Simple Solar For A Simpleton

[cwyckham]

A rule of thumb that I've seen quoted many times is to take your daily use in amp hours and multiply by three to get array size in Watts.

The issue becomes mounting and overall size, I think, especially on a monohull. Some of the new panels are very efficient.

I'd skip the ones you can walk on. They seem pretty useless from what I've heard.

I think you're better off with all panels the same to get max efficiency from the mppt controller as all the panels will have the same voltage-current curve.

[Celestialsailor]

Originally Posted by atoll

when choosing panels it is a good idea to be able to tilt them to face the sun.

...Yup

[SunDevil]

Keeping even little shadows from covering a few cells is more important than angling them. Unless you have a house boat parked in a single marina, it is also hard to know what direction to angle the panels and when, and that is a lot of work multiple times a day for a few more amps.

[Celestialsailor]

The newer panels are better than the old ones ofr this.

[Jimbo485]

[QUOTE so basically you will need to average about 350-400 amp hours a day if you dont want to run a generator.

so basically you want enough panels to charge at 50 amps for 8 hours a day. so 600watts of panels QUOTE]

Sorry, Gents, I reckon that is real conservative and more than half of the energy needs will need to come from non-solar sources. Call it 400 Ah per day of consumption, so Vic, you need a total of 1600W of panels if you don't want to run a generator. Maybe 1200W if they all have the same open circuit voltage and you have a good MPPT and are prepared to run that generator sometimes.

Vic, you also need 1600 Ah of total battery capacity to become independent of your generator. 1200 Ah if you rely on your non-solar sources of energy. What is your aim? What do the local prices and the maths tell you?

As Atoll pointed out, the maths is clear. And the conclusion is inevitable, until you run out of deck space or decide to reduce your electrical consumption.

Some may roll their eyes and think WOW, TOO EXCESSIVE. But here we are, no generator, a 6A alternator on a little 8 HP outboard (ie. zero) and those are the real figures if you don't want to rely on internal combustion engine backup energy supply.

Vic, you have the backup, so go for the 1200W of solar and the 1200 Ah of battery capacity and use your generator every now and then. Or go for 1600 if you can fit the panels and batteries and throw away that generator.

[Jimbo485]

Congratulations on getting to the core of the current loss!

A little more maths for the previous post. In very general terms, your theoretical panel output in W should be divided by 12 (some will say 10) to give the output in A. That is applicable, averaged out over the day, with a bit of cloud, low angle, etc, for 4 hours. 3 hours to be conservative. You want 400 Ah? That means 400 Ah divided by 3 hours, equals 133 A, multiply by 12, and it roughly gives you 1596W of panels. In real life, you will never see 133A, you might see about 80 A at midday, but over the course of the day, these average rule-of-thumb figures work out.

In conclusion, if you want 400 Ah, you multiply by 4 and that is your solar requirement in W. This is real life. With 970W, we see a max of 50A and consume about 250 Ah when on passage.

You ideally use your batteries from 60% to 85% capacity, so that is 85% - 60%, ie 25% or one quarter of your consumption in Ah. So multiply your consumption by 4 to get the battery capacity.

Hope this helps thru the next stage, Vic.

[noelex 77]

These rules of thumb are often quoted and while they are useful those planning solar installations keep in mind that solar output is very dependent on location and time of year.
In my current location solar production varies 4:1 between summer and winter and this is without changing geographic location.
The rules of thumb don't take these large variations into account. Battery capacity, no matter how large, cannot even out the solar production between summer and winter.

Insolation values are available, for most locations, over different seasons. Using these to plan the size of your solar array, is much more accurate, particularly if planning a boat and trying to decide if you need a generator, have enough power to run a Watermaker, or microwave etc.

In the OP case fitting the most solar possible is the sensible alternative, but it's not clear, without more information, what the practical effects of this will be.

[Jimbo485]

Yes, Noelex 77, they are for the tropics with occasional cloud cover and are based on our own real experience with no energy supply from internal combustion engines.

And these estimations give panel sizes that are far greater than those on most boats. Which just indicates how much most boats use their generators or engines to supply energy when they think otherwise.

