Solar panels - what does the watt really mean?

Well what practicle difference is there in the ability of a 4hp outboard motor and a 6hp motor to push a 12ft tinnie along ?......you need to tell the audience here just what you want to power to get a reasonable responce, and do some more reading.

I bought a book from Collyn Rivers entitled 'Solar that really works'.... explains it all, in detail. His site is [ www.caravanandmotorhomebooks.com ]. There are of course other references on line. As I understand the points you raise - the hotter the actual solar wafers, the less efficient (and they get quite hot inside that 'glasshouse' even on a mild day - imagine the temperatures, lunchtime, Birdsville in January :-o). As for W ratings - work on about 70% of the nominated rating as the best you could ever get ..... cold day, bright sunlight directly overhead. The rating... 80W - 140W etc. is established under perfect flash testing in the factory....unlikely to approach anything like that in the field.... but fair enough as a performance guide between panels. Over to others re performance on partial shading.... some panels are better at it.

I think you'll find amorphous panels are more forgiving in partial shade than is the case with mono/polycrystalline, however they are generally larger in area for the same size output.

Obviously size matters (isn't that usually the case). The bigger output panels will charge your battery quicker + run your fridge e.g. at the same time.

Krooznalong
I am not an expert, just speaking from experience in using a panel.
Get the biggest you can! Size does matter! I believe shade will affect all panels and it does not take much shade. Ideally for the best performance, during the day you need to move the panel to face the sun. Some caravans have multiple panels fitted to the top so although they do not face the sun, the combined size produces enough power.
It also depends on how you are going to carry the panel. Do you want a folding panel as an example, or are you going to fit it to your camper/caravan? Remember the cable size also matters.
Heat will affect the panel. We carry a 120 watt folding panel and the best performance we have got was in Tasmania on cold days. In the Kimberley not so good.
Our panel has 10 metres of heavy duty cable so we can place the panel in lots of positions so it can get the best sun. We also have a 10 metre length of security cable.
Do your research and remember, some people will have different opinions on what is best. From reading lots of threads I am not so sure price does matter. Some get brilliant results from the cheapest panels. Don’t forget you will need some sort of regulator/controller. We have a MPPT Solar Regulator fitted to the panel and a STECA Solar Charge Controller fitted to our camper. Generally when camping I just plug the panel straight to the STECA Solar Charge Controller. Again, lots of different opinions on what is best there and some people on this site are very passionate on their opinions.
Again, before someone attacks my opinion, it is just from my experience.
Kevin

Mate,
I will give you a real life instance of the output versus temperature.

our little grid connected house system puts out a a maximum of 12.5 Kilowatts on good days in September. Now we are in November and getting hotter the maximum is around 11 Kilowatts even though we are getting more hours of sunlight. This will go down to around 10 Kilowatts as summer progresses.

Guess you know it is not the ambient temperature but the direct sun temp that is the biggest killer. Panels will reach 60 degrees easily. You will notice they are never mounted directly onto anything. They are mounted higher so as to get air under them to help cooling.

With the price of panels now go as big as you can if the panel will fit or can be easily carried or stored.

RA.

It is true that Solar panels are more efficient when they are cool but it isn't going to be a big issue. They lose about 4% of their output for every 10 degrees. So a panel at 45 degrees will output about 15% less than it would at 25 and it will output about 5% more at 15 degrees.

The fix for this is get 20% more than the rating and use a MPPT regulator. You lose 30% off the top if you get a PWM regulator anyway.

Also yes, Mono and Poly panels will drop to almost zero output when shaded.

The fix for this is get 2 - 3 smaller panels instead of 1 big one and wire them in parallel so if one is shaded then the other 2 will still work at capacity.

Most people with a fridge and lights and light stereo will need about 160W of rated capacity. If you have 2 fridges or use fridges on freezing then that may go up to 240 - 300W ( freezing).

They are cheap now so it is a case of the more the better without being silly.

Oh, Amorphous panels will work relatively better in the shade but cost more and are less efficient to start with. Forget them.

If you are really worried about shading, get some panels on the roof and some portable ones and have about 200AH of battery.

I have 115W on the roof and a 80W portable but have managed to last 5 days and still have 100% charge in Melbourne in winter, and that is with a big fridge water pumps, lights, and big stereo.

Boobook's comments about performance v's temperature and shading are spot on.

Just how much solar capture and battery capacity you need depend on your individual usage patterns. To run a few LED lights you'll need very little, but to run any fridge for long you'll need over 100W of nominal capacity plus at least 100 Ah of battery capacity. To run as a freezer, double that.

Suggest you'll find Electricity for Camping a useful read.

Cheers

John

Thanks for the replies. I know know a lot more and I think I know what I need. Christmas present for self coming up. Off to ebay!

You can get involved in all the minor technicalities, but the single message you neet to take away is that you need a lot more pannel and battery capacity than a lot of poeple want to believe for reliability and long term performance.

If you want to run an efficient single 40 litre fridge run as a fridge on a continuous basis, you need a absolute minimum of 160watts of pannels and 100AH of battery.

Any less and all you will be doing is prolonging the run time between charging from other sources.

The more pannel and battery capacity you have the easier life your battery will have.

