Hello
The capacity of the 6V dynamo/alternator is 60 W.
Regarding that you might put on the light sometimes, You have 40 or 45 W front light, 10 W back licht, 2 x 21 W Indicator and sometimes 21 W brake light
So you see why MZ recommended a big battery:
Because you consume more than is produced by the dynamo.
For a short time the dynamo can produce up to 90 W, but even this is not enough.
And this maximum production is only at a high rev. drive.
So when you drive mostly short distances the battery won´t charge enough.
The ETZ has about 210 W...
By the way:
Here in germany it is obligatory to drive with lights on all time.
This helps to be seen better by other drivers...