solar confusion

hi there guys and gals i am new to this site so please be kind.i have read more forums on solar setups than i can poke a stick at and still am filled with doubts and worries. So i thought id just ask you the experts my self so here it goes.i am wanting to run a 12v compressor fridge 100l Primus Mammoth on a solar setup, now the website says they draw 0.98 a/h which from what i have read is far from correct (maybe in the north pole) and the real figure i am looking at is around 3 a/h.I want to purchase a set of 240 watt panels and a 280 a/h deep cycle battery to power the fridge accompanied by a Ctek smart charger.
how long realistically could i expect to run the fridge for ? is this a sufficient setup ?
i have read sum equations on how to work it out but have failed miserably from what i have read it goes like this please correct me im far from a math wiz.

fridge uses 3a/h x 24 hours in a day =72 amps per day
solar charge gives me 12a/h x 8 hours of light =96 amps each day
battery discharge is 72a/h-96a/h=-24a/h
which in theory my battery wont be discharged and should have an extra 24a/h

are my maths correct? wont surprise if im wrong please help me and share your thoughts cheers until then Happy & Safe Travels :)

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Reply By: Member - LeighW - Sunday, Jan 12, 2014 at 18:36

Sunday, Jan 12, 2014 at 18:36
Your panels are 240W, allow 14.4V for charging that's about 16.6A or 132Ah for a 8 hour period. Assuming a good quality MPPT will give you more again, so not sure how you calculated the 86Ah?

Keep in mind though that if ambient is above 20C or there abouts you need to derate your panels. Also keep in mind you could get a few overcast days etc.

Given sun though you should have no problems.


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Follow Up By: Member - LeighW - Sunday, Jan 12, 2014 at 18:39

Sunday, Jan 12, 2014 at 18:39
Forgot to add, unless your cost cutting I would go with a good quality mppt charger.

Even if you intend to use the ctek to charge via the car, reason being the ctek is a compromise solution, it will not perform as well as a good quality purpose built mppt charger. Also if you intend to go bush with two seperate units you have some redundancy, with a combo unit if dies you loose everything.

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Follow Up By: Member - Andrew W14 - Sunday, Jan 12, 2014 at 21:18

Sunday, Jan 12, 2014 at 21:18
i think you need another look at your figures:

1. The solar panels will put out at around 17 volts. The solar controller will then regulate this to about 14 volts.

So the max that 240 watts of panel can put out is around 14 amps. However NO solar panel is 100 % efficient and most run around the 80% efficiency mark by the time it gets to the battery.

This gives you a total output of 11 amps.

I am not sure where in Australia you are but the accepted average for maximum solar out put is 5- 6 hours per day (as the sun sets and rises and moves away from 90'C to the panels the output drops greatly).

So for any given circumstances the best panels in the world rated at 240 watts would give the OP around 50-60 amps on a day of full sunshine and no cloud cover at all. Somewhat less than the optimistic if unrealistic 132 ah you unrealistically claim.
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Follow Up By: Member - LeighW - Monday, Jan 13, 2014 at 10:35

Monday, Jan 13, 2014 at 10:35
For my calculations I assumed a good quality mppt charger would be used.

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Follow Up By: Member - LeighW - Monday, Jan 13, 2014 at 10:52

Monday, Jan 13, 2014 at 10:52
Andrew W14, unlike you I don't have a crystal ball, and yes my figures were a guesstimates as I have no idea as to what panels he is using could be 12V/24V or grid connect type.

His panels are moveable so can be re-pointed! Assuming sunrise at t around 6 and sunset around 9 that's 15 hours the panels could be will be producing some output.

Using a good quality mppt charger, the power output at 14.4V should be pretty similar to what it would be at the panels max power output voltage.

From memory the CTEK is supposed to be around 96% efficent, a good quality mppt charger should also be around this.

