How much power does solar panels actually generate?

Hi acei,

2oldman has just moved over to 24 volts and has a dc to dc converter from Victron to meet his 12 volt needs.

So far it appears he is getting a larger solar harvest, but he has not run it in winter time yet.

BTW Victron makes a better power meter than anyone else--and it is a bit cheaper than the nearest competitor. Talk to mena about his.

I'm a bit of curmudgeon so I do not have an amp-hour counter. I'm also way out in left field on batteries as almost no one successfully uses a multiple 12 volt array.

I believe in saturation charging, which means 14 to 16 hours at a fairly low amperage (1/5 of an amp). That's usually impossible to do with solar, for the sun is rarely up for 16 hours. Because of having two banks and appropriate switching, I can use one bank to do a saturation charge on the other, if I choose to.

Hi acei,

There are solar insolation maps available for the whole world--just google for a link.

24 or even 36 volts has many advantages over 12 volt. Here are some:

Wire size is rated in amps--so a #10 wire @ 12 volts can only do about 30 amps = 360 watts. Where as #10 wire @ 36 volts can do 1080 watts.

six six volts in series removes the issue of parallel balanced wiring.

The components in an inverter don't have to work as hard and are therefore likely more efficient when going from 36 volts DC to 120 volts AC.

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The parallel issue can be mitigated somewhat by using balanced wiring (see the smartgauge site if you wish to understand that better).

Statistically speaking the greater the number of cells the more likelihood there is of a failure. The other side of the coin is that when in series each cell must provide the full number of amps demanded. In parallel each cell only caries part of the load. For example:

6 series 6 = 12 volts; put a 100 amp load on and each cell must produce 100 amps. The weakest cell will limit the flow of current--so if one cell is "weak" and can only do 95 amps the bank will have that as an upper limit.

12 parallel 12 = 12 volts: put a 100 amp load on and each cell needs to produce only 50 amps. The weakest cell will NOT limit the flow of current so if one cell could do only 40 amps the other paralleled battery would do 60 amps to make up the difference (hopefully that never happens).

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The other problem with a 12 volt bank is that one weak cell may cause that battery in the parallel array to act as a parasite on the others, fairly quickly drawing the capacity of the other batteries down to the weakest cell.

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Right now on another thread someone is having issues trying to run a microwave on twin six volt jars. It runs for about four seconds and then the microwave shuts down. The inverter doesn't appear to be going to fail over condition. His problem is simple. Six volt batteries, with their thicker plates, are not intended for high amperage loads. Twin 12 volt batteries would easily be able to do what he wishes, if they are wired like this:



As it often doesn't cost a dime more to do this, I think it is worth the trouble.

smkettner wrote:
Smaller controller at 1/2 the amps. (45 vs 80)
Smaller battery cabling to inverter.
Better balance with four 6v batteries in series. Eliminates parallel battery issues.
Yes the higher power items will be more efficient. Even the controller has better efficiency.

Not saying it is end all better but larger systems tend to use higher voltage.

Sounds interesting. So some components will be cheaper but there will be an additional converter (24->12v)?

What is this "parallel battery issue" you are referring to?

Also, I wonder if there's any Danfross 24V fridge around...

Smaller controller at 1/2 the amps. (45 vs 80)
Smaller battery cabling to inverter.
Better balance with four 6v batteries in series. Eliminates parallel battery issues.
Yes the higher power items will be more efficient. Even the controller has better efficiency.

Not saying it is end all better but larger systems tend to use higher voltage.

smkettner wrote:
With this much power I would think about a split 24v / 12v system.
Just need a 45 amp solar controller to feed 24v. Inverters would also run off 24v directly. Use a 24/12 Dc converter to keep a 12v battery charged to run the other systems.

Does the fridge run on DC? Is it available in 24 volt?

I think 2oldman just went this route and seems pleased.

What would be the advantage of this split system? Less power loss?

westend wrote:

I believe your AH estimate for a fridge is on the high end. If you go with the Danfoss 12v compressor unit, you should see about 20-40 AH/day, depending on use and ambient. I use a small dorm fridge and it uses 80w at a 1/5 duty cycle. The newer Samsung 10 cu. ft. models use just a bit more.

To each, his own, on the module choice. I don't see that the Sanyo HIT modules are that much more efficient to harvest 20% more power. I chose to buy a discontinued 235 watt USA made panel for about a third the price of the Sanyo. I'm about 99% sure any warranty is out the window but the Mfg. is still building modules. If warranty is important, then a Sanyo or Sharp module makes sense.

Good luck with your solar installation!

