Solar panel - does this sound reasonable??

[BAtoo]

I have 140Ah battery. Boat mainly used weekends. Assume I use half of my 140Ah in a weekend and want to replace this by next weekend.

Recharge Mon-Fri. = about 70 hours daylight. = about 1A per hour to replace used 70Ah.
1A is about whats produced by a 20w panel

So here's the question - how efficient in practice are panels in the UK ??

Do I need a 20W, 40W, 60W or 80W panel to achieve this??

Boat is on a swinging mooring. Panel will be mounted (probably) shadow free on the stern but not always pointing South !!

 

[BabaYaga]

I believe the rule of thumb says that during the summer months you will get a charge from a panel mounted flat that is 25 percent in Ah of the panel rating in watts.
So to replace 70 Ah in five days you would in theory need a 56 watt panel.
In practice a bit more, as you might have five cloudy days in a row.

 

[BAtoo]

Thanks, hadn't heard that "stat".

Hopefully will be able to mount it at about the right angle......if not direction all of the time

Although I suppose that asks another question - is flat better than the right angle facing the wrong way some of the time

 

[Bav34]

Your maths sound reasonable as there is obviously a lot of variance between no sun, morning/evening sun and full midday sun. It's all a bit of guesswork.

I recently fitted 2 x 12W panels flat on my coachroof. Swinging mooring.

Variable week ... more overcast than sunny.

My maths went as follows:

Potential output from panels 2 amps per hour (Ahrs)

Rated output 1.36 AHrs.

Observed output , total cloud cover 0.6 AHrs

Observed output, midday sun 2.2 AHrs !

My guesswork said 12 hours a day at 1.36 AHrs x 7 days = 114 AHrs a week.

The panels are amorphous and were DEFINATELY giving a charge when there was no visible sun.

The week prior to fitting them I had run our fridge (30 AHr a day) for three days plus taps, instruments, lighting and the 220 Ahr battery rested at 12.2 volts which I understand is half charged.

A week later it was sitting at 12.8 volts

Accepting that these figures are rough and ready, bottom line is the battery charged in a week.

 

[William_H]

Solar

You probably realise that a solar panel is usually rated at 20v output so a 10watt panel gives .5 amp.

I don't know that it matters much if the panel is flat with random azimuth orientation or angled. I go for flat.

Is it vital that your batteries are fully recharged over a week between uses?

Solar capacity tends to depend on the space you have to fit panels and the amount of money you are willing to spend. Excess capacity can be wasted in a regulator. The actual amount of AH you get over a week is so variable with weather, time of year and boat orientation that you can only fit and hope.
Further generally if you want more capacity 2 at 10watts is similar to one at 20w so it is not a waste to add more capacity later depending on your actual experience. good luck olewill

 

[Bosun Higgs]

.

The week prior to fitting them I had run our fridge (30 AHr a day)

Now that does impress!. MIne, and its not an old one, is at least double .

 

[Bosun Higgs]

Do I need a 20W, 40W, 60W or 80W panel to achieve this??

Its fun spending other people's money but I would go for the 80w. Firstly to give some spare capacity, secondly to allow for the natural internal self discharge of the batteries and thirdly to charge asap to minimise sulphation.

 

[Bav34]

Now that does impress!. MIne, and its not an old one, is at least double .

Based on this:

http://www.tcschandlery.co.uk/5864/Waeco-ColdMachine-CU-84.html

Worst case is 60 watts which is 5 amps (AHrs) x 24 hours = 120, but fridge only runs for '10' minutes an hour so call it 15 mins to allow for the evaporator current = 30 AHrs a day.

Again, rough and ready, but the battery life seems to support my calcs.

 

[Even Chance]

I use my boat at the weekends and in the evenings. I have a 20w panel, mounted flat and my battery is always charged and ready for use. I always have my plotter/fishfinder/VHF/stereo radio/wind instruments/tillerpilot etc turned on when sailing as well.

 

[vyv_cox]

Based on this:

http://www.tcschandlery.co.uk/5864/Waeco-ColdMachine-CU-84.html

Worst case is 60 watts which is 5 amps (AHrs) x 24 hours = 120, but fridge only runs for '10' minutes an hour so call it 15 mins to allow for the evaporator current = 30 AHrs a day.

Again, rough and ready, but the battery life seems to support my calcs.

I suspect those consumption figures are measured under ideal insulation conditions, and maybe in a temperate climate. My Waeco 65 litre fridge has a Danfoss BD35F water cooled compressor and runs, in Greece, between 30% and 40% of the time. Using a top lid, well insulated refrigerator would no doubt reduce this but 10 minutes per hour in a warm climate seems very optimistic.

My unit pulls about 4 amps when it first switches on, gradually reducing to about 3 amps towards the end of the 'on' period.

My solar panels total 125 watts, which is plenty in June and July but not in May and September, with fridge running 24/7.

 

[noelex]

My guesswork said 12 hours a day at 1.36 AHrs x 7 days = 114 AHrs a week.

I do wish solar panels worked like this, but you are not going to get anything like this many AHrs from 24W of solar panels in the UK.