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Follow Up By: new boy - Monday, Jan 13, 2014 at 11:07

Monday, Jan 13, 2014 at 11:07
You say the panels put out 17 volts regulated to 14, but only work at 80% there for only 11 v effective but wouldn't the drop in efficiency be taken up in the excess between 17 to the regulated 14 .What I,m trying to say is 80% of 17 is 13.6 under the 14 therefore it would get the full 13.6 not your 11.Just trying to get this clear as to why the drop in efficiency is from the 14 not the 17 volts or does the regulator only work at 80% of input.
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Follow Up By: Brian 01 - Monday, Jan 13, 2014 at 12:29

Monday, Jan 13, 2014 at 12:29
New boy, maybe I can offer a little bit of clarification here and hope it's not tooooo long winded.
There are a number of different efficiencies talked about in relation to solar installations.
Efficiency of a solar panel really relates to its ability to convert light energy (photons) into electrical energy (free electrons).
This ability is affected by such things as the substrate being used for the cells (which controls how much of the light spectrum that the cell can transform into electricity), the reflectance of the covering membrane (which controls how much of the available light actually reaches the cell substrate), plus a number of less important parameters.
The best solar cells have an efficiency of about 21%, with 18% being the norm.
We then talk about the efficiency of the solar panel to deliver what energy it is producing, and that's where the 80% figure comes from, it can be far lower in some conditions.
If you were to begin with a 100% efficiency, and then deduct losses due to panel temperature, angle of inclination, solar azimuth, dust on the panel, etc. you come up with a rough rule of thumb of 80%
Almost all panels that we term to be 12v will have a working voltage (Vmp, under an impedance matched load) of about 18v (this is the absolute maximum voltage of the panel when connected to a load) and an open circuit voltage (Ocv) of about 21v .
This Ocv will appear at the terminals of the solar panel even in low light conditions when there is insufficient photon energy to produce any meaningful current flow in the charge circuit.
The amount of current that any particular 12v panel can deliver will depend on the wattage of the panel (how many individual cells it contains), the intensity of light, and how well the impedance of the load is matched to that of the panel.

The efficiency of a solar regulator/controller relates to how much power is lost in converting the incoming energy into the outgoing charge to the battery.
A standard (non MPPT) controller is known as a series or pulse width modulated (PWM) device. Put simply, it is an electronic switch that just connects and disconnects the panel directly to the battery according to a set of rules embedded in the electronics of the controller.
When you connect a solar panel directly to a battery, the output voltage of the panel is immediately reduced to that of the battery.
So, if the battery is deeply discharged (say to 11.5v), then the panel will be pulled down to that voltage. This means that a 100w panel that that is outputting 5.5 amps Imp (its maximum current flow) is only delivering 64 watts to the battery (5.5 x 11.5 = 64). As the battery voltage rises, the impedance matching becomes closer to unity (but will never get there), so the panel voltage rises as does the overall efficiency of the conversion.
You can see from the above that connecting a 24v or higher panel to a 12v battery via a PWM controller will result in more than 50% of the panel's output being lost.

An MPPT controller works differently, it does not connect the panel directly to the battery but instead acts as an intermediary and keeps the panel working as close as possible to the 18v. This means that the 100w panel that is supplying 5.5 amps to the controller can now be converted by the controller into 11.5v at 8.7 amps which is still 100 watts (the figures will be slightly less due to some losses).
A 12v MPPT controller can convert any input voltage (within its design limits) to charge a 12v battery with little loss of power.
Hope this helps.

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Reply By: oztrippin - Sunday, Jan 12, 2014 at 18:45

Sunday, Jan 12, 2014 at 18:45
thanks for your reply i believe the Ctek dual charger has a built in mppt charger in the unit itself ??
the reason i calculated 96a/h for solar was thinking they produce 12a/h instead of 16 a/h

keep the responses coming thanks people cheers
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Follow Up By: Member - LeighW - Sunday, Jan 12, 2014 at 18:48

Sunday, Jan 12, 2014 at 18:48
Yes it has one in built but it won't be as efficient as a good quality stand alone unit.


