries better off than when you arrived. We don't into marinas if we can help it

So I have two batteries (110 Ah) with a 1 2 both switch ....
I have a tiller pilot, led lights, an led dusk to dawn anchor light, a tablet for Nav and a handheld garmin GPS, a VHF and seperate AIS receiver, and depth and wind instruments. If I did night sailing it would be under motor. My normal day sails are 3 to 4 hours up to 7 absolute max. My fag packet calculations are based on a six hour sail and I reckon about 25-30 Amp hours over 24 hours allowing for a night at anchor. But I'd be lucky to get 15 sails over the summer of that duration, half the time we are inside Chichester harbour with minimal electricity demands. And we use the motor and this summer without any shore charging the batteries are still fully charged.

So what size panel, 20W, 50W, 100W or more? I can squeeze a 100W panel in front of the spray hood. Would appreciate some opinions as you can do all the calculations based on equipment and a rigid 6 hour sail but guestimating actual usage seems more random. Think of our use as Solent pootling rather than routine cross channel raids...........

 

[lw395]

More is better, but it's diminishing returns.
The benefit of the first 20W panel is massive on a basic yacht, the additional gain of each 20w is less.

Before solar was cheap, I used to alternately take the batteries home in winter for a proper charge. Might still do that if we had a weekend using lots of battery and very little diesel. It may be easier to do that a couple of times a year than to fit more solar that's only going to earn its keep twice a year.
Lots of solar really comes into its own when you have a fridge or use the boat for long periods often.

 

[CPD]

Do the sums as you have then compare the guestimate to the largest you can reasonably fit. Hopefully the guestimate will be less than what you can fit. Then fit the largest you can. The "spare" availability will be consumed by gadgets). That is exactly what I did and I have no regrets .

 

[alahol2]

Your setup and sailing sound very similar to ours. Mostly 3 day weekends in or around the Solent. The odd week away down to Poole when the weather looks set fair.
We have a 30W panel ahead of the sprayhood, boat on a mooring. That 30W is enough to ensure the batteries are fully topped up each time we arrive at the boat. The battery capacity, minimal engine use and panel means that we never got below 50% on the batteries this year.
I'd say a minimum 30W panel but any more would give extra resilience.

 

[William_H]

Solar panels are relatively cheap however real estate on the boat to conveniently site a panel is the real problem. You say you can fit 100w in front of the spray hood so that might be your answer given that being horizontal and on a swing mooring total power will be a lot less than ideal. On the other hand a smaller panel will be less susceptible to damage but perhaps that is not a concern. It will never create an electrical problem with too much solar power if you have a regulator. ol'will

 

[VicS]

Very likely I will install solar as I am over-wintering on a swinging mooring. I have done some reading around but still not got "the answer" as to the size. As far as I understand it if all you are doing is keeping your batteries topped up with a small panel then your recharge time is significant if you have a good weekend away on the boat. The impossible thing to factor in is how much you might use the engine, which might in fact leave the batteries better off than when you arrived. We don't into marinas if we can help it

So I have two batteries (110 Ah) with a 1 2 both switch ....
I have a tiller pilot, led lights, an led dusk to dawn anchor light, a tablet for Nav and a handheld garmin GPS, a VHF and seperate AIS receiver, and depth and wind instruments. If I did night sailing it would be under motor. My normal day sails are 3 to 4 hours up to 7 absolute max. My fag packet calculations are based on a six hour sail and I reckon about 25-30 Amp hours over 24 hours allowing for a night at anchor. But I'd be lucky to get 15 sails over the summer of that duration, half the time we are inside Chichester harbour with minimal electricity demands. And we use the motor and this summer without any shore charging the batteries are still fully charged.

So what size panel, 20W, 50W, 100W or more? I can squeeze a 100W panel in front of the spray hood. Would appreciate some opinions as you can do all the calculations based on equipment and a rigid 6 hour sail but guestimating actual usage seems more random. Think of our use as Solent pootling rather than routine cross channel raids...........

Your profile says you have a Konsort ?

Very probably , and your experiences this summer agrees, you will be able to keep the batteries charged from the normal engine use. You can always extend that if necessary.

Your two batteries on a 1,2, both switch mean that you either need two panel and controllers, one for each battery, or a dual output controller in order to avoid having to leave the boat with the switch set to "both"

Two 10 watt panels or a single 15 to 20 watt panel will probably be the minimum sensible choice for maintaining 2 x 110 Ah batteries. However I would opt for the largest I could fit comfortably in the space available. That may be 1 panel somewhere between 50 and 100 watts or two panels somewhere between 25 watts and 50 watts each. Final choice may depend on dimension proportions as much as on watts
output.

You might like to consider adding a dual sensing VSR to your system to free you from some of the downsides of a 1,2, both switch and the need for two solar panels or dual output controller

 

[duncan99210]

I’m with Vic on this topic. Look at the available space for panels and fit as much as will fill it. Linked to a controller you’re not worried about overcharging and you’ll welcome the additional output in use. I’d opt for a number of smaller panels rather than a smaller number of large panels to minimise the effects of shadows. Final thought; if you putting panels on top of the coachouse roof, look at securing the boom off to one side so as to minimise shadowing.

 

[Mistroma]

I'd agree with others about fitting as much as you can. Most people run out of places to fit panels before they end up with a huge surplus.

