Solar Panel size advise

[Phil23]

I have installed a 12volt Danfos (Fridgetech GE150) refrigeration kitset in my fridge. I have a off engine piston driven compressor which is connected to both my freezer and fridge which does a great job however overnight my fridge temp increases enough that I either need to run my motor or go for a bit of a cruise which on days we just want to stay put is a bit of a pain in the Butt, hence the 12v unit to help maintain fridge temperature.


My fridge draws 6amps when running when running add say another 5amps for overnight incidentals etc my query is what size solar panel should I install. I have 2 deep cycle batteries giving 200amp hours.
On asking this question along my marina I got 6 different responses varying from dont bother to 300amps which seems overkill to me. I am limited on mounting area which is also a consideration.

 

[NWSeadog]

I'm just starting down this road myself. What you need to do is figure out how many amp/hours you need. In my case the refrigerator is the primary user. When running it takes about 4 amps. I'm guessing it only runs about 25% of the time so I need 24 amp hours to keep the batteries charged. A 160 Watt flexible solar panel can provide a maximum of 8 amps in full sun. If I had 8 hours of full sun that would give me about 64 Amp / Hours. Realistically I'm not going to get maximum light. So maybe derate that number to 60% or 38.4 Amp/hours per day. That would give me only 14 Amp/hours for other requirements on the boat. I doubt that will cover everything but would probably give me another day to 2 at anchor between running the generator.

Those are my thoughts. Your numbers will be different but the process should be the same. Someone will come along shortly with some real world numbers.

 

[Frosty]

Well, it depends (I know, don't you hate to hear that?).

A few thoughts:

-- I live off grid at home, and in my experience, 200 watts is enough to run a 50 liter 12-volt compressor refrigerator, plus other smallish loads (device charging, LED lights) 24/7. BUT, that's in a sunny climate with no shade on the panels and with them angled to face the sun when necessary (after a cloudy day, etc.). (Not flat on a boat and with railings and other shady bits.

-- Since on most boats there is shading from rigging, rails, etc. - and the panels will be mounted flat - you may need, say, 400 watts to "equal" a perfectly placed 200 watts in other setups. That's an educated guess not pure fact.

-- The rule of thumb used to be to match panel wattage with house bank amp hours, but now that panels are so cheap (and batteries still aren't), and given the disadvantages with insolation on a typical boat I'd say 400 watts would not be too much. OTOH, 200 watts should be noticeable and better than none. (And maybe you have a great location on your boat.) Pretty much whatever you can fit would likely be useful. It's not like a house where you could toss on 2000 watts (we wish!) So in some ways while figuring out draws and such is great, in most cases on boats you can put on whatever fits and still not have too much "Gee, what am I going to do with all this extra power?" is not something you hear often (altho possible in some setups I suppose).

-- I have tried a number of "flexible" panels, and even babying them (ventilated back, full support, no bending) just haven't had much longevity (granted I have not used super premium ones such as Solbian). So I've gone back to rigid-framed glass panels. I still hope to see that change though.

-- Back to the fridge, I don't know which compressor unit you have but the ones I run (Danfoss BD35 or BD50) have control units that are very sensitive to voltage drop, so don't skimp on wire size. Speaking of which, solar power is precious and that wiring is another place to pay close attention to voltage drop. I shoot for 1.5%. That may sound nuts but with limited space for panels I don't want to give up any hard-won power. Also, unless your controller has a voltage sense lead (many don't), any voltage drop between controller and battery bank changes your precious pre-set charging specs (maybe not as important with some battery types).

I've skipped or smoothed over some things but it's already maybe a wall of text.

 

[BruceK]

Phil23, Are your batteries 200ah each or 200ah total(ie 2x100)? Do they serve house only?
Panel sizes are usually expressed in watts. To get amps output divide by 12, but that`s ideal theoretical max, I`d assume output of 50% of that.
If you work out how many amps you want to generate you can size your panels accordingly.
Frosty is pretty much "on the money". Your compressor unit is probably drawing around 4 amps, x 24 = 96A a day.
You`ll need a regulator, and some allow you to connect a load direct, ie without the power going via the battery, and allow falling back on the battery, say at night or when the panels are not producing. The regulator would likely give you a way to set that up, it can be a little complex.
If you`ve got the space for panels you won`t regret going bigger rather than smaller. Panels have got a lot cheaper. But beware of the stated output, check ebay and check comments from panel users about actual output compared to claimed output. Not all panels are created equal. Most panels and regulators come from China so there could be an availability issue.

