I'll venture with this one, tho your usage seems to be a fee or charge. But first, flödestaxa doesn't seem to be a 'usage' fee (whether special or not). It seems to be a cost/fee/charge related to flows of the district heating water (transfer medium). Sometimes called a 'födespremie' -- is that in your text?

Here's a description:
"Flow cost is a less common price component. It is generally only based on the volumetric flow of the DH water through the customer heat exchanger, but occasionally also include the temperature of the return water." ( http://publications.lib.chalmers.se/records/fulltext/149216.... )

"Flow cost components, whose purpose is to provide incentives for customers to hold their equipment in condition, are also difficult for customers to understand. When
using a flow cost component, companies should keep in mind gathering information about flow rates requires a lot of effort and to provide easily understood information explaining why low return temperatures are important."
( https://energiforskmedia.blob.core.windows.net/media/21111/k... )

This also talks about "power and flow components" -- and you'll find 'effktavgifter' is translated to 'capacity fees' but which seems more generally translated 'capacity charges.' The 'effekt' being measured here is energy -- generally the incoming temperature vs the outgoing temperature for the facility. The difference in temp = the energy consumed -- which is the 'heat' provided and charged for.