Solar....how much wattage is enough

[Horace42]

7 hours ago, WotEver said:

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’

So roof size creeps into it now! - something you previously dismissed along with battery.  The battery size might be irrelevant to you and Mike (as you say - but are you his appointed spokesman? - I am sure he can speak for himself) as he has done with some useful facts - but to anybody trying to select a suitable solar panel - the battery size is important - and a balanced system cannot be properly designed without taking the battery size into account.

 

Even if you think battery size does not matter, the MPPT does - and will depend on it.

 

I am sorry you don't want to see that.

3 hours ago, WotEver said:

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. 

I am afraid it does. Solar generation is linked to battery size. But specifically for your usage I don't know. But by your rules you need 3 x 'usage' batteries. Which I assume you want fully charged. 180Ah will need 18A for 10 hrs. That you might get from a 216Wpanel if the sun shines for 10 hrs. 

With so many variable to consider it is only possible to give advice to make sure the demand is accommodated entirely by the batteries and the means of charging matched accordingly - which is linked to the solar panels - and relying on the MPPT to balance the load.

Edited May 28, 2018 by Horace42

[Theo]

6 hours 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.

Have a greenie sir!

 

If I could have a set of panels which would give 3kW in the middle of winter I certainly would get them.  I could then use the washing machine on a hot filll as often as I wished, heat the boat electrically, have all the hot water that I could possibly want, have an electric motor instead of the BMC 1.5, charge the batteries for the electric pig levitators and lots more.

 

However, 240Wp is all I have and the best I have seen recently for a few hours is the middle of the day is about 150W.

 

N

[Horace42]

7 hours ago, Alan de Enfield said:

Repetition adds emphasis :

 

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

 

 

SHOUTING does not make you right. I have explained why battery size matters.

Please explain how your solar systems would work in practice if you had no batteries.

 

[Alan de Enfield]

7 minutes ago, Horace42 said:

180Ah will need 18A for 10 hrs. That you might get from a 216Wpanel if the sun shines for 10 hrs. 

That is totally incorrect.

 

You are working on the assumption that the solar panel controls the charge rate of the battery - false !!!

The charge rate reduces and the voltage increase as a battery gets to a higher SoC.

At (say) 90% SoC the battery(s) may only accept 5 amps - if your solar panel were to be putting out 50 amps, only 5 amps would be going into the batteries. It may take you 4 hours of charging to get to 90% SoC, another 4 hours to get from 90-95% SoC and a further 8 hours to get from 95-100% SoC as the current the battery(s) will accept falls lower and lower.

 

As a guide a battery can be considered to be fully charged when THE BATTERY has reduced the current it will accept down to 1% of the battery capacity. So for a 100Ah battery as it approaches 'full' it will only take 1 amp - irrespective if you try to force 10 amps or 100 amps into it.

 

You are posting rubbish and not only making yourself look silly, but trying to give those asking questions the wrong information which WILL cost them money.

 

I'll leave you to it now - I can do no more.

 

[WotEver]

7 minutes ago, Horace42 said:

Please explain how your solar systems would work in practice if you had no batteries.

What an absurd suggestion. 

 

When you’ve completed the three steps outlined in my earlier post, you will have a suitable battery bank for your boat. Note that solar forms no part of these steps as it does nothing worthwhile in the winter. 

 

Solar will now now help with step 2 during the summer but will be of no use in the winter - get as much or as little as you can afford. 

[Theo]

Mr Horace, you said " Even if you think battery size does not matter, the MPPT does - and will depend on it. "

 

Could you explain, please?  I understand about impedence matching for maximum power transfer but once the batteries are fully charged you are not interested in transferring any more energy to them.

 

[WotEver]

17 minutes ago, Horace42 said:

Solar generation is linked to battery size.

No it isn’t. Not in any way whatsoever. I’m done wasting my time with you. 

[Dr Bob]

23 minutes ago, Horace42 said:

 

Even if you think battery size does not matter, the MPPT does - and will depend on it.

17 minutes ago, Horace42 said:

I have explained why battery size matters.

 

 

Horace, I totally disagree with what you are saying.....although as I dont understand a word of what you are saying .....I may be wrong (but I'm not)!

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

I dont think you understand it properly the MPPT controller doesnt try and match anything. It tries to put power in until it cant - because the batteries cant take it and then goes into float. Nothing to do with balancing anything.

I do not understand your expanation. Please try again.

 

Edited May 28, 2018 by Dr Bob

[Richard10002]

11 minutes ago, Horace42 said:

SHOUTING does not make you right. I have explained why battery size matters.

Please explain how your solar systems would work in practice if you had no batteries.

 

Horace, I have 490W of solar panels, and a 40A Tracer BN MPPT controller and am at the point where I need new batteries. My power useage is about 100Ah per day, and I have the means to charge by engine or generator in the winter or if there is no sun.

 

What range of battery bank sizes would my panels be OK for?

[rusty69]

1 minute ago, WotEver said:

No it isn’t. Not in any way whatsoever. I’m done wasting my time with you. 

But, but what about those house dwellers wiv panels? Where do they put their batteries? 

[Dr Bob]

1 minute ago, rusty69 said:

But, but what about those house dwellers wiv panels? Where do they put their batteries? 

In their torches?

4 minutes ago, WotEver said:

No it isn’t. Not in any way whatsoever. I’m done wasting my time with you. 

Are whatsoever and wotever related?

[Theo]

The progress of this discussion is distressingly familiar.

 

Does any one remember the chap who threatened to take Daniel to court because his erroneous electircal opinions were hidden from general view ?

 

Makes me wonder if Horace is his reincarnation.