 

[Insequent]

For boats its simple: add as much solar panel capacity as you can!

Use a reasonably good charge controller. Then you don't need to be concerned about over-charging the batteries. Do not just connect the panel to you battery bank, or you could cook it.

I am towards the other end of the spectrum to some of you. I have 2070W of solar and a 1284 Ah house bank on a bit bigger boat. I don't worry about power management, leaving lots of smallish loads on. But refers are the killers. My main galley fridge plus freezer typically uses 85 Ah/day (based on actual measurement). Its fairly old, but has a Danfoss compressor. On a 5 month Great Barrier Reef trip we had 2 x Waeco 110 litre freezers (Danfoss compressors also) with meat, home-cooked meals, milk etc and they would swallow 130 Ah/day, each, @ -10°C setting. Freezers use a heck of a lot more energy than refrig. does. Indeed, measurements for 24hr for 1 x Waeco were: +3°: 21 Ah. -5°: 62Ah, -10°: 131Ah.

One thing I noticed was that the Waeco's would lose efficiency when the house bank voltage dropped a bit. When plugged into shore power at marina's they would stay at -10 easily. At anchor, even with <50% DoD on the house bank the Waeco's would struggle to keep at -5. Plugs/cable size were all upgraded but no difference. I figure the Danfoss units are really sensitive to voltage drop, pretty much their only flaw! I don't have a solution for it at this point.

 

[DavidM]

There are several articles in this forum's Library on solar panels including one I wrote on solar panel sizing which covers the issues discussed above.


David

 

[mvweebles]

Short answer: each 100w solar panel (glass vs flex) will produce about 30ah of power each day. But that is average for the year - mid summer will be closer to 40ah; mid winter will be closer to 20ah. Assumes flat mounted panel in mid-low latitudes.

Linger answer: there are a ton of articles describing calculation and conversion. General rule in sizing solar system is you can take a panel rating (watts) and multiply by 5 (hours) for the daily output over a year. Because mounting flat, you will see about 80% of that. I have data from a large residential solar array mounted flat over a year to support this, but I'm sure others will chime in with lengthy calculations. In the end, you will be pretty close to 30ah produced for each 100w of solar panel. Coincidentally , 30ah is about what most marine refrigeration units in a reasonably well insulated box consume (I take note though at the post with very good data on freesers being 2x-4x that)

Your info on your battery is vague. As someone else said, unclear whether you have 2 x 200ah = 400ah total, or 200ah total. Assuming lead acid, you have half usable of whatever the total is. If 200 total, you're on the low side. Problem is that at 200 (100 usable) , you may not have enough holdover reserve for below average days of sunshine. That said, I would install the solar and see how it goes before upgrading batteries. Spend a few extra bucks on a decent Victron MPPT controller with Bluetooth and you will be able to pull a lot of data off your system to see production. Coupled with voltage meter on your batteries (or off your inverter depending on model and features) , should give a decent clue on what's going on.

Good luck

Peter

 

[DavidM]

Well, I strongly disagree with 30 Ahs required each day for a marine fridge.

My Novacool packaged 3 cu ft fridge used about 60 Ahs. A 6 cu foot well insulated icebox with 1/3 divided as a spill over freezer and a Frigoboat keel cooled system with a compressor speed control module used about 75 Ahs.

Both of these were measured with a battery monitor and were corrected for lighting and other house DC loads.

But I totally agree with your solar output rules of thumb. Just use 40 Ah per 100 watts or a flat mounted panel on a sunny day in the summer at moderate latitudes and 20 Ah per 100 watts for a the same in the winter.

You can get fancier by using solar insolation maps but the above will put you in the ball park.

David

 

[boathealer]

"Man, I put one too many solar panels on my boat."

......said no one. Ever.

 

[mvweebles]

Well, I strongly disagree with 30 Ahs required each day for a marine fridge.

My Novacool packaged 3 cu ft fridge used about 60 Ahs. A 6 cu foot well insulated icebox with 1/3 divided as a spill over freezer and a Frigoboat keel cooled system with a compressor speed control module used about 75 Ahs.

