How much power from Solar? In the uk.

[onesea]

I know there is no black and white answer but….

Having written this I guess the question really how much Solar do you need to run a fridge?

Given 2 vertically mounted 100watt solar panels (they are in effect back to back as side screens) connected to a battery how much power do you get from the sun during the summer period? So unless the sun is behind I count as one 100watt panel.

I know the numbers are variables orientation, weather etc. Just looking at this as a simple roughly how much.

Be aware lots of bad maths and approximations following:

Would you expect it to handle constant averaged 3 amp power draw? that’s about 36 watts or 72 amps.

More bad maths:
Solar generation in raw numbers 100watts at 12v is 8 amps. So say 6 hours good sunshine that should give about 100 watts or 96 amps a day?

Our present set up:
With current draw of about 2 amps (or 48 watts) will handle by the time we have motored for an hour or so a day after 3/4 days the battery voltage is still 12.8v give or take by the time we have motored 30minutes to our berth.

We have never needed to bother orientating the solar panels to gain more energy we like this. We can tilt them if required but if there’s been no need why bother?

The fridge conundrum:
The addition that has added to our power consumption is the fridge. Estimated to draw about 1amp well actually 3 amps when the compressor is running estimated to be 1/3 of time? So that’s 12 amps or 12watts?

At the moment this is considered a luxury and something to keep the milk and beer cold as we are not used to having it. We also have not had chance to really monitor/ optimise get a feel for its power consumption.

Has anyone any practical experience they can draw on?

 

[rotrax]

We have 400W of solar, two 1 metre square rigid panels mounted on the pilothouse roof. Controled by an EPEver 30.

We run the two large OE refrigerators and a Dometic 40 litre freezer 24/7 during the period May to October. We spend days-and nights- on the hook, a bright home made anchor lantern is powered from the battery bank and is often on for twelve hours. It is plugged into a connector which needs disconnecting. It draws just under an amp.

Since fitting the Solar the 440AH house bank has not been charged externally, only by the main engine when in use and the Solar.

Winter the output is hugely reduced, but the batteries are always at float when we are not living aboard. The only current draw is the bilge pump and a couple of led warning indicator lamps

I dont believe our Solar is enough to supply the required power for year round living aboard, but is perfect fo summer long distance-average 1500-2000 NM's- cruising. I fitted an inverter last year, it was not used often and appeared to have no effect on the battery condition. I suspect greater use would require more Solar.

 

[B27]

Last Summer, a single horizontal 50W panel enabled us to run the fridge (must be similar model, average about 1A) continuously for up to 3 days with no significant motoring. Single, old, 110Ah house battery.

Depending on where we were moored I estimate the panel could give about 6Ah on a fairly sunny day. Less if you're in the shade up the Dart. Maybe more if you spend all day aiming the panel. Best day might have been twice that.
Plenty of days much less than that.

Aside from the fridge the rest of our boat is nothing like the OP's 3A average draw. Charge a few android devices, phones and ipad, some lights in the evening. Nasa instruments. Diesel heater at the edges of the season, but not for hours at a time.

 

[onesea]

@B27 thanks that gives me some confidence.

The 3 amps adds up with:
Fridge,
Autopilot,
Plotter (12"),

Depth,
Wind,
WiFi for NMEA 2000
Nav lights/ cabin lights (all led),
charging phone/ tablet,
Hoover (yes I sail with dogs)

Hour a day motoring I just estimate as berthing 30 minutes from end of river to berth and back.

We do have option of charging in Marinas it's just planning the summer cruising and estimating if we have enough with the fridge.

 

[Sea Change]

If you want a definitive answer you could plug the relevant numbers in to this:

JRC Photovoltaic Geographical Information System (PVGIS) - European Commission

That will give you an idea of the actual insolation at your location, so it's a starting point. Your vertical panels on a moving boat are a bit hard to model though!

 

[vyv_cox]

With 125 watts of solar in Greece at anchor we are able to run the fridge 24/7 in mid summer but not after end of August. Fridge is a Danfoss unit running about 1/3 of the time, average a little more than 1 amp/hr.

