Panel to Battery Ratio?

[clayswen]

Is there a general ratio that is common for battery charging? I want to eventually keep adding 100ah of battery every few months. Right now I have 9.2kw of array with my sol-ark12k. My home consumption averages about 15kwh on a cool day to about 40kwh on a hot day. Right now I am only using 28 panels but I purchased 40 so I can add another 4k of panels as needed. What kw per ah of battery is a good solid ratio?

 

[BrightCandle]

I don't think there is one particular ratio because it really depends on your goal with the batteries and how much power is used when the sun isn't shining and the cost differential of export verses importing from the grid. Assuming the goal is optimal economics to make sure all the batteries are positive return I found the battery has to be used every day in all seasons. It would thus not be enough for the 1 day in 30 through the spring/summer/autumn where sufficient power was not produced and wouldn't carry much forward to the next day. This worked out to be about half the consumption in the winter, which with night time cheaper tariff charging and solar that on average at least makes up the other half is the most economic point. The production of the panels annual is about 20% higher than annual consumption (which by their end of their life should be about the same as annual consumption).

There are various calculators out there on the web like Gary does Solar's that can give you can idea how a system will perform day to day as well.

So my rough distillation of what I found to be the most economic for me was Annual Solar production 20% higher than annual consumption and a battery capacity around half of average daily consumption, which is enough to cover 98% of the days through the April ->September where there is enough solar produced most days and the batteries are enough plus some spare to make it through the night. In the other months the strategy switches to charging the batteries on cheap nightly power and the solar + that charge survives the other 20 hours on average.

 

[hwy17]

Ratio used to be an important factor with lead acid, because there is a target charge rate that will keep lead acid healthy.

Nowadays with lithium ratio can be whatever you want. You can imagine what the limitations are of being heavier on either one. The best ratio is always more of everything.

 

[Hedges]

For AC coupling, it takes a couple of seconds to shift frequency and curtail production. For Sunny Island, SMA recommends 100Ah of 48V battery per kW of AC coupled GT PV (e.g. Sunny Boy).
I'm using AGM, which can accept a high charge current, and have maybe 30% the recommended size.
It should actually be more an issue of how many kW of "load dump" (load suddenly taken offline), rather than actual load. But since my system exports to grid, dropping grid may count as load dump, or maybe Sunny Boys disconnect due to loss of grid before Sunny Island takes over.

With DC coupled PV such as SolArk, it can curtail production faster.

Lithium batteries will run toward over-voltage when charging occurs, so they need SoC and balancing such that system remains on line.

What charge rate do your batteries accept? 0.5C? You could plan bank sized based on that, or you could program inverter to limit charge current despite an oversize PV array, as I did. PV is able to supply all loads during sunny days plus recharge battery.

If your lithium batteries can get cold, max allowed charge current drops toward/to zero amps as temperature approaches freezing. So 0.5C charge rate would be excessive if batteries were 10 degrees C.

And then it depends on your objective.
For me, it was just enough battery to make it through the night during power failure.
Depending on time of use rate schedules, you might want to store 100% of production to export or offset loads later in the day, so battery capacity about 6kwh for every 1kW of PV.
For extended off-grid operation, traditionally people aimed for 3 days worth of storage.

 

[clayswen]

If I add more batteries in the future in parallel, does the max continuous discharge change of stay the same? If I run two 48 volt 100ah batteries in parallel, besides the ah adding up, does the max continuous stay 100 or add up to 200? Still learning!!

 

[Hedges]

The currents would add if current split equally, which it may not.
You can try to wire with equal resistance path to each which will help.

If one disconnects due to over-current or any other reason, then you would get cascading disconnects.

You should be able to count on some, but don't push it.
The discharge currents are fairly limited, so larger inverters do require multiple server-rack batteries in parallel.
I would look at 280 Ah like PowerPro, if not already using some 100 Ah.
Mixed capacity would probably be more imbalanced in current draw.

 

[clayswen]

Thanks for your input. I was gonna try to keep the batteries all identical starting with one 100ah battery and just adding another one every few months till I reach about 30-40 kwh.