Solar....how much wattage is enough

[Murflynn]

9 hours ago, Mike the Boilerman said:

.......................................

 

(Notice battery size didn't come into the calculation?!  ) 

 

but now you leave us (them) nothing to argue about.  

[Gareth E]

Tips for someone who's obsessive about reducing power usage and therefore generator running time in winter:

 

Get of rid of your electric fridge, get a gas one.

 

Only recharge phones etc. when the generator is running. The generator always produces more power than the batteries can take therefore this energy is effectively free.

 

Fill a couple of bottles with water when the gennie is running to make brews etc, same theory as above.

 

For the truly obsessive plan your generator running to coincide with your favourite t.v. program.

 

Only have 1 leisure battery. This is possible because, having done all the above your daily usage (from the battery rather than 'free' generator excess) is unlikely to exceed 20-30AH. On a sunny winter's day run the generator early in the day. The weak mid day winter sun might only give you 1 or 2 amps but with a single battery, this will be enough to fully charge the battery, saving hours and hours of genny running time.

 

Obviously you'll also need t.v., computer etc. with the lowest possible power usage along with led lights throughout, and only ever have 1 light on.  

[WotEver]

10 hours ago, Horace42 said:

Sorry, but I don't need to get it!  It is not me asking for advice.

Yes, you do need to get it. Either that or stop offering poor advice like “it depends on the size of the battery bank”. The bank is sized as I described above (step 3). Solar generation is sized according to how much generation is required, which is a balance of daily CONSUMPTION and space available for mounting the panels. 

 

As Mike clearly demonstrated, bank size is irrelevant in the calculation of ‘how much solar’

[WotEver]

10 hours ago, Horace42 said:

On the face of it, it sounds more like a rule of thumb for estimating battery size - a factor of 'three times'?  - I guess comes from 8 hrs use per day.

No, it comes from never taking the batteries below 50% and allowing a small margin. You missed that I wrote “at least” three times. You can go as big as you like. Where does 8 hours come from?  Neither step 1 nor 2 mentioned a third of a day, I referred to DAILY usage and generation. 

[WotEver]

10 hours ago, Horace42 said:

Anyway I am sure the OP, not versed in the finer points of these things, would like to know how the results of stage 2 can be used to get the size of the solar panel.

Geez... it can’t!  Let me explain very slowly again... The. Bank. Size. Is. Irrelevant. To. The. Amount. Of. Solar. 

[Alan de Enfield]

4 minutes ago, WotEver said:

Geez... it can’t!  Let me explain very slowly again... The. Bank. Size. Is. Irrelevant. To. The. Amount. Of. Solar. 

Repetition adds emphasis :

 

Geez... it can’t!  Let me explain very slowly again... The. Bank. Size. Is. Irrelevant. To. The. Amount. Of. Solar.

 

 

[WotEver]

10 hours ago, Horace42 said:

on the basis of you statement that 'this is true for any boater' it suggests boats without solar panels are included. Therefore any boat designed to your rules - and without land lines or running the engine, or using anti-social generators - or windmills - has anybody invented a wave-generator (operated by passing boats that don't slow down) - then if to rely on solar panels will need power equal to 1/3rd of the battery bank. 

NO!  It would require solar generation equal to the daily consumption plus around 10-20%.

 

It would also require the boat to move much closer to the equator to enable the generation to continue through October to March. The Mediterranean might do, but the Indian Ocean would probably be better. 

 

Besides, they’re not ‘my’ rules, they’re just the facts of life if using lead acid batteries. 

[Alan de Enfield]

11 minutes ago, WotEver said:

No, it comes from never taking the batteries below 50% and allowing a small margin. You missed that I wrote “at least” three times. You can go as big as you like. Where does 8 hours come from?  Neither step 1 nor 2 mentioned a third of a day, I referred to DAILY usage and generation. 

Just a guess but :

I reckon he is taking your 'factor of 3 times' and assuming that to get 24 hours usage the factor of 3 is used to multiply the 8 hours to get 24 hours (or one days usage)

[MtB]

8 minutes ago, WotEver said:

Geez... it can’t!  Let me explain very slowly again... The. Bank. Size. Is. Irrelevant. To. The. Amount. Of. Solar. 

 

That's not really an explanation though is it? Its a statement.

 

An explanation is a series of steps explaining the logic which arrives at an outcome. 

[WotEver]

Just now, Alan de Enfield said:

Just a guess but :

I reckon he is taking your 'factor of 3 times' and assuming that to get 24 hours usage the factor of 3 is used to multiply the 8 hours to get 24 hours (or one days usage)

Now I’m confused

Just now, Mike the Boilerman said:

 

That's not really an explanation though is it? Its a statement.