Go in undergunned and your battery will have a short life and your beer wil not be cold.

cheers

There are some good points mentioned above, and some less good.

1/ Battery life, assuming a traditional lead/acid battery (wet cell, AGM or gel) depends on the depth of discharge of batteries being kept at minimal levels. "Deep cycle" batteries will still last longer if they are not regularly discharged more than about 25%. For example, they may be rated at 2000+ cycles at 20% depth of discharge, but that may decrease to 1000 cycles at 40%, and far fewer at higher discharge levels. The batteries need sufficient capacity to power the load for the longest foreseeable time between full charges, and still stay at above 50% charge levels. So a 100Ah battery should be regarded as be able to provide a maximum of 50Ah (and preferably less) if you want it to last. Lead acid batteries also need to be fully charged, their life will be significantly diminished by under-charging.

2/ MPPT is a technique used to maximise the power from the solar panels. Solar panels produce a fixed maximum current, but their voltage is driven by what they are attached to. A discharged battery may have a low voltage, say 10V, which will become the output voltage of the panel if the it is connected directly to the battery. For a 200W solar panel, the maximum power will be (for a "12V" panel) about 11.1A at 18V. The 11.1A is a fixed maximum, so if that voltage is reduced to 10V by a discharged battery, the power becomes 11.1A x 10volts = 111W. The MPPT process allows the panel to operate at it's "best" voltage, no matter what the
voltage of the battery, so the panels can produce 11.1A x 18V = 200W. The charger then re-regulates the output voltage to meet the batteries requirements.

3/ PWM is a battery charging technique. Traditional battery chargers produce a set voltage (or several set voltages). This causes current to flow to the battery. As the battery charges, its resistance increase and so the current decreases. Eventually the current drops to near zero, at which point more complex chargers enter a second charging stage at a higher voltage. Good chargers have three such cycles, some have more but that is just marketing. PWM is a technique that monitors the battery and uses short pulses (PWM= pulse width modulation) of higher voltage to charge the battery. However this technique is only appropriate to the later stages of charging, the initial "bulk" charge is still at fixed voltage. PWM therefore allows a slightly higher rate of
charge, but only of the last "top up" stages of charging.

4/ Panel rating. Solar panels are rated at a set temperature under set illumination. Output decreases slightly with increasing temperature. The light levels used to specify full output are only available for a few hours around midday in full summer, so for most of the time they will output less than their label rating. The amount of light hitting a panel is dependent on its angle to the sun, so whilst a horizontal panel (on a vehicle roof) will be ideal at midday in the summer when the sun is directly overhead, at other times it will be far less than optimum so for best results the panels should be realigned every hour or so to track the sun. When averaged over the whole of Australia, and the whole of a year, a panel will output around 4 times it's label rating in watt-hours per day. This average will obviously include far higher values in northern Australia in summer, and far lower ones in southern areas in winter. But on average a 200W panel will produce around 800Wh per day. At 12V that equates to 6Aah, but losses in the system and battery will reduce that by perhaps 20%.

5/ Shading. All panels are badly affected by shading, and the differences are not worth worrying about. Nominal 12V panels often do not have diodes built in, so a partially shaded panel can allow current to flow backwards through the shaded cells, reducing output still further or (for really poor/cheap designs) allowing the battery to discharge through the panel.

6/ Other points of interest. Flexible panels are easy to store, but tend to be soft and have lower efficiency (less power for a set area). Grit and dust can scatch the surface, reducing the effciency even further. Dirt, dust, shading and misalignment will reduce the output of any panel by a considerable degree.

7/ Electrical measurement. Amps are a measure of the rate of flow of electricity. Amphours (not "amps per hour") is a measure of the amount of current that has flowed, where 1amp for 1 hour is 1 amphour. This means than 1 amp for 10 hours is 10Ah, and 10A for 1 hour is also 10Ah. Watthours (Wh) follows the same concept as amphours, i.e. it is watts x hours
watts = amps x volts, So to get from Ah to Wh you multiply Ah x voltage.


What this means is that you need firstly to work out how much power you need. Whilst travelling, your batteries should be charged by your vehicle (if set up correctly). If you are travelling every day, you probably do not need solar at all. If you are stationary at an unpowered site for a week, on the other hand, then your solar may - if you don't run a generator or your vehicle - need to provide all the power. A fridge will use more or less power depending on ambient temperatures, and manufacturers often quote useless numbers (like "max amps") rather than anything useful. My Engel fridge has a maximum current of 2.5amps. If the thermostat causes it to run 100% of the time, it would use 2.5Ax24h=60Ah per day. If I allow a conservative 70% duty cycle (the time the thermostat runs the fridge motor), that comes down to 42Ah per day. So my solar panels, if they are the only source of power, need to provide more than 45Ah per day to run the fridge. That is 45Ah x 12.8V = 576Wh. In a northern Australian summer, that would easily be provided by 576/5= 115W (so 120W) panel. In a southern winter I may need 576/3=192W (so 200W) or more to do the same job. Given a cloudy sky causing a 50% reduction in panel output? Or a few lights that suck more power?

Any solar panel will add some power, but you'd need a lot of panels to power a couple of fridges, water pump and camp lighting under worst case scenario with no other power supply. What particular level you can survive with, or afford, or require comes down to where you are going and what you are powering!