I recently purchased a caravan and ordered 2x100 watt solar panels to be put on the roof. However, recently I went to another premises to purchase a 3rd deep cycle battery and was told by the proprietor that the solar panels will only be 80 each and I should have at least 300 for the 3 batteries.

I measured the solar panels and counted the squares – 4x9 squares and went into JCar and they also told me they think I have only 80 watt panels.

If anyone out there could please help me to identify and verify whether I have 100 watt panels as I ordered and paid for.

Thanks for your help in anticipation.

The Major
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Reply By: - Johny boy (NSW) - Wednesday, Apr 23, 2014 at 15:35

Wednesday, Apr 23, 2014 at 15:35
I didn't think that the squares were how you measure them but surly you can measure Wattage with a multi metre??

cheers John.
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Reply By: HKB Electronics - Wednesday, Apr 23, 2014 at 15:36

Wednesday, Apr 23, 2014 at 15:36
I'm no expert with regards to solar panels but I just did a quick search of Ebay and the 100W panels advertised there have 4x9 squares as you put it.

A check of the specs shows max power current 5.187A and max power voltage 19.28V

P=ExI = 5.187x19.28 = 100W

As for how many watts do you need, battery count has nothing to do with it really, it is the demand that determines who many watts you require ie if you take 100Ah out of your batteries overnight then you need to put it back in the next day. You'll need to work out your demand and size your panels accordingly.

Fixed panels won't be as efficient as a pointable setup would be.


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Reply By: Member - Bruce C (NSW) - Wednesday, Apr 23, 2014 at 17:46

Wednesday, Apr 23, 2014 at 17:46
The rule of thumb and experience tells many of us that 100 watts of panel for each 100 Ah of batteries is a good starting point and will suffice in most average situations.

More panel capacity is better if you can afford and have the space for them.

If running all the normal devices such as small compressor fridge (Waeco etc.), lights and ancillary equipment the above rule of thumb works well and 200 Ah of batteries will usually suffice.

If running a large compressor fridge such as a Vitrifrigio then 400 watts of panels and 400 AH of batteries would be best to avoid potential lack of storage and generation capacity.

Cheers, Bruce.
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Reply By: Member - John and Val - Thursday, Apr 24, 2014 at 07:09

Thursday, Apr 24, 2014 at 07:09
As Bruce says, 100W of panel for each 100 Ah battery is a good rule of thumb. If your panels are the ones Leigh refers to then they are spec'ed at 100W. They are rated to deliver under optimum conditions about 5.2 amps at 19.3 volts.


most batteries require charging at up to about 14.4 volts. Using a simple PWM charger the battery will load down the panels enough to drop the panel voltage down to 14.4V, with the current still at about 5.2 amps. That's about 75 watts. Using a MPPT controller the panel will operate at it's optimum voltage (19.3V (sounds a bit high, but let's accept it)) and will convert the voltage so as to deliver 14.4V to the battery. The excess will be traded for higher current, so that you may still get close to the 100W.


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Follow Up By: Mike S2 - Thursday, Apr 24, 2014 at 09:09

Thursday, Apr 24, 2014 at 09:09
Hi john,I have 2 x 180w panels on my van going thru a 30amp mppt regulator to 2x 120 ah batteries, the most current I have seen on the amp gauge is 14-16 amps going to them,I would have thought closer to 23-25amps on a decent sunny day would be the norm,maybe a problem with one or both panels?,what is your take on this,regards Mike
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Follow Up By: Member - John and Val - Thursday, Apr 24, 2014 at 12:29

Thursday, Apr 24, 2014 at 12:29
Hi Mike,

There are a lot of variables involved so I wouldn't initially suspect your panels.

First there' s the question of where your current (amps) meter is installed. If it's on the panel side of the regulator, then under optimum conditions, and provided your batteries will accept it, I'd expect to see about 20 amps maximum. If it's on the battery side of the regulator, then 10-15% more, ie your 22-23 amps under optimum conditions, and again, if the batteries will accept it.