The ubiquitous Danforth BD35f. At 12VDC, it draws right at 4.5-5.0 amps when running. Bring the duty cycle down to 25%-30%, and you're in the ball park of 30AH. Yes - definitely varies based on usage (e.g. door openings), insulation, and ambient weather. So 75Ah is possible, but that means something like a 60% duty cycle (luckily, should coincide with brighter sunshine for solar). Key is decent insulation, and a top-loading box is very helpful too. My refrigeration has both a front-door for occasional use (once or twice when meals). And a top-load hatch for drinks and quick-grab condiments.

 

[mvweebles]

My fridge draws 6amps when running when running add say another 5amps for overnight incidentals etc my query is what size solar panel should I install. I have 2 deep cycle batteries giving 200amp hours.

If 6A (probably Danforth BD50f) is correct, assume 50% duty cycle for both fridge and freezer. 6AH x 50% x 24H = 72AH per day, which is pretty close to DavidM's observation. You will need 2-3 100W panels to replace that amount of power (two during summer, three during winter). 200AH of battery is sufficient if they are in good shape, but not much leftover for a cloudy day or other usage, especially if they are not deep-cycle rated such as Trojan T-105s.

Suggestion would be to install two 100W panels but purchase a controller that will allow you to add a third panel if needed. Wait to upgrade your batteries until you see how the system responds after upgrading the solar. Be aware that during the winter, due to shorter days, your battery bank may be undersized as the fridge will spend more time drawing the 6A from the battery vs instant solar replenishment.

Good luck!

Peter

 

[bogranjac1]

300 watts solar on roof by 1 x 300 amp hr AGM house battery x Xantrax 40 amp charge/ load controller = 1x 45 litre Engel & 1 x 50 litre Waeco run 24/7.
This combination has been successfully used in Queensland winters for last 2 years.
Remember even in sunny places expect cloud.
My house battery will support this arrangement under totally cloudy conditions for 5 days before needing to start engine for alternator charging.
No one likes the obnoxious thick skinned boater who runs a hideous antiquated petrol generator for endless hours.

 

[kchace]

If 6A (probably Danforth BD50f) is correct, assume 50% duty cycle for both fridge and freezer. 6AH x 50% x 24H = 72AH per day, which is pretty close to DavidM's observation. You will need 2-3 100W panels to replace that amount of power (two during summer, three during winter). 200AH of battery is sufficient if they are in good shape, but not much leftover for a cloudy day or other usage, especially if they are not deep-cycle rated such as Trojan T-105s.

As someone who has gone through this I can say that these numbers are just about spot on AND I agree that 200AH of battery capacity is definitely a bit on the low side.


I have 400ah of house battery capacity. At first I added a single 100w panel, I added a second 100w panel a year later. The 2 panels give me about 70-80ah per day during normal sunny weather. Much much less on cloudy rainy days. My older fridge uses about 70ah per 24hrs. Add other normal boat uses when at anchor and I still have to run my generator about 1.5 hrs per day.


I'm planning (hoping) to install 700W on 2 350w panels above my bimini. With that amount of power, I'll be able to run my (750W) water heater off solar (on sunny days - via the inverter) and should never have to run my generator except for AC.

Like someone said above - nobody EVER said they have too much solar.

Ken

 

[syjos]

.
No one likes the obnoxious thick skinned boater who runs a hideous antiquated petrol generator for endless hours.

I agree 100%

Especially when rafted to the offending boat.

There are also many boats with noisy diesel generators not equipped with a Gen-Sep water/exhaust separater.

 

[Frosty]

Just as a data point, I have a 50-liter, "cooler shaped" refrigerator. I mention the shape as these are not super well insulated as a boat ice box might be. It has a Danfoss BD35 compressor. I set the thermostat for 30ºF which keeps things about as cold as they can be without freezing (box is not all at 30º when set at 30).

If it's hot inside (80ºF and up), I'd think I use around 48ah per day for the fridge. When coolish (say outside temps are 65º day and 40º night and I don't heat space overnight), probably down near 25ah per day.

In my experience it's not so much about how many amps the compressor draws as it is all about duty cycle. i.e. can you reduce duty cycle?

