"[...] variationer i gasprisen per forbruger[...]"

I have to confess I have never actually seen it used in practice but I looked it up in Fremmedordbogen:
an adv (merk) til, "skylder til" (sættes i bogholderiet foran debetposter); MODSAT: per.
Ordbog over det danske sprog (vol. I, p. 530, "an" 1(3)) has "som en slags partikel brugt i bogholderi for at betegne en person ell. konto som kreditor.

I suggest the translation "paid (or owed) to"...

Good luck.

Short term natural gas production marginal costs. It is much more difficult to estimate a cost-specific production variable for natural gas production than for power utility production given that in natural gas production, among other factors, its variable cost is relatively small compared to its fixed cost and given that natural gas resources are exhaustible and that therefore there will be some balancing element between its current and future sales price in determining the natural gas sales price. Hence a ’minimum natural gas price’ is more appropriately operational, with a set ceiling. Fluctuations in this (minimum natural gas) price severely impact as natural gas price fluctuations (including scarcity payments) ***affecting*** the consumer, especially during the summer months (use of air conditioning in buildings perhaps?).

The inference seems to be that use of natural gas by the power utilities increases during the summer months with the cost of its additional consumption being transferred by these power utilities to the end consumers as added seasonal scarcity payments to be made.

--------------------------------------------------
Note added at 12 hrs (2010-08-22 21:47:42 GMT)
--------------------------------------------------

the explanation comes in the next paragraph of your source text:

Natural gas production capacity. The North Sea has a limited natural gas production capacity. The natural gas price for the end consumer is determined, as just mentioned, within a combination of the minimum natural gas price and the scarcity payment. Natural gas production capacity is found to be limited during the October to March winter months so that the natural gas price ***affecting*** the end consumer during the April to September summer months is primarily determined by the minimum natural gas price, while during winter a surcharge is added which is determined as a natural gas production capacity scarcity payment.