Edit: Yup, 900AH for those Trojans, I should have caught that. Still, a heck of a lot of capacity.

The fridge in reference is a 20cuft residential one. This is based on the yellow sticker that all fridges come with. But I agree that if I choose a Danfross compressor based one, it will consume maximum half this, if not less.

As for the Sanyo panel choice - one of the bigger reason for this choice is its use of amorphous cells which are better under overcast days. I didn't want to buy 100% amorphous panels because they are quite a bit less efficient under good lighting conditions. Hence initially, I thought about mixing mono-crystal panels and amorphous panels but apparently, that's not a good idea unless I go with multi-controller route which I don't want to consider. At least in theory, the Sanyo panels with both types of cells sound pretty good. having said that, I don't have a proof that these panels work significantly better than 100% mono-crystal panels under shades. Need to do more research. If it only generates 5% more power under shades as well, then I would not go with this panel.

pianotuna wrote:
Hi Acei,

It is better to work with "worst case" and have a pleasant surprise than to be over optimistic and have a nasty surprise in the form of a short fall of energy. Obviously we get many more hours of sun in June than in December. But energy use may often be much greater in December.

Here is a place to do some "look ups" on solar insolation rates:

Photovoltaic potential

Since you are in the Toronto area this may be useful:

Toronto, Canada

Acei wrote:
I will need close to 1000w of solar panel according to the sheet (based on the assumption that I only have 4 hours of sunlight per day - is this a good assumption?)

Interesting links! Is there an equivalent one for US as well? :)

With this much power I would think about a split 24v / 12v system.
Just need a 45 amp solar controller to feed 24v. Inverters would also run off 24v directly. Use a 24/12 Dc converter to keep a 12v battery charged to run the other systems.

Does the fridge run on DC? Is it available in 24 volt?

I think 2oldman just went this route and seems pleased.

Acei wrote:

westend wrote:
Have you done the calculations for daily use? 1800AH of storage is huge, also 500 lbs. of added weight.

The Sanyo panels are nice but I doubt the price will bear out any increased efficacy over the life of comparable modules, sold at much lower prices. Of course, value may not be your criteria. There is worth in buying the higher priced item, sometimes.

Edit: Saw where you used the spreadsheet to calculate daily use. Something is probably amiss (like refrigerator duty cycle?), you are about double what most power gobblers report for daily use.

I've simply done it based on a residential fridge power consumption. I took annual power consumption number from bestbuy.com then figured out the daily consumption from that. It worked out to be about 100Ah. Does this sound about right? Alternatively, I can go with that compressor based one which appears to use about half that, but obviously the capacity is far smaller.

And the Sanyo panels - I have three reasons for this panel. First is, as you mentioned, for additional few % efficiency, which I agree isn't enough to justify the price difference. The second, is that the pannel _should_ (I didn't confirm this yet) work better under less optimal lighting conditions due to its hybrid use of amorphous and mono cells. Third, is more of an insurance. This combo overall may only return an additional 20% power over a panel that costs half as much (inc. cloudy conditions), but if I do end up using that extra 20% once a month, I would pay for it. Besides, I do plan to keep this trailer for 8+ years, so I don't think the price difference is going to make that much difference at the end.

Having said that, if I made wrong assumptions, please point them out!

I believe your AH estimate for a fridge is on the high end. If you go with the Danfoss 12v compressor unit, you should see about 20-40 AH/day, depending on use and ambient. I use a small dorm fridge and it uses 80w at a 1/5 duty cycle. The newer Samsung 10 cu. ft. models use just a bit more.

To each, his own, on the module choice. I don't see that the Sanyo HIT modules are that much more efficient to harvest 20% more power. I chose to buy a discontinued 235 watt USA made panel for about a third the price of the Sanyo. I'm about 99% sure any warranty is out the window but the Mfg. is still building modules. If warranty is important, then a Sanyo or Sharp module makes sense.

Good luck with your solar installation!

Edit: Yup, 900AH for those Trojans, I should have caught that. Still, a heck of a lot of capacity.

Hi Acei,

It is better to work with "worst case" and have a pleasant surprise than to be over optimistic and have a nasty surprise in the form of a short fall of energy. Obviously we get many more hours of sun in June than in December. But energy use may often be much greater in December.

Here is a place to do some "look ups" on solar insolation rates:

Photovoltaic potential

Since you are in the Toronto area this may be useful:

Toronto, Canada

Acei wrote:
I will need close to 1000w of solar panel according to the sheet (based on the assumption that I only have 4 hours of sunlight per day - is this a good assumption?)