 

[noelex]

Do I need a 20W, 40W, 60W or 80W panel to achieve this??

70Ahr to be replaced in 5 days=14Ahrs day
To average this in summer / spring in the UK you will need about 60w.
If you leave the solar panel up while sailing you get an extra 2 days
70Ahr in 7 days =10AHrs day so 40w would be OK.

These numbers are averages and do not provide any reserve for a week of bad weather, so if you can fit something larger it would help.

 

[nimbusgb]

A 140 Ah battery half flat ie. 70 ah used, will require about 90 ah to get back to full charge.

Assuming worst case - autumn or even winter you could have as little as 5 days at perhaps 2 or 3 hours per day useable charging time in overcast conditions. ( one assumes when it gets worse than that you'll be going out less )

So the short answer is get the biggest panel you can afford/fit and fit a proper charge controller.

There are some really screwy figures in this thread as usual.

Saying a panel put out 2 amps per hour is like saying you drove your car at 100 miles per hour per hour.

A panel puts out 'n' amps. If it does so for an hour thats 'n' Amp hours. If it does it for 15 minutes it's n/4 Amp Hours.

It may sound pedantic to some but if you said fill the tank with 70 miles per hour of diesel you'd be just as confusing!

 

[Pete7]

I believe the rule of thumb says that during the summer months you will get a charge from a panel mounted flat that is 25 percent in Ah of the panel rating in watts.
So to replace 70 Ah in five days you would in theory need a 56 watt panel.
In practice a bit more, as you might have five cloudy days in a row.

In practise our 45w solar panel gives us about 10AH per day. The panel is mounted flat on the coach roof so the boom casts a shadow during part of the day loosing some efficiency. However, 10 AH is enough to make up any weekend shortfall and the panel spends most of its time trickle charging the batteries.

We use a similar amount of power to Bav34, about 30AH a day with fridge and lights.

The good news is the panel is fit and forget and whilst not cheap neither is replacing batteries.

Pete

 

[Bav34]

I DID say rough and ready but the bottom line is after the useage I described i.e 3 days of fridge on constantly etc, using a top loading fridge, in a temperate climate, after a week our battery was as fully charged using the solar panels I described, as it woud be after being plugged in to shore power .

That's all I need.

Good grief.

Sorry that my degree wasn't in electronics.

 

[Scotty_Tradewind]

I have two old 130W panels fitted on a gantry.
In the very brightest day this April they were providing just over 7 amps at the very best but around 5.2A just approaching and after the middle of the day.
In the grey skies of the last week they were struggling to get much over 1A at times.

I think its a bit of a lottery and often a compromise as to what is switched on.
Having the space I've added a Rutland 914i wind genny to try and cover most situations.
Although highly unlikely, I could have a fairly high demand if used all at once of fridge, radar, Navtex, GPS, plotter, vhf, Eberspacher Hydronic, tiller pilot, lights, occassional pumps.

If in the U.K. I think it's generally recognised that a wind genny is possibly more advantageous, whereas in the Med' it's better to go for solar.

footnote...
I cleaned my friends 2 large panels on his boat in Turkey last summer by washing them off. His charge almost doubled immediatly.

 

[haydude]

I believe the rule of thumb says that during the summer months you will get a charge from a panel mounted flat that is 25 percent in Ah of the panel rating in watts.
So to replace 70 Ah in five days you would in theory need a 56 watt panel.
In practice a bit more, as you might have five cloudy days in a row.

My panel is 5Ah, it is mounted flat on the foredeck. The top I got in the last three weeks was 4.5Ah. When cloudy it gets down to 2Ah. Last Monday (bank holiday) it was cloudy and at around 6pm it was giving about 0.5Ah.

I hope this helps.

 

[haydude]

If in the U.K. I think it's generally recognised that a wind genny is possibly more advantageous, whereas in the Med' it's better to go for solar.

I am afraid this is a common misconception. When oriented to the sun, on a sunny day panels in the UK give 100%. If not directed to the sun only about 10% is lost.

A solar panel provides a small charge even under the light of a street lamp.

 

[Scotty_Tradewind]

I am afraid this is a common misconception. When oriented to the sun, on a sunny day panels in the UK give 100%. If not directed to the sun only about 10% is lost.

A solar panel provides a small charge even under the light of a street lamp.

Not sure we get quite the amount of sun that some parts of the Med' do, yet we can get a fair amount of wind .....especially on the forum .????

 

[haydude]

Not sure we get quite the amount of sun that some parts of the Med' do, yet we can get a fair amount of wind .....especially on the forum .????

Hmmm, let me think ... I have been around the Med for three weeks starting from the Easter school term and beyond May bank holiday. A low pressure system has been hanging over the Iberic peninsula, extending into the western med for the whole time. Wet, windy, cold, sea state rubbish. For THREE WEEKS!

At the same time in the UK they had fine weather for the whole time, nice, warm, sunny.

More-over, I visited southern Spain at least once at month during a weekend, where my boat has been until the end of April. Every time I have been there it rained for at least half day, whilst a whole day was the norm.


Any more about "the med"?