Spot or average
Converting a balance uses the closing rate; converting a flow uses the period average. Getting them the wrong way round is invisible in the output, because a plausible number comes out either way.
For USD, the two answers diverge most in 2003 by 11.7% of the annual average. A full year of flows converted at the closing rate instead of the average is misstated by that much, which is often more than the margin of the business doing the converting.
The averages are taken over published fixings, not over calendar days. Averaging the gap-filled daily table would count every Friday three times, since Friday, Saturday and Sunday all carry Friday's rate, and four or five times around a holiday weekend. That weights the mean toward whichever weekday sits next to a closure. There is a second trap in the same table: the average of euros-per-unit is not one divided by the average of units-per-euro, because the mean of reciprocals is not the reciprocal of the mean. For EUR/USD the two disagree by 0.07% in a calm year and 0.53% in 2008.
Which currency a price is counted in
The warehouse holds one euro-denominated measurement, Eurostat's household electricity prices, beside GDP in dollars. The FX table is what lets the two be compared, and the comparison matters.
Price rise, 2021-S1 to 2022-S2, in EUR
... the same rise, in USD
The euro against the dollar, same months
The EU's average household electricity price, Eurostat's own figure for the 27 members, rose 25.6% in euros and 5.7% in dollars over the same eighteen months. The euro fell from 1.205 to 1.014 against the dollar while that was happening.
So what. Both numbers are right. A household paying in euros did face a 26% rise, and a dollar-denominated buyer of the same electricity did face 6%. A chart titled "European electricity prices" with no stated currency is reporting the exchange rate alongside the energy market. This warehouse already carried that warning in prose, from the case where Japan cut emissions 21% between 2010 and 2024 and still scored 10% worse on carbon intensity measured in current dollars: the yen lost 42% of its dollar value, so Japan's GDP counted in dollars fell 28%. It is a column now, not a paragraph.