 

An explanation is a series of steps explaining the logic which arrives at an outcome. 

Fair enough.

 

You’ve already explained it clearly though (with a series of steps) but he still repeats ‘bank size’. 

 

 

So repetition of the earlier point seems valid. 

[Alan de Enfield]

1 minute ago, WotEver said:

Now I’m confused

It is always a concern when information / advice is provided by folks who do not have even a basic understanding of the subject and base it on their knowledge gained 30 years ago when they 'tried solar and it didn't work'.

One wonders if it 'didn't work' because they tried to apply their own advice ?

[WotEver]

Just now, Alan de Enfield said:

It is always a concern when information / advice is provided by folks who do not have even a basic understanding of the subject and base it on their knowledge gained 30 years ago when they 'tried solar and it didn't work'.

One wonders if it 'didn't work' because they tried to apply their own advice ?

Indeed

[Robbo]

Most of working out how much solar you need is really irrelevant in most cases especially on a Narrowboat.   Just have the max amount you want to give roof space for!  Some days it will be too much, some days it won’t be enough.

[MtB]

4 minutes ago, Robbo said:

Most of working out how much solar you need is really irrelevant in most cases especially on a Narrowboat.   Just have the max amount you want to give roof space for!  Some days it will be too much, some days it won’t be enough.

 

This sums up the calculations quite well, and is actually what I did. 

 

I kept adding panels until the roof space available was mostly used, then developed a separate way of charging the batteries adequately in winter. The input from the solar is simply too variable and unreliable to err, rely on.

 

 

 

Edited May 28, 2018 by Mike the Boilerman
To add a bit

[WotEver]

4 minutes ago, Robbo said:

Most of working out how much solar you need is really irrelevant in most cases especially on a Narrowboat.   Just have the max amount you want to give roof space for!  Some days it will be too much, some days it won’t be enough.

Absolutely. 

[reg]

4 minutes ago, Robbo said:

Most of working out how much solar you need is really irrelevant in most cases especially on a Narrowboat.   Just have the max amount you want to give roof space for!  Some days it will be too much, some days it won’t be enough.

I hope you are aware that Pragmatic answers spoil debates

[Ssscrudddy]

I once read somewhere to have 1AH of battery for 1W of solar. But in reality unless you're 1 of those that turn everything off & don't use electric, then you simply won't have enough roof space to cover your electric needs in the winter with solar.

[gbclive]

1 hour ago, Robbo said:

 

Most of working out how much solar you need is really irrelevant in most cases especially on a Narrowboat.   Just have the max amount you want to give roof space for!  Some days it will be too much, some days it won’t be enough.

 

Do this.

 

Then think about this.

2 hours ago, Gareth E said:

 

Tips for someone who's obsessive about reducing power usage.......

 

Sorted!

(Me - about 500 tiltable + 40A MPPT Tracer. Very happy, but of course I wish I had a little more ?)

 

[Alan de Enfield]

11 minutes ago, Ssscrudddy said:

I once read somewhere to have 1AH of battery for 1W of solar. But in reality unless you're 1 of those that turn everything off & don't use electric, then you simply won't have enough roof space to cover your electric needs in the winter with solar.

So if I have 60w solar panel I only need a 60Ah battery ?

 

Lets say that it actually relates to electrical usage (say) 100Ah per day - which is a not-unusual figure.

Do I therefore only need a 100w panel and a 100Ah battery ?

 

As had already been said (a number of times) it all starts with an electrical usage audit.

1) Work out how much you use, then

2) Work out how you are going to replace it and, then

3) Work out how you are going to 'store' it (with sufficient capacity to cover 3 days usage in case of rain / clouds etc)

Edited May 28, 2018 by Alan de Enfield

[Ssscrudddy]

Ok then other way round, if you got 600AH battery bank get 600W of solar.

But hey go for it, if you only want a 60W solar panel then by all means just have a 60AH battery.

Methinks you are just being awkward on purpose

[WotEver]

18 minutes ago, Ssscrudddy said:

Ok then other way round, if you got 600AH battery bank get 600W of solar.

Why?

 

If my usage is only 60Ah/day do I really need well over 300Ah/day solar generation?  As has been repeatedly stated, the amount of solar bears no direct relationship to the amount of storage. 

[Horace42]

12 hours ago, Mike the Boilerman said:

Ok for simplicity, lets assume the OP uses a steady 100AH at 12v per day, every day of the year. 

 

Let us also assume my own solar behaves typically. I use 30AH per day at 24v, equivalent to 60AH at 12v.