 

[AngusMcDoon]

average a little more than 1 amp/hr.

Aaaarggghh! No engineer should ever use that unit. Not even a mechanical one!

 

[bergie]

The Victron MPPT calculator gives decently good estimates of solar output for a given set of panels in a given location (as well as telling which of their MPPT devices would work for that)

MPPT Calculator - Victron Energy

 

[B27]

@B27 thanks that gives me some confidence.

The 3 amps adds up with:
Fridge,
Autopilot,
Plotter (12"),

Depth,
Wind,
WiFi for NMEA 2000
Nav lights/ cabin lights (all led),
charging phone/ tablet,
Hoover (yes I sail with dogs)

Hour a day motoring I just estimate as berthing 30 minutes from end of river to berth and back.

We do have option of charging in Marinas it's just planning the summer cruising and estimating if we have enough with the fridge.

We don't tend to sail 24 hrs a day, so plotter and autopilot would be part time?

There is a balance between solar and battery.
You always need enough battery to cope with consumption on dull days, so for most people you don't need enough solar to always balance the books.
I'm happy to finish a short cruise with house battery low, knowing it will get charged in the next day or three.
I'm not expecting to never motor, so the alternator is always going to be a significant player.

I've got a couple more small panels to improve the solar situation in the Autumn, but there is no way I can mount enough panels to run the fridge and autopilot 24/7 in the rain.

What solar does for me is:
Recharge on the mooring
Extend the time I can go without motoring
Extend the life of the battery.
Reduce engine running.

I haven't needed to run the engine purely for battery charging, but I'm not above nuancing a decision to motor because the battery is a bit low. If I do ever get in a situation where I need to motor for a couple of hours to charge the battery, that doesn't bother me. It would annoy me if it was a frequent occurrence. I think we motor less than most people, we like sailing and don't give ourselves schedules where we must motor to make progress, so many people will find they have fewer days where solar is such a big player?
It's a different scenario from the liveaboard types who stay still long term.

Different people will have different gains from increasing either solar power or battery capacity.

I feel that vertical panels are not ideal, we'd have too much shading and too little time they were pointing in a useful direction. An unshaded horizontal panel works for us away from our home mooring, it's doing something whenever there's sun and several hours of 14.4V is better than one hour of more power than your batteries will accept. On our home mooring we angle the panel to roughly South at about 45 degrees.

 

[bergie]

We don't tend to sail 24 hrs a day, so plotter and autopilot would be part time?

When we did our original power budget, we included three scenarios with different amount of hours of use per device:

• Under way (autopilot, plotter, etc on)
• Anchored and working remotely (laptops used 8h per day)
• Anchored and not working

Quite drastic difference in power consumption between the three. The spreadsheet said 1.4kWh per day under way, 1.3kWh when remote working, 1kWh when not. That's not too far off from the real-world average power consumption of 1.28kWh per day on last summer's cruise.

To mitigate some of the problems caused with high power consumption, we added a hydrogenerator for when under way, and a self-steering windvane.
With that setup we didn't really need shore power between early May and late August last year.

(Numbers in the graph are average Wh produced/consumed per day in a given month)

 

[Stemar]

On Jazzcat, 200w of solar, mounted horizontally, keeps up with our use including a fridge and radio most days in the summer. A bit short on a dull day, but catches up on sunny ones.

 

[vyv_cox]

Aaaarggghh! No engineer should ever use that unit. Not even a mechanical one!

It wasn't supposed to be a unit, simply a shortened way of writing the typical consumption. I apologise if it offends

 

[onesea]

There is a story behind our vertical mounted solar panels, they where cheaper than Side screens. They can be tilted but then we would need side screens as well.
The fridge was an accidental buy at the boat Show

The Victron MPPT calculator gives decently good estimates of solar output for a given set of panels in a given location (as well as telling which of their MPPT devices would work for that)

MPPT Calculator - Victron Energy

That works will it gives me a yield of about 0.3 kWh per day. Can some one give me that in a useful unit?

eg amps/hr. google gives me 25 amp hours/ day.so maybe 1/3 of my requirement?

