For annual baseload and monthly baseload: Are the volume and balancing risks left with the seller?

Yes, that is correct. In both cases, a volume is predefined in the contract. In the case of an annual baseload, the volume is equal for every hour in a year. In the case of the monthly baseload, the hourly volume is equal for every hour in a given month, but monthly volumes vary according to the historical production profile for a given technology. In essence, one can expect a higher volume in the summer months when referring to solar production. The balancing cost and corresponding risks remain with the seller.