Secondly, the location of the regulator is far more important than generally recognised. It should be close coupled to the batteries by decent heavy cable so that the voltage drop between regulator and batteries is minimised. The regulator takes its cue from what it sees as the battery voltage and if there is a significant voltage difference between regulator and battery not only will this result directly in reduced charging, it will upset the controller's calculations so that it will likely reduce the charging rate.

Thirdly, the batteries are an important element here. If they are in good condition and well below being fully charged, then with your battery capacity they should accept (at least initially) all the current your controller can supply. They will not accept so much when approaching fully charged, indeed the controller will taper the charge rate anyway at this time in response to the increasing battery voltage.

Fourthly, the metering is important. If using an analogue style (moving needle pointing to a scale) accuracy may be pretty awful (probably within 10%). If it's a digital meter, properly installed, is probably within 1%.

A really important measurement is the battery voltage. When charging strongly, voltage will be below 14.4 volts for almost all deep cycle batteries. Voltage will rise as charge builds up until it reaches about 14.4 when the batteries are about 80% charged. The controller will then maintain this voltage while the charging current declines to a low level. The battery is then considered fully charged and good controllers drop the voltage down to about 13.4, which provides a trickle of current. (I've referred to 14.4V, though this is a typical figure and different battery types and different controllers may have slightly different thresholds.)

Another variable is of course just how optimally oriented are your panels? If horizontal and you are in southern Australia, you will not achieve anything like full output. (Reduction from the maximum is related to the sine of the angle from normal incidence, so near enough is ok, but larger departures from optimum can have a substantial effect.)

Suggest hang a voltmeter on your battery. If everythings behaving roughly as above, but you never see 20+ amps going in, then maybe panels, maybe batteries have an issue. It would be worth temporarily separating the two batteries - do both perform the same way? accept the same charge under similar conditions?

Sorry to be so verbose, but there really isn't a quick answer!



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Reply By: Member - Boobook - Thursday, Apr 24, 2014 at 09:58

Thursday, Apr 24, 2014 at 09:58
The panels should have a sticker on the back.

included in the specs should be something like

Voltage Peak 21
Current Peak 4.9

Multiply the two values together and you should get 100 ( w)

As to the total power you want, there is no real relationship between the battery sizes and the solar sizes other than more of both is good.

The key factor is your load, and power use and sun availablity. Google richards solar spreadsheet. You really do need to calculate several parameters and using a rule of thumb will leave you with a system with undetermined capacity.

For example 300Ah and 200W is fantastic if you are in Alice Springs in Winter and have a fridge, some Lights and say a TV and computer. It would be near useless and be lucky to last 1 1/2 days with a freezer, fridge and same TV / computer in a riverside camp in the High Country or at Cape York at Easter.

It only takes a little effort to get it right so you know what you capacity you have. Do it right.
AnswerID: 531108

Reply By: The Bantam - Thursday, Apr 24, 2014 at 22:47

Thursday, Apr 24, 2014 at 22:47
Further to what has already been said..

There is a deal of variance about how solar pannels are specified.....but without exception solar pannels will only deliver their rated pouput under, full sun, arround midday and in a high solar radiation area.

If for example you are in Tasmania in winter..ya best case will be 25 to 30% of rated and for a short period in the middle of the day.

Running pannels thru an inefficient network with light cables and a poorly located charger and you may only realise half the output of your pannels.

As for the people in shops.....well unless that shop was a good solar specilaist and an honest one.....their experience will be limited and their opinion of little value.

There are all sorts of people who will sprout simple rules of thumb for solar systems.....even your truly.....but the fact is it is not that simple.

If you want to make a properly informed decision you need to factor may things.

Where you will be traveling and when.

what your electrical demand is.

How long you will be expecting your system to stand up on solar alone.

What other charge sources there are.

A rough rule of thumb is.
If you expect to support a 40 litre co pressor fridge run as a fridge, on an ongoing basis and in reasonable solar radiation, you will need about 200AH of batteries and at least 200 watts of solar pannels.

but that is just a rough rule of thumb.

many of the off grid solar systems have very large battery capacity compared to their solar capacity...the intention is that the batteries will hold up over several days of poor sun and good sun and poor sun will average out...they do not rely on being able to fully charge the batteries in one day

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