Opinions on battery sizing

Hi Bill, thanks for the speedy reply

Seeing as my panels give a max output of 16amps, that puts me at less than 4% rate of charge with a 450ah bank.......If I get another panel at 140w (8amp max), that will still only put me at 5% in ideal conditions which we get plenty of here in portugal in the summer.........the winter can be blue skies for weeks on end or grey skies for weeks on end! Firing up the generator is an option but only very occasionally, i dont want to be relying on it more than a couple of times a year.

However, if I get a 225ah bank - With my 280w of panel i am sitting at 7% in ideal conditions, then adding one more panel would give me 10.6%.

It looks to me like the 225ah bank is the better option as my charge controller is limited to 30amps at 12v which means one more panel max.  I could switch to 24v and get twice as many panels out of my cc, but that means getting a new (more expensive) inverter and losing my 12v loads which I use for the majority of my lighting.

The last option I can see but dont really understand how to set up, is getting another 30amp cc and wiring it alongside my existing cc to get more panels into the system.  I can't see how this would work properly when boost or eq charges were triggered? what if one triggers and the other doesn't?

Thanks for reading, Chris

Thanks for joining the discussion vtmaps,

For me, this is where it gets a bit difficult.  I live completely off grid, and whereas most people can easily add up the wattage of their appliances and calculate how long they  run on an average day - I don't actually have any of these things, instead power is something of a luxury that I use when I have it for things such as Laptop, 12v lights and a fishtank heater to heat a small tank of water as a thermal mass and provide heat to the roots of tender plants through the winter. All of these things are essentially luxuries and as such are hard to calculate a typical daily draw........ Also my cc tends to show some sort of "phantom load" during the day so using my cc's  load ah log to estimate typical usage is not accurate!

This is what's lead me to specifying my peak load, which I would definately not use all day.  I'll try and break down my individual loads and their probable hours of use:

Laptop - 70w, around 2 hours per day (but sometimes i'll use it more if I have surplus power) = 140wh
12v lights - 2 x 4 watt led's max 4 hours per night  = 16wh
220v Lights - 2 x 20w cfl's max 4 hours = per night = 160wh
Fishtank heater - tricky to estimate as it runs intermittently and depends a lot on ambient temp - 75w for maybe 6 hours per day = 450wh
Circulating pump - 5w constant = 120wh

That means that my daily energy load could be 886wh/day.

As I said, most of these are luxuries so if the solar is low I'll usually sacrifice the laptop or the 220v lights to keep the aqaurium heater on - or cut the aquarium heater and pump and rely on a small gas heater until the sun returns enough to get the water up to temp.

This makes my decisions around system size more about financial viability (I have very limited funds and dont want to make a situation where I have to keeping putting money into my system for it to survive) and making the best use of the investments I have already made in panels, charge controller and inverter.

But, back to the point - My daily use could fluctuate between maybe 500 - 1000wh/day.  Obviously if I expanded my system i could find a use for extra power (which is obviously tempting!).

Thanks again for reading, all opinions/suggestions welcome,  Chris

Wow, thanks Bill - what a excellently comprehensive reply!!

I can see straight away that I have been quite drastically underestimating the amount of cash needed to build a reasonable setup based on 1kw/day!

I think you're right about trying to minimize using the aquarium heater, financially speaking i'm going to be unable to get a 392ah battery bank AND 4 more panels AND upgrade my charge controller (my current one is rated at 30a 12/24v).  I could theoretically switch to 24v and keep the same cc, but i've heard bad things about running the steca 3030 on 24v close to it's amp limit. I would then also need to buy a new 24v inverter and lose my 12v leds which are pretty good for their low power use.

So, with these things in mind, and looking at the figures you kindly worked out - I think my best plan of action is to try and half my projected 1kwh/day. I think this would be do-able as the figures I gave for usage were based on the max I would use, and I would rarely do all of these things in one day, and definately not every day (especially if i'd invested in quality batteries and had a v. strong incentive to never discharge below 50%). 

Cutting that 1kwh/day in half would allow me to get quality batteries, afford one more panel AND keep my current cc and inverter. 

This would make my system 225ah T105-re's, 420 watts array - looks like it sits comfortably between full time off grid usage and typical cost effective maximum (assuming I can just halve the ah and array figure you provided as i dont fully understand the equations for this - if you feel like breaking it down for me i'd love to learn  )

It also looks like it would put me comfortably over the typical December figures minimizing winter genny use (hoping for near average weather). 

Then, by the time the batteries have given all they've got in maybe 5-10 years, I could have piece by piece upgraded cc and array ready to give a bigger battery bank the kind of amps they'd be after..................

While i'm musing about compromising on my *ideal* system, I suppose I could always forget the quality batteries for now - get cheaper 100ah 12v SLA's which i would feel less precious about (and run them lower than ideal) and use the saving and the 2 years while i kill them to get more panels and a bigger charge controller.  This would double my current capacity without locking me into a 5-10 year period where I can't expand the capacity of my bank if i find the means to.................

Hmmmmmmm! It seems there are quite a few options and many different angles to approach from!

Or would it be better to wire the panels in series to 48v for the mppt charger?