@B27 I am with you on sailing style and not motoring, we have just got used to 12 months of being totally of grid electrically. This summer will tell how much we are in debit buy.

Sailing overnight is not a norm for us but some longer passages planned it’s something I or we will be doing. Worst case I can double up the domestic battery capacity.

 

[Alan S]

It wasn't supposed to be a unit, simply a shortened way of writing the typical consumption. I apologise if it offends

1 Amp would have been a shorter way of writing the average consumption.
Sorry for being pedantic, but it's no surprise that beginners get confused by energy and power units when people who do understand them get sloppy or careless.

 

[ctva]

I have exactly the set up you have, a 100w one on each side as as spray dodger on a 34' AWB. This is in 'sunny' Scotland.

When we go sailing we switch the fridge on and it satys on, we charge iPads / iPhones etc.. When sailing a chartplotter and full instruments / autopilot. No hairdryers (I have a proper sailing wife), kettles, inverters or any of the other domestic or entertainment rubbish that we go sailing to get away from.

Prior to have in them, I carried a honda10eu generator which I maybe ran every few days depending on wind for sailing or motoring.

Now with the two panels, I have ditched the generator for the last few years. No figures as it either works or does not. Actually I'll be replacing the three 85ah domestic batteries this year as they are now 8 years old and probably do not hold charge very well.

 

[onesea]

@ctva thank you, I just failed to mention my 100ah battery, but by all accounts it should be big enough. If not I might double it.

However with only a 15 amp alternator that might struggle.

 

[William_H]

OP question is understandable but almost impossible to answer. So many variables. Cloud cover panel orientation and fridge insulation temp setting and use.
I think the whole question should be approached from the other way. ie solar panels are relatively cheap for wattage and returns. What costs and is difficult is finding a place for them and mounting them in an acceptable way. On a small boat there is simply a reasonable limit. Plus it is not usually very practical to move the panels for orientation for max power. You can get some increase in charge by using an MPPT controller. (not a pulse width controller) So all OP can do is go with what he has and observe actual results and use engine for charge as necessary. Another battery in parallel will increase charge from engine and of course give more time before depletion after shore charge. An amp meter on solar system would be really useful.
OP calculation says 100 watts gives 8 Amps. Usually solar panels are rated at max current multiplied by max voltage (18 -20v) so max current will be quoted on the panel but expect 5 amps. A PWM controller in effect connects panel directly to battery. (until battery is fully charged). This means 5 amps at best conditions of sun and orientation into a LA battery.
A MPPT controller converts the voltage of the panel (be it full or little output) to exactly what is wanted to charge the battery. ie about 14v. So max current might be 100w divided by 14v is 7.14 amps which in practice is discounted by inefficiencies in MPPT converter. So perhaps 6.5 A. NB the panels were originally designed before MPPT so 20 v max was chosen as best compromise of getting enough voltage to put some charge into a 12v LA battery at marginal sunlight using direct connection or pulse width controller. ol'will

 

[vyv_cox]

@ctva thank you, I just failed to mention my 100ah battery, but by all accounts it should be big enough. If not I might double it.

However with only a 15 amp alternator that might struggle.

When we had 2 x 110Ah batteries the voltage first thing in the morning was sometimes down to 12.0. Adding a third made a huge difference, typically 12.4 v. I think two batteries is the absolute minimum if they are going to have a reasonable life.

 

[onesea]

When we had 2 x 110Ah batteries the voltage first thing in the morning was sometimes down to 12.0. Adding a third made a huge difference, typically 12.4 v. I think two batteries is the absolute minimum if they are going to have a reasonable life.

I am starting to think that a second battery is required.

 

[Slowboat35]

What an astonishing dogs dinner of mangled, garbled and plain imaginary units this thread contains!
A little understanding of the most elementary basics of electricity and power consumption are necessary before any coherent discussion can thake place. (With apologies to those who do understand the units involved).