To even out the summer / winter effects, you need more solar panels, not increased battery capacity. So in your case, you need 16 times your consumption for the panels and still 4 times for the battery capacity in order to survive the winter, without burning fuel thru a piston.

Vic is up north right now but will be heading south into the tropics on his way back to Australia. What factors do you think he should use?

[noelex 77]

I think in Vics case even with the most solar he can fit he is still going to using the generator for a signifficant time, but this is is only a guess.

Generally there is nothing wrong with ignoring day to day variations. The insolation values are averages anyway An adequate battery bank will even out most day to day variations.

However, the battery bank is not large enough to even out seasonal or geographic variations. Since you still need the power, it's sensible to use lowest insolation value in your calculations when determining solar output and consider how you will manage in these conditions.

I have crused extensively around Australia for example. As my boat had no generator at that time, I had to size the solar array so that I had adequate power for southern Australia during winter time. Solar output here was about 11AHrs a day for every 100w.

My concern is people often apply the "solar panel rules" of 30-40Ahrs per 100w and make decisions about what appliances to fit, or the need for alternative energy sources like wind or generator based on these averages. The averages are realistic, but they don't help in the middle of winter away from the equator. On the other hand they are too pessimistic for a boat that spends all its time in sub tropical waters, or only cruises reasonable latitudes during the summer.

Data is available to make a much better assessment of the likely solar panel output based on location and time of year. I think given the wide variation in insolation values many cruisers would be better served using this more accurate data and applying that to their individual circumstances.

Quoting solar output averages is a bit like saying the average tide worldwide is 4 feet. Accurate, but probably not a great deal of help.

[atoll]

good imput there from guys actively cruising!
just had a look at that calculation i did which was calculated at a 100w panel putting out 8amps,this in fact is not the case,a 135 w panel is rated at near 8 amps,a 100w at 5.7 a or there abouts.

so to revise,as a minimum 6x 135 w panels,giving 800 watts.

this is still not enough for during periods whilst under way,but sufficient if at anchor!

what i would reccomend is in addition to the 6 panels is fitting a wind generator on the mizzen mast,such as an aerogen,they are fantastic and in the tradewinds pump out on average 6-8 amps 24/7,but have seen as much as 35 amps on windy days!!!

from my 22 years of living on anchor and 100,000 miles of passagemaking, i have observed that generally when there is a lot of cloud cover it is also windy!

[Jimbo485]

All good points, boys!

But what is Vic gonna do?

[atoll]

Originally Posted by Jimbo485

All good points, boys!

But what is Vic gonna do?

i think he might have gone into boat dollar$$$$$$$$ shock!!!

[atoll]

i found a good article comparing 4 types of wind generators here;

http://www.naviclub.com/Test_compara...nes_marine.pdf

[noelex 77]

Good find atoll I have not seen that artical before.

It would be worth starting a new thread with this so someone with an interest in a new wind generator can find it easily.

[VirtualVagabond]

Originally Posted by Jimbo485

All good points, boys!

But what is Vic gonna do?

Mmm...
Ok... firstly thanks for the input. All very helpful stuff.

Firstly a report back.
With the mystery ignition switch turned off, we're down to -11.8 amps on the Xantrex. When the top load fridge cycles it drops to -8.1, but I'm not sure how often that happens.
Last night I turned the top loading fridge off for 10 hours and we're watching fans and lights.
The Isotherm folk emailed back and confirmed that the other fridge/freezer we have does consume 7 amps, and the only way to reduce it is to maximise air circulation to the back, which is not easy to improve in the short term because it's very built in and already has a fan assisting.

So, it appears we may be able to reduce our consumption to somewhere around 230 amp hours per day at anchor. That is a vast improvement.
I also only had to run the genny for 1 hr 35 mins this morning.

It makes sense to me to get as much solar wattage as I can a) afford and b) fit, without looking like an aircraft carrier. I do need to be able to move around on top of the doghouse occassionally so will have to leave footpaths.

I always assumed we would have to run the genny a bit as well, but we will be doing most of our sailing in the tropics and sub tropics so hopefully that will be minimal.

Vic