According to the energy certificate, the building should require 55 kW primary energy. That would be about 6 tons of pellets per year. And not almost three times as much.
Not at all. It is already stated on page 1. What you are looking for is the final energy demand. With this, you can calculate how much fuel you will need to heat the place and provide hot water. The primary energy demand extends the formula for calculating the final energy demand by the primary energy factor of the fuel. This factor expresses how much additional energy is required in the entire process chain to obtain the energy carrier. It is defined by standards and is variable. Mining coal is more complex than producing pellets. Furthermore, it is politically desired to prefer burning domestic wood (as it is more climate-friendly) instead of Chinese lignite. Coal therefore has a factor of 1.1, wood 0.2. By the way, electricity was still at 2.4 until the end of 2015 – so rather unfavorable – and since the beginning of 2016 at 1.8. This leads to lower primary energy demand ratings for heat pumps, thus making it easier to achieve e.g. KFW standards. Political intention. Conversely, this means that you can barely achieve KFW 55 with a gas heating system anymore without mixing in an additional renewable energy source (-> solar thermal). If you take the final energy demand here and multiply it by the primary energy factor, you arrive at the primary energy demand. Specifically, this means that what you have so far calculated as consumption in tons of pellets based on the primary energy demand must be divided by 0.2 or multiplied by a factor of 5 in order to make a consumption forecast based on the energy certificate.
Final energy Qe = useful energy Qn + system losses Primary energy Qp = final energy Qe × fp
But aside from the calculations, I am still surprised by the "surprise" you seem to be experiencing. If you burned 0.5 tons per month in summer with solar thermal support, an increase to 2 tons per month in the cold season is far from surprising. After all, the heating now has to warm the house (which requires significantly more energy than hot water preparation) on the one hand, and on the other hand cover hot water preparation, which was previously (almost) completely handled by solar thermal. Heating load calculations were read here in the forum, in which 6 kW heating load was confirmed, of which about 1.5 kW was for hot water (if my memory does not deceive me). Just to get a feeling of how much more demanding heating the house is in relation to hot water preparation. Just wait until it really gets cold. It will not stay at 2 tons per month.