Hi, is my 4 years old fully charged for 12 hours Varta Silver battery bad or need replacement if it dropps to 12.25V turning engine key to on but not starting the engine?
What is the normal voltage on a good battery having key to on wo starting the engine?

 

Is the X350 garaged or kept outdoors?

 

normal

 

12.25V is enough voltage. My XJ can also start the engine with only 11.5V. (which is a battery close to death )
In the state before starting the engine, the car drains about 300W from the battery, so voltage is not comparable to idle voltage.

 

A better test is what does the car read after sitting all night? Jaguar wants 12.6 VDC or more.
As posted above when starting it will vary.
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Exactly. Car batteries are preferred to have 12.6V at rest. 12.4V is acceptable but iffy.

I asked the question if the OP kept the car in a garage or outside. His signature indicates he is in Stockholm.

Both cold and hot weather can affect a battery's voltage. Cold slows the electro-chemical process, reducing available cranking power—especially below freezing.

Engines require more power to start in cold, just as batteries are producing less voltage.
Cold doesn’t degrade the battery as quickly as heat, but it exposes weaknesses in older or undercharged units.
Oil is thicker as well and the starter motor has to work harder even with 5W-30 engine oil. It's recommended by Jaguar in Europe to use 0W-30 when temps get to –35°C (–31°F).

Conversely, heat speeds up internal chemical reactions, which can temporarily boost performance but also hastens wear.
Electrolyte loss is common in high temps, especially above 122°F, leading to reduced capacity and potential internal damage. I don't think Stockholm gets 100+F temps.

Too, charging systems may misread battery status in heat, causing overcharging and thermal stress.
For those in hotter climates batteries often last under 41 months, compared to 58+ months in cooler regions.

 

I bought my Jag when it was 4 years old and it had the original Varta battery
When the battery was 16 years old I removed it only because I was afraid
that it would fail and I would get stuck somewhere.
I bought a new Varta battery for about $400 and as soon as it was out of warranty
it failed.
That is progress

 

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Yep, it's the old 'pay me now or pay me later' law of probability; closely related to Murphy's Law.