The 1,5 kWp PV to 100 Ah Lithium rule

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Mark asked Oct 18, '20

The 1,5 kWp PV to 100 Ah Lithium rule

Some questions about this rule to those who understand all the implications,

Is the PV array size, for the purpose of calculating this rule out, the STC rating of the array or the NMOT rating (more realistic real world performance)? I am guessing most would say use the STC rating but can we safely use the NMOT rating or not? Does Victron say anything about this? We all know we dont get the rated power of the PV panel, so why calculate battery size based on a wattage we will never achieve?

For example, say a PV array of 60 panels at 450w.

In STC terms this is 27 kWp (60 x 450 = 27000). In NMOT terms its 20.3 kWp (60 x 338 = 20280)

So if we follow the 1.5KWp PV to 100 Ah lithium rule for the STC rating of this array it would be ;

STC PV 27 kWp / 1.5 kWp = 18 | 18 * 100 Ah = 1800 Ah | 1800 Ah x 50 V = 90 kW battery .

So if we follow the 1.5KWp PV to 100 Ah lithium rule for the NMOT rating of this array it would be ;

NMOT PV 20.3 kWp / 1.5 kWp = 13.5 | 13.5 * 100 Ah = 1350 Ah | 1350 Ah x 50 V = 67.5 kW battery.

Thoughts ?

Lithium Battery

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Answer 1 · 2020-10-18T16:57:35Z

wkirby answered · Oct 18, 2020 at 04:57 PM

I don't think that rule holds much water. 100Ah is only half of such an equation, the other half is the battery bank Voltage. Then you should consult the battery datasheet to find out charge current.

You have used a 48V bank for your example.
Let's use the 100Ah Victron lithium battery which has a recommended charge current up to 50A.
So for a 48V 100Ah battery bank (four in series) you could safely charge at 2.4KW,
If only one 100Ah battery for a 12V system, then it would only be 600W.
Cut those calculations in half if you use a 48V Pylontech lithium battery whose recommended charge power is 1.2KW.

STC is a good rating to go by because most of the time you'll be charging below the recommended maximum power, which will help to prolong battery life. Then you are still safe when the STC power is achieved and then when the array produces more than STC then you'll still be below the absolute maximum charge power, Array power greater than STC is rare, but when it does happen you don't want to damage things.

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Answer 2 · 2020-10-19T05:30:11Z

Mark answered · Oct 19, 2020 at 05:30 AM

Good information, thanks.

What about at those high battery sizes though? Even a Pylontech battery of 3.5kW x 16 is a big charge rate overall.

What if the battery is a 48v 100Ah battery x 16 ? That's 76.8kW overall. Even with a charge rate of 0.3 C its a significant charge rate for a battery bank that big.

So if we get into big battery banks how much of a difference does it make if the battery bank is slightly undersized for a given PV array size? (for example using NMOT instead of STC ratings)

Of course this depends on where the installation is. In Canada on a brilliant bright sunny morning in winter with snow on the ground the STC would probably be exceeded. In a steamy jungle valley were its only ever hot and dry and hot and wet NMOT would be safe.

My interest is in a large battery bank at 48v. Would be great if we could use higher voltages but Victron does not go there, so......

There is a lot in this rather then just figures. One has to have a feel for what works and what wont, hence asking here.

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