 

My 560W of MPPT solar supplies ALL my electricity from about the beginning of May to the end of September. That's five months of the year. In addition at each end of the season I get about a further month where I get 50% of my electrical consumption and a further month each end where I get perhaps 20% of my electrical consumption. The remaining three months I get nothing. 

 

So assuming the OP wants to replicate my solar performance in his own boat, he will have to scale up 560W of panels by 100/60. He will need 930W of panels and an MPPT controller.

 

Does that shed any light?

 

(Notice battery size didn't come into the calculation?!  ) 

 

I would say that was useful practical information and the sort of thing that could help the OP select a viable 'working' system.

The battery size did not come into your calculation because your example relates your average panel output to match to your average usage.

The battery comes into the equation (and size really matters) when the MPPT controller tries to match the instantaneous output of your panel to your instantaneous demand.

I doubt if such a system would work with no batteries - unless the user stood there constantly switching things on and off to suit the weather.

So a battery makes sense - but what size ?

To enlarge on my decision logic behind this, consider what use would a 1Ah be, or 2Ah, or 3AH, - not much I imagine - so go on adding 1Ah to the battery size until a sensible practical size is reached.

Using your numbers for the OP - this would be 100Ah to exactly match - or by the 'x3' rule is 300Ah.

The batteries however are subject to a nominal max charging current (heat/gassing problem) whereabouts a ten-hour rate is used - where 930W at 12V is to 77A,  and  x 10 = 770Ah, but knock off 100Ah being consumed - then a battery of about 670Ah would maximise the panel output. So pick a battery between 100Ah and 670Ah.

A nominal 600W is more akin to 300Ah - similar to yours maybe.

Although not given - battery size does matter to the MPPT controller.

 

 

[Dr Bob]

1 hour ago, Horace42 said:

The battery comes into the equation (and size really matters) when the MPPT controller tries to match the instantaneous output of your panel to your instantaneous demand.

I doubt if such a system would work with no batteries - unless the user stood there constantly switching things on and off to suit the weather.

So a battery makes sense - but what size ?

To enlarge on my decision logic behind this, consider what use would a 1Ah be, or 2Ah, or 3AH, - not much I imagine - so go on adding 1Ah to the battery size until a sensible practical size is reached.

Using your numbers for the OP - this would be 100Ah to exactly match - or by the 'x3' rule is 300Ah.

The batteries however are subject to a nominal max charging current (heat/gassing problem) whereabouts a ten-hour rate is used - where 930W at 12V is to 77A,  and  x 10 = 770Ah, but knock off 100Ah being consumed - then a battery of about 670Ah would maximise the panel output. So pick a battery between 100Ah and 670Ah.

A nominal 600W is more akin to 300Ah - similar to yours maybe.

Although not given - battery size does matter to the MPPT controller.

 

 

Sorry Horace, but I havent a clue what you are going on about.

You say "when the MPPT controller tries to match the instantaneous output of your panel to your instantaneous demand."

How does that relate to battery banks size?

You dont need complicated science. 500W of panels will put out say 20-30A max which they will do if you battery bank is 50Ahs short of full capacity. As the bank becomes full then the mppt will go to float and very little if any charge will be put in. Any bank size over 100Ahr would be fine and most of us have at least 100Ahr capacity. Why an upper limit? 

 

I seem to remember 15 years ago, people telling me to make sure my bank was big enough so the solar didnt fry the batteries.....but that was before mppt's.....when we had very cheap controllers. Its a bit more sophisticated these days.

 

Someone earlier said it is good to have a bigger bank 'so you dont waste the extra energy from solar when the bank is full and the sun is shinning. That too is rubbish as it just means that you work at the top end of your battery capacity which is now higher with an extra battery. I run a largish bank (660Ahr) as with my 100-120Ahrs per day usage, I want to always remain above 85% capacity to reduce the cycles and prolong battery life plus it should be more robust to the 60 secs of Nesspresso heating at 100A......the size is nothing to do with my panels or my panels nothing to do with battery bank.

[WotEver]

44 minutes ago, Dr Bob said:

the size is nothing to do with my panels or my panels nothing to do with battery bank.

Repetition is generally a good idea but sadly it doesn’t appear to be sinking in...

[Alan de Enfield]

2 minutes ago, WotEver said:

Repetition is generally a good idea but sadly it doesn’t appear to be sinking in...

 

Maybe 'Fisicks' has different rules in Horace's parallel universe - his ideas certainly don't work in ours.

 

It the other universe it appears that the size of the panels determines the charge current going into the batteries !