I helped a buddy install a 150 liter "normal refrigerator shaped" unit that includes a 30 liter freezer compartment. It runs a BD50 compressor. I don't know his exact power usage in 24 hours, but I bet it's no more than 1.5 times mine (but he has three times the box space and rock-hard ice cream in the freezer). Difference is we put 4-5" of extruded polystyrene insulation around every side but the door (none added) and the bottom (1" added). His compressor also has more advantageous cooling position set up (and with the BD you can easily plug in an extra fan that comes on when the compressor comes on if desired)

Another point is to make sure the compressor can't suck in any of the hot air coming off the output side. You can often make something to help this in situ. Picture one of the old radar face snorkels made of Coroplast. Likewise plenty of space for the compressor heat to exit.

Power you don't use doesn't have to be collected (not to say deprive yourself, but more along the lines of can the fridge run at a lower duty cycle?)

 

[Comodave]

A simple thing to do that will reduce power consumption of the fridge and help the fridge cool better is add ventilation to the fridge box. Our Vitrifugo fridge didn’t cool well. I added a vent both high and low in the box that the fridge lives in. Also added a 12 volt 50 mAmp fan that blows into the box at the bottom. So hot air will be blown out the top vent. The fridge works much better now. I don’t have statistics as to run time but the fridge freezes well now so it must be working more efficiently.

 

[Frosty]

Our Vitrifugo fridge didn’t cool well.... Also added a 12 volt 50 mAmp fan that blows into the box at the bottom.

You may already be on to this, but if you have a Vitrifrigo, it probably uses a Danfoss compressor. At least on the BD35 and BD50, if you hook two fan wires across terminals + and F* on the compressor controller, it will come on whenever the compressor is running and shut off when it is not (or C and F specifically on unit 101N0600). This can help ventilate a stuffy compressor compartment.


Another option is to fit an LED light across terminals + and D. The light will give flashing codes if there are errors.

I found a good manual on Danfoss' site (mine is a few years old or I'd post it) that explains this and more.

You can see the terminals on this photo of a BD unit:

 

[WayneDana2]

"Man, I put one too many solar panels on my boat."

......said no one. Ever.

Truth you speak.

 

[Comodave]

Frosty,

That is exactly what we did. And it did make a big difference.

 

[FoxtrotCharlie]

Our boat came with a household (stainless) 10 ft fridge with freezer on top. The old NovaCool had evidently died and PO bought this one at Walmart. Turns out to be very very efficient. We have 400 AH of house bank (4 golf cart bats) and a 3,000w pure sine wave inverter. When we are cruising we either have the gennie or the inverter on - has worked great for the last 900 miles on the rivers - anchoring out about 30%. Had planned on getting a 'marine fridge', but if it ain't broke, don't fix it.

 

[firehoser75]

Phil23,
Despite the fact that you are located on the opposite side of the world, I will relay my experience with my solar system as you should find similar results.

I have 2 panels totalling 570 watts mounted flat on my pilothouse roof. They are controlled by one MPPT controller. Don't skimp on the controller, they are very important, and it is even better if each panel has it's own controller. Separate controllers minimize any shading impact on one panel.

I have a 600(+) amp/hr battery bank (golf carts). With this set up, I don't need to run my generator at all on sunny or partly cloudy days in the late spring through to early fall. My batteries make it back to 100%. On hot days they drop to about 90% overnight. Days where fog does not lift til the late afternoon, different story.
My main draws are refridgeration (Novacool RV style fridge), Waeco freezer, lighting (LED), electric toilet, some electronics, and some other small loads. We use propane for cooking.
Definitely put the largest wattage of solar on the boat as you can fit and afford. It will give you more options. However, even if your solar does not regularly get you back to 100%, solar will reduce the frequency and/or length of time you need to run your generator (unless you cook with electricity). If you have to run the gennie, run in the AM and then let the solar "top off" the batteries.
Hope this helps,

 

[FF]

" I have a off engine piston driven compressor which is connected to both my freezer and fridge which does a great job however overnight my fridge temp increases enough that I either need to run my motor or go for a bit of a cruise"

Something is wrong! Most likely you are not pulling the holding plates cold enough to freeze the solution inside. OR the box insulation stinks.

An hour after engine operation, take the temperature of the plate surface . That will be the temperature the eutetic solution inside the plate melts at.

You must see at least 10F or 15F on the plates surface BELOW the off holding temp to properly freeze the eutetic solution with the engine on.

Save your money , fix what you already have.