I see that you are wintering afloat on a mooring. Ah/day will drop a lot in winter months, table below gives approx. output for an unshaded 100W panel lying flat on deck in the Solent area. The figures are daily Ah during each month in an average year. Obviously heavy snowfall would completely kill output if panels are covered but I imagine that wouldn't exactly be an average year in the Solent.
Month Ah/Day
Jan 4
Feb 8
Mar 17
Apr 25
May 29
Jun 31
Jul 30
Aug 24
Sep 18
Oct 10
Nov 5
Dec 3

You will get a fair bit of shading from the boom but still get good results. I have 3 fitted that position and output has always been reasonably close to predictions. Having 2 or 3 smaller panels in parallel helps reduce impact of shading in that situation. I'd suggest 2x50W as a start or 3 smaller panels to help fill available space.

You can always pro-rata above figures to meet your expected needs if you think 100W is too much. Certainly, 3-4Ah/Day will more than keep up with self-discharge during winter. However, 25-30Ah in summer wouldn't be excessive for many people.

 

[cpedw]

Ah/day will drop a lot in winter months, table below gives approx. output for an unshaded panel lying flat on deck.

That's a useful guide (about a factor of 10 from mid winter to summer) but what size is the panel it's based on and what location (does location make much difference?)?

Derek

 

[LadyInBed]

Consider
using two smaller panels rather than one large - more locations possible, redundancy and choice of series or parallel wiring.
semi flexables - less of a trip hazard and can take a slight bend.
dimensions of the panels - long thin / short fat.
Fit the largest capacity you can afford / have room for, you can never have too many watts!

 

[seumask]

On a similar boat also on a Chichester harbour mooring, weekend use and a couple of weeks away in the summer we have found a 50W pannel on the coach roof works well. IMHO you wont need much more. It brings the battery back up after a weekends use and allows the fridge to be left on for a few days in sunny weather at anchor without much worry.

 

[lw395]

On a similar boat also on a Chichester harbour mooring, weekend use and a couple of weeks away in the summer we have found a 50W pannel on the coach roof works well. IMHO you wont need much more. It brings the battery back up after a weekends use and allows the fridge to be left on for a few days in sunny weather at anchor without much worry.

But the OP doesn't have a fridge.
He's just got the stuff we used to have before anyone had solar.
The first panel I bought IIRC was 10W. That made a huge difference to keeping my old batteries afloat during the winter.

 

[Graham376]

But the OP doesn't have a fridge.
He's just got the stuff we used to have before anyone had solar.
The first panel I bought IIRC was 10W. That made a huge difference to keeping my old batteries afloat during the winter.

The OP, assuming one of his batteries is kept for engine starting, only has 110ah domestic. Whilst a small panel will top up the batteries during the week (we had 25w for that purpose when weekend sailing) , it won't keep up during a summer holiday cruise so I would suggest fitting the largest he has space for.

 

[Mistroma]

That's a useful guide (about a factor of 10 from mid winter to summer) but what size is the panel it's based on and what location (does location make much difference?)?

Derek

Apologies, I was in a bit of a hurry earlier and forgot to add these vital details. Calculation results were for a 100W rigid panel flat on deck in Solent area. You might exceed these figures in an odd year but that's unlikely. Shading will obviously reduce the output. Other losses have already been included.

 

[ghostlymoron]

I had a single semi flex 20w panel for similar sailing profile and it kept the battery well charged and returned to full charged in between visits to boat. However, times change and prices have fallen so I would now install 2 x 50w panels which would allow limited use of my cool box. Get a good pmw controller too.

 

[solent clown]

We just bought a 50w panel, and controller for £47 from a marine supplier. Our intention is to buy another 50w panel and controller later as for our own reasons we want two stand alone systems.
Had I realised how cheap things had become we would have done it before.
Our boat on swing moorings on the hamble has a huge leak due yo the coachrood being hacked about, so knowing the battery stays charged is one less thing to worry about. It is just the float switches on the pumps that keep me awake at night now

 

[peteK]

I started off with a 10w panel when prices were expensive,this kept the batteries topped up in the summer but struggled in the winter,then added another 10w panel which maintained them all year round.I have just recently purchased a 30w lensun panel
which will replace the original 10w panel on the hatch garage so will end up with 40w total.
The boat is 23ft with basic electronics and two 85amp batteries.

 

[doug748]

.....

So what size panel, 20W, 50W, 100W or more? I can squeeze a 100W panel in front of the spray hood. Would appreciate some opinions as you can do all the calculations based on equipment and a rigid 6 hour sail but guestimating actual usage seems more random. Think of our use as Solent pootling rather than routine cross channel raids...........

I seem to have similar requirements to you and usually top the batteries up via a smaller Aerogen which works exceptionally well on a swinging mooring. On a marina, this season, I have swapped it for a 10W panel which has been very good but this has been a sunny season and I expect it will be of marginal use over the winter. This does not matter much as I shall be ashore.

As noted above, you probably don't need anything at all if you use the engine regularly but it is nice to have. On long slow sunny passages the little panel halves my power consumption; I have it on a movable mount to roughly catch the best light. However I have used the engine so much, in this summer of no wind, that it hardly mattered.

Do fit something like the Nasa Battery Monitor, if you don't have one already, a very practical piece of kit and greatly entertaining if you have an adjustable panel.

Cutting to the chase, I would fit a 20W panel and see how I got on, I think you would probably be happy with that but, if not, it is easy to fit a second one later. The coachroof is a prime spot for a folded dinghy but if you seldom make longer passages this may not be a consideration.

 

[FairweatherDave]

This is all really helpful stuff. Thanks. Really haven't decided which way to go yet. Since we don't actually use the sprayhood the panel is not exactly hidden so ideally from an aesthetic point of view I'd like a 50w, but we can squeeze a 100w if we wanted. Need to head to the boat with my measuring tape. Wish I didn't sit on the fence so much, but a nice problem to have

 

[Bobc]

I have semi-flexible ones, and in the winter I find they work better hung over the dodgers than on the top because the sun is so much lower.