How Many Watts of Solar?

[Stu Jackson]

Originally Posted by john61ct

On many boats, solar may be important, but especially using LFP, may not be the primary contributor.

May I ask why?

[newhaul]

Originally Posted by Stu Jackson

May I ask why?

on some boats if you fit a large alternator with Lfp you can charge quite fast due to acceptance rates of the Lfp battery.
For instance I have a 100ah Lfp bank with a 120 amp alternator I can charge the battery from the 20% dod to full charged in less than an hour.
Or running my little 800 watt generator in conjunction with a 40 amp charger it would take about 2 hours and a quart of gasoline.

[john61ct]

Originally Posted by Stu Jackson

May I ask why?

If you mean how does LFP make such a big difference?

Yes it's the crazy fast CAR, coupled with a high amp source, can put in an hour or two more Ah than a pile of solar contributes in a full week.

Coupled with the fact that there is zero need to get to Full,

that's a different issue, but can eliminate **all** need for panels if the owner prefers

[markic]

Originally Posted by newhaul

Ok there has been so much posted its really getting confusing for some.

Here is my personal opinion ( as an installer)
first you need an idea as to how much power you will use . Size battery bank accordingly ( for this you want to plan about three times your expected usage to allow for days when solar is not good)

( there have been a couple of sites with budget prep papers posted Already)
next imo you will want about a watt per amphour of battery capacity for charging.
This equals to be about 100 watts per 100 ah capacity. (50 ah usable power for Fla for longevity)
this is if solar is your only charge source.

Love the simplicity of this approach, KISS.

thanks for posting

[StuM]

Originally Posted by captainwd40

...

Simple Math:

If you count on 5 hours of full sun 5/24 = .21

...
If your battery is depleted and needs recharge (1100 watts) and you have a 1000 watt solar panel that will provide full charge for 5 hours per day you will get 1000 watts x .21 = 210 watts per day to recharge your batteries.

Oh Captain, my Captain! (What sort of a captain are you?)


Simple, but totally incorrect.

Firstly, your units are wrong. You don't get Watts per day (nor do batteries store Watts). Watt is a unit of instantanous power. You get Watt hours per day.

That mistake results in your second mistake. You are effectively dividing your daily solar output by 24. You get an hourly average of 210Wh, not a total of 210Wh per day.

If you get 5 hours equivalent full sun with 1000 W panels, you get 5000Wh per day which is approx 350Ah @ 12V nominal.

[john61ct]

Yes "watts per day" is not a thing.

nor amps per day, both those units are flow rates with a time element already included.

Wh or Ah are the units for energy stored, or power consumed "per time unit"

[Reefmagnet]

Oddly enough, the victron connect doohickey keeps a history of exactly that. Wh per day. Which makes some sense, because simply dividing the total by battery voltage results in daily amp hours.

[StuM]

Originally Posted by Reefmagnet

Oddly enough, the victron connect doohickey keeps a history of exactly that. Wh per day. Which makes some sense, because simply dividing the total by battery voltage results in daily amp hours.

What battery voltage?



Bulk, absorption, float charge voltage or voltage at 4:00am after your batteries have been drawing down all night?


Somewhere between 14.8 and 12.3?

[newhaul]

Originally Posted by StuM

What battery voltage?



Bulk, absorption, float charge voltage or voltage at 4:00am after your batteries have been drawing down all night?


Somewhere between 14.8 and 12.3?

hopefully somewhere well below 14.8 for me and well above 12.3
With either of those voltages I have issues with my Lfp. So for me lets say 12.6 at daybreak.

[Reefmagnet]

Originally Posted by StuM

What battery voltage?



Bulk, absorption, float charge voltage or voltage at 4:00am after your batteries have been drawing down all night?


Somewhere between 14.8 and 12.3?

And 10% or 20% charge losses? I take it as a relative approximation @ 14vdc. The battery monitors tell me actual amp hours at the battery as the sum of in and out but as a single number the mppt record breaks it down into bulk, absorption and float which gives an indication of supply and demand.

[Adelie]

Originally Posted by john61ct

It would be **very** rare for solar to get a bank back to true 100% Full before noon, unless LFP chemistry.

True 100% Full is what's required for longevity, can't take the controller dropping to Float as meaning anything,

needs to be verified with an ammeter, adjusting the controller AHT until current tapering below .005C or whatever your bank mfg specs for endAmps.

Most lead banks take 6+ hours, even if the source puts out more current than the maximum amps accepted by the bank.

So by 9am? Only in latitudes where the sunrise is before 2am. . .

If the bank was only pulled down to 85-90% it could be back to 100% by 0900.

[newhaul]

Originally Posted by Adelie

If the bank was only pulled down to 85-90% it could be back to 100% by 0900.

to many variables to make a statement like that . In my opinion.

What chemistry of battery?
What ah capacity of bank?
What size solar array?
What type of solar controller?
Location and time of year?

To many variables but yes if everything is lined up just right it is possible .

With correct solar sizing and lifepo4 battery then it is possible
With Fla battery it is not possible period.

[Adelie]

So if an FLA bank was at 99% at a sunrise of 0700, it couldn’t get to 100% by 0900?

[sailorboy1]

Originally Posted by sailorboy1

600W assuming you also are going to run your refrigeration

Post #7

The OP ended up here and you guys are still arguing BS theory. Nothing on a boat follows theory and all you can do is plan around the experienced usage. My 640W doesn't ALWAYS cover my usage, but I bet overall would take care of it 90+% of the time. Yes sometimes it is real for cloudy for 3 plus days, but no energy survey is going to help with that. If I'm not on the boat and all that is running is my LED anchor light and refrigeration I bet there still isn't any problem (haven't tried it with the 640W as in past I only had 290W and after a week I came back to the boat once only to have 97% SOC)

[john61ct]

Originally Posted by Adelie

So if an FLA bank was at 99% at a sunrise of 0700, it couldn’t get to 100% by 0900?

Not if the C-rate is much lower than what the batt can accept

(kidding, but serious 8-)