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Reply By: Brian 01 - Sunday, Jan 12, 2014 at 18:48

Sunday, Jan 12, 2014 at 18:48
The Primus Mammoth 100 litre unit draws 4.4 amps on 12.5v, so at a modest 50% duty cycle you can expect a daily draw of 55Ah up to around 80Ah in hot ambient.
With this in mind, your battery, without any charge input, would support the fridge for around two and a half days at best and two days at worst without taking the battery below 50% Soc.
With cool clear weather and diligent regular alignment of the solar panels, you can expect an input of pretty close to 90Ah.
This means that you should be pretty self sufficient provided the weather doesn't let you down. At a pinch, running the battery a bit lower won't hurt, but remember that you have to put it back in, and whilst you have a little up your sleeve, that's all it is.
Hot overcast weather can be a bitch.
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Follow Up By: oztrippin - Sunday, Jan 12, 2014 at 18:53

Sunday, Jan 12, 2014 at 18:53
thanks heaps mate you defiantly put the mind at a little ease much appreciated cheers
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Follow Up By: Brian 01 - Sunday, Jan 12, 2014 at 22:03

Sunday, Jan 12, 2014 at 22:03
I definitely wasn't trying to be defiant. lol
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Follow Up By: Ross M - Sunday, Jan 12, 2014 at 22:04

Sunday, Jan 12, 2014 at 22:04
Many miss the predictive text and what it does. It selects "Defiantly" when you try to type in "Definitely".
A lot of people on many forums are being defiant when they are really definite.
I am defiantly definite about this. Just joking. Technology may mean you say things you don't mean cos it thinks it's smarter.

Ross M
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Follow Up By: Adam H - Sunday, Jan 12, 2014 at 23:02

Sunday, Jan 12, 2014 at 23:02
Hi oztrippin

yes it can be a mind field when trying work out if you have enough power. buy the sound of it you should be fine. though if you can rotate the panel throughout the day this will help to maximise the solar you can collect.
i have a 100 amp hour battery with a 160 folding panel and over new years in the river land South Australia i collected 48 amps of solar on clear 36 degree day. so your system should see you though without 2 many drama's. though i also collected 15 amps on a 42 degree day when it was cloud cover all day. though the 2 days did balance them selfs out.
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Reply By: hazo - Monday, Jan 13, 2014 at 09:14

Monday, Jan 13, 2014 at 09:14
Other things to consider are, the ambient temp around the fridge and how often the fridge is loaded with food/beer whatever, and how often the fridge is opened or closed.

Also the fridge insulation itself, some imported fridges leave a lot to be desired as they are designed for countries with less severe heat than we get here.

All these variables will affect any calculation as to how long you can run it for.

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Reply By: Member - Bruce C (NSW) - Monday, Jan 13, 2014 at 10:19

Monday, Jan 13, 2014 at 10:19
Hi Oztrippin
My suggestion is to buy the most solar capacity you can afford and have room to fit.
Once you have it you will be thinking of using more power for different things any way.

My calcs suggest that your proposed system is just on the good side of a borderline capacity with not a lot of reserve capacity for suggested use should you encounter really hot days or cloudy days.

According to my calcs.
Daily frig consumption = around 83 Ah per day (average worst case)
Daily generation = around 91.42 Ah per day (average worst case)

This suggests to me that your proposed 240 watt system could be bigger for best results.
I also recommend a stand alone regulator as has been suggested by others.

Cheers, Bruce.
At home and at ease on a track that I know not and
restless and lost on a track that I know. HL.

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Reply By: robert s4 - Monday, Jan 13, 2014 at 18:11

Monday, Jan 13, 2014 at 18:11
hey oztrippen I have a 80 ltr engle a 720 marine pro battery a red ark a 120 watt solar panel we sit for days no worries we have led lights for camp and a led on the awning i got my lecky to make a spare battery box up for me as we r going up north in September just in case I buy a 3rd battery to through in the back .we freeze everything before we go and also put a frozen bottle of water in the freezer it helps youll be laughen I reckon our basic set up works for us
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Reply By: Geoff in SA - Tuesday, Jan 14, 2014 at 11:41

Tuesday, Jan 14, 2014 at 11:41
Hi Newboy
This solar debate is interesting to watch.
My simple answer is this....
only take 50% of the output folk think you should get and you will be a lot closer to the real value.
Take what you think you may need and add 50%

And then pay for a good controller and you will be OK
Remember to have reasonable batteries as well.
I only use $100 truck batteries x3 I bought from Paramount Browns here in Adelaide
Now I expect to get about 3-4 yrs of work out of these batteries.
About the same as 3oo to 400 dollar batteries

and if I only get 50% of their life I am still way way ahead

My response is


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