 

N

[WotEver]

3 minutes ago, rusty69 said:

But, but what about those house dwellers wiv panels? Where do they put their batteries? 

I’ve got 4kW on the roof but the sun never shines when I need it. I’ve seriously looked at batteries but they make no economic sense right now

[Horace42]

7 hours 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. 

It was an assumption on my part that your 'factor of three' related to 8 hrs sunshine charging per 24hr day - but and whatever factor is used - the batteries will need charging - presumedly by the solar panels.   I have no idea what this third day is.

 

[Alan de Enfield]

I reached a decision :

 

 

[rusty69]

6 minutes ago, WotEver said:

I’ve got 4kW on the roof but the sun never shines when I need it. I’ve seriously looked at batteries but they make no economic sense right now

Floodlights is wot you need. Never again will you're panels be without light! 

[Theo]

1 minute ago, Alan de Enfield said:

I reached a decision :

 

 

Probably a sensible sanity preserving decision.  I think that the OP, or reading the whole of this topic will realise that Horace has his opinion which is differenct from all the others contributing to this thread so while we needed initially to state our reasons to avoid the OP's being led astray, the thread has run its useful course.

3 minutes ago, rusty69 said:

Floodlights is wot you need. Never again will you're panels be without light! 

One of the interesting things about the perpetual motion machines that you can see on You Tube is that it is not always obvious why they don't work.

 

[rusty69]

Its also how Obiwan and Darft vaders light sabers are powered.

 

You'll never see them fighting in the dark, cos the panels are on their backs. 

[Horace42]

10 minutes ago, Theo said:

Mr Horace, you said " Even if you think battery size does not matter, the MPPT does - and will depend on it. "

 

Could you explain, please?  I understand about impedence matching for maximum power transfer but once the batteries are fully charged you are not interested in transferring any more energy to them.

 

That is right. The MPPT needs to know the batteries are fully charged - and then stops overcharging them. As is likely if they are too small - or if the right size for the demand - then the solar panels might be oversized. But the latter is the logical solution - enough solar power to fully charge the batteries is the ideal situation.

To be honest I don't know much about the 'innards' of an MPPT - but outwardly it is the thing that makes everything work in harmony to cope with all the instantaneous variables.

 

[Theo]

4 minutes ago, Horace42 said:

That is right. The MPPT needs to know the batteries are fully charged - and then stops overcharging them. As is likely if they are too small - or if the right size for the demand - then the solar panels might be oversized. But the latter is the logical solution - enough solar power to fully charge the batteries is the ideal situation.

To be honest I don't know much about the 'innards' of an MPPT - but outwardly it is the thing that makes everything work in harmony to cope with all the instantaneous variables.

 

Blimey!

 

[Horace42]

27 minutes ago, WotEver said:

What an absurd suggestion. 

 

When you’ve completed the three steps outlined in my earlier post, you will have a suitable battery bank for your boat. Note that solar forms no part of these steps as it does nothing worthwhile in the winter. 

 

Solar will now now help with step 2 during the summer but will be of no use in the winter - get as much or as little as you can afford. 

Of course it is an absurd suggestion. It is a decision logic that sets an extreme condition from which to move in the right direction to focus attention on what matters to lead to a satisfactory answer.

Something practical like 'getting as much or as little as you can afford' is simple advice and much easier understand.

[Dr Bob]

4 minutes ago, Horace42 said:

Of course it is an absurd suggestion. It is a decision logic that sets an extreme condition from which to move in the right direction to focus attention on what matters to lead to a satisfactory answer.

 

...but can you explain again why battery bank size is dependent on solar size? Your logic flow did not make sense to me so I guess there is one or more assumptions wrong. Please explain it again in words I can understand, maybe including all your assumptions.

 

I think your error in logic is in the assumption "when the MPPT controller tries to match the instantaneous output of your panel to your instantaneous demand." which I have referred to 3 times now in separate posts, but you have not clarified it.

[Tony Brooks]

24 minutes ago, Horace42 said:

That is right. The MPPT needs to know the batteries are fully charged - and then stops overcharging them. As is likely if they are too small - or if the right size for the demand - then the solar panels might be oversized. But the latter is the logical solution - enough solar power to fully charge the batteries is the ideal situation.

To be honest I don't know much about the 'innards' of an MPPT - but outwardly it is the thing that makes everything work in harmony to cope with all the instantaneous variables.

 

No it does not and neither do battery chargers. Those that think they are clever enough to work out when the batteries are fully charged usually get it wrong and  take the charging voltage down too soon.

 

The controller MPPT, PWM, or any other sort just controls the charging voltage to what ever is considered safe and that depends upon battery type and materials but nowadays it will typically be around 14.5 volts. However when the batteries are well discharged the current THEY allow to flow tends to pull the charging voltage down. As the batteries charge the current falls so the voltage rises until the controller (of whatever sort or battery charger/alternator)  has to control the voltage so the batteries are not overcharged. At any given charging voltage it is the BATTERIES that determine the charging voltage

 

Many controllers of whatever type allow the user to set the absorption and float voltages. It is up to the user to get this correct for optimum results.

 

Whilst living aboard that is all that is important but if you leave the boat for long periods with the electrics turned off I suppose there is a chance that any controller might overcharge the batteries but those that drop to a float voltage usually do it on the basis of TIME above a certain charging voltage not bank size.

[WotEver]

1 hour ago, Horace42 said:

The MPPT needs to know the batteries are fully charged - and then stops overcharging them. As is likely if they are too small - or if the right size for the demand - then the solar panels might be oversized.

Every word of this is complete and utter tosh!

 

Ooops, I said I was done with this...

[reg]

9 hours ago, Robbo said:

 

Edited May 28, 2018 by reg