Solar System Sizing...

Steve

From my limited investigations I reckon you'd probably need an 80 watt job. That should run the fridges and top up your battery during a good day's sunshine. 80W should give you 6.6 amps when full on so there's a fair safety margin.

That's the way I considering heading ... when I have the spare ching ching $$$.

Cheers

Engel fridges usually draw about 2.5 to 3 amps each when running so maybe your sparky got it wrong. You can check each one individually with a multi meter that has a 10amp plug on it.
I would suggest that you would need about 120watt panel total to build the battery up when stopped as the draw overnight from both units would lower the battery considerably.
I have a 64 watt panel and it manages to run the my engel during the day and when the fridg cycles it puts the power into the battery. Just enough to keep the battery going when stopped for a couple of days.

Steve,
I believe an 80 watt panel will only give you 'realistically' 5 Amps, times 5 hours at full sun = 25 watts, PLUS the few hours each side of 'full sun' will give you a further (maybe) 10 to 22 Amps daily. I have noticed some Isolator systems nowadays state they are suitable to be used with Solar panels, what this means I am not sure, maybe some are not ? I don't know the answer.
A 75 watt DC battery I believe is too small to do the job you are asking of it on a three month trip, yes, only my opinion, I had one 80 a/h and then added another in parallel, giving 160 a/h total and now don't have battery problems.
I don’t use a regulator as the power 'in' is a similar number to the power 'out' and the batteries are Calcium which can take the extra current when offered.
I also believe the 3.6 amp current draw of your two fridges combined to be a bit low!
(Quote) Hopefully someone out there with EXPERIENCE can fill me in on what I should consider/buy in the line of:
suitable panel(s)-> I use Solarex
a good controller-> depends on how deep your pockets are… do you NEED one?
battery monitor that will let me see the state of charge etc. of my battery-> a $30, 30 amp in/out gauge wired between the battery and the fridge shows the battery power to the fridge, also if the panel is wired via the fridge it will show the Solar power going to the battery.
(plug) if you want a good Isolator and Monitor I have both presently for sale, as I have recently upgraded...

Steve, rather than rehash what's been said many times over here, why not google for the answers re the different types of solar panels? The 'net is absolutely full of info about them.
About your panel wattage, it also depends how you are going to use the panel(s). Like, are they going to be fixed onto the roof rack and remain oriented flat? Are you going to site them at each stop so they face the sun for maximum output?
It *will* make a difference. Having them fixed on the roof is the least hassle but it also means you'll have to park the cruiser in the full sun every time you stop.

If you use mono or poly crystalline panels you have to make sure no part of the surface is shaded, a shade from a branch or something means the panel output drops to zilch. Amorphous panels give you a proportional output, shade half the panel and you get roughly half the current of the full panel in sun. These panels are also near twice the size of the other tyes, for comparable outputs.

If it was my trip I would use at least 120W of solar power AND make sure the panels get lots of sunshine whenever the engine is not charging the battery.

The best thing you can do is getting a little needle type Ampmeter ( rated for the maximum panel output current), fit it into a plastic project box and let it monitor the panel output all the time. It will give you a *very* good idea what's going into your battery. If you get a three stage solar regulator ( recommended) the meter will also show you when the battery is getting near full as the needle will start 'pulsing' rather than showing a steady charge current. When the battery is almost completely full the 'pulses get gradually shorter and the pauses longer until the charging current drops down to the trickle value which is most likely lower than the meter will register.
A simple Ampmeter is *much* more useful than a voltmeter for this purpose. It also draws virtually no power for itself and requires no batteries to work.
Klaus

Hi Steve, I have just gone through the same thing myself.

If your fridges draw 3.6 Amps for the 2 running at the same time and you work out a conservative duty cycle of approximately 40% on 60% off then over 24 hours you will consume around 35 Amps / Day. Now the duty cycle does change quite a bit depending on ambient temperature.

A solar panel has a max current rating and for an 80W panel this is 4.55 Amps @ 17.6 Volts (4.55 * 17.6 = 80W). This is what the manufacturers claim.

In reality however a solar panel will only produce around 75% of the claimed current. This is because a panel is rated under test conditions and things such as panel temperature, atmospheric conditions, orientation, cable loses etc will have an impact. Also the fact that we are using the panel in a 12.8 Volt system (even though this contradicts ohms law of power = voltage x current) will add to the problem.

So for an 80W panel you will most likely get around 3.4 Amps. You may get more or less but there is no way of exactly working out what you will get until on the day.

Now work out what you will get for the day by multiplying the panel output (3.4 Amps) by the number of Peak Sun Hours in the day. This will vary on things such as Latitude and season. During Summer 6 PSH is usual and Winter around 3 PSH.

Therefore an 80W panel will produce around 21 Amps (3.4 x 6) on a sunny day. If you plan to stay in one spot indefinately then 2 panels should do the trick (providing it stays sunny) but if you only plan to stay in the one spot for a few days you should get away with one.

Regarding regulators I would recommend a 3 stage and the best I believe is the Plasmatronics PL20. This has everything but is around $350. I bought a Manson SBC-7120 off ebay for $130 and it does the same thing as the PL20 except does not show Ahrs out. The PL20 may have a couple of addition features but these are options which cost $. Do a search on the Solar sites in Australia for a better understanding of features, costs etc but also setups.

Regards
David