The emission factor of Slovak electricity for 2025 is 350,70 gCO₂/kWh, and at the same time 259,19 gCO₂/kWh, and at the same time 87,04 gCO₂/kWh. All three values are published on the same page by the same register and each belongs to a different mix: the first is the residual mix, the second the supplier mix and the third the generation mix (OKTE, a. s.; Guarantees of origin — National energy mix section, annual page 2025, retrieved on 23 August 2026).
This text takes apart the composition of the residual mix and sets it against the two other published mixes. It is not a methodological manual for reporting and not advice on choosing a factor for a specific company: what is set out are the published values and their mutual ratios, not a recommendation as to which of them to use.
The headline figure: 350,70 gCO₂ per kilowatt-hour
The residual mix is one of three mixes the register publishes for every year, and in 2025 it carries an emission factor of 350,70 gCO₂/kWh. The supplier mix holds 259,19 gCO₂/kWh for the same year and the generation mix 87,04 gCO₂/kWh; the ratio between the outer values is 4,03-fold.
The same gap runs into the second published indicator. Radioactive waste comes out at 1,2 mg/kWh for the residual mix, 0,91 mg/kWh for the supplier mix and 2,36 mg/kWh for the generation mix — here the order is reversed, because the indicator is tied to the nuclear component.
The order of the two indicators therefore follows no single rule. For carbon dioxide the residual mix is the highest and the generation mix the lowest, for radioactive waste it is the other way round: 2,36 mg/kWh for the generation mix against 1,2 mg/kWh for the residual one, that is 1,97-fold in the opposite direction. Anyone comparing two countries or two years on one of these indicators alone gets a different ranking than on the other.
What the residual mix of 2025 is made of
The table below sets out the full composition of the residual mix for 2025 as the register publishes it, and beside it the share of the same item in the generation mix. The difference between the two columns is what left the generation side to holders of guarantees of origin and no longer figures in the residual mix.
| Component | Residual mix | Generation mix |
|---|---|---|
| Renewable sources — total | 11,93 % | 18,17 % |
| Biomass | 4,22 % | 4,41 % |
| Solar | 5,12 % | 3,05 % |
| Wind | 1,03 % | 0,00 % |
| Hydro | 1,53 % | 10,71 % |
| Nuclear sources | 35,68 % | 67,39 % |
| Fossil sources — total | 52,39 % | 14,44 % |
| Gaseous fuel | 29,06 % | 11,83 % |
| Hard coal | 18,20 % | 1,00 % |
| Oil products | 2,35 % | 1,31 % |
The sum of the published sub-items gives 11,92 % for renewables and 52,40 % for fossil in the residual mix, that is a hundredth away from the published totals of 11,93 % and 52,39 %; the difference arises from the rounding of the individual items. All three mixes close at 100,00 %. The values sit on the annual mix page for 2025.
The third published mix, the supplier mix, stands between the other two: renewable sources hold 34,40 % in it, nuclear 26,87 % and fossil 38,73 %. The volume it relates to is 23,44 TWh, while the generation mix relates to 26,58 TWh; the difference of 3,14 TWh is 13,40 % of the supplied volume and reflects that generated and supplied quantities are not the same measure.
Gaseous fuel: 29,06 % of the residual mix
The largest single item of the residual mix is gaseous fuel with a share of 29,06 %. In the generation mix the same item holds 11,83 %, that is 17,23 percentage points less. The difference between those two figures does not arise because gas is burnt twice, but because part of the low-emission generation is no longer in the residual mix.
For comparison: in 2024 this item held 23,80 % of the residual mix and in 2023 it held 21,05 %. Over two years it therefore rose by 8,01 percentage points, while the total fossil share of the residual mix moved in the same period between 47,94 % and 52,39 %.
Hard coal: 18,20 % of the residual mix
The second largest fossil item of the residual mix is hard coal with a share of 18,20 %. In the generation mix the same item holds 1,00 %, so the difference between the two mixes is 17,20 percentage points — the sharpest disproportion between them at all, if the nuclear component is left aside.
Here too it is a persisting state, not a one-off swing. In 2024 hard coal held 17,33 % of the residual mix and in 2023 it held 18,86 %, while in 2019 it was 6,86 %. The data for 2023 sit on a separate annual page.
Nuclear sources: 35,68 % of the residual mix
The nuclear component is the only one exceeding a third of the residual mix, and at the same time the only one markedly higher in the generation mix: 67,39 % against 35,68 %, a difference of 31,71 percentage points. It is this component that also sets the order for radioactive waste, where the generation mix at 2,36 mg/kWh sits above the residual mix at 1,2 mg/kWh.
Movement over time is sharpest here of the whole table. In 2019 the residual mix held a nuclear component of 55,49 %, in 2022 it held 69,06 % and in 2025 only 35,68 %. The span between the outer years is 33,38 percentage points.
Renewable sources: 11,93 % of the residual mix
The renewable component of the residual mix is 11,93 %, which is less than the 18,17 % of the generation mix and considerably less than the 34,40 % of the supplier mix. Inside it, one figure stands out: solar sources hold 5,12 % of the residual mix against 3,05 % of the generation mix, that is more.
With hydro it is the other way round: 1,53 % in the residual mix against 10,71 % in the generation mix. The difference between these two pairs says which generation is covered by guarantees and which is not — without any price or volume of guarantees being derivable from these figures. All four values come from the annual mix page for 2025 and relate to the calendar year 2025.
The solar component moves over time more sharply than any other renewable. In 2024 it held 0,98 % of the residual mix, in 2023 it held 1,86 % and in 2025 already 5,12 %; over two years it therefore rose by 4,14 percentage points. The wind component holds 0,00 % in the generation mix and 1,03 % in the residual mix, a difference that appears in the tables of earlier years as well.
Where the error sits in models: three mixes, three factors
The error does not arise in the calculation but in the choice of factor. If consumption is multiplied by the generation-mix factor of 87,04 gCO₂/kWh instead of the residual factor of 350,70 gCO₂/kWh, the result is 4,03 times lower — and the difference has nothing to do with how much electricity was actually taken.
The gap between the two factors is moreover widening. In 2019 the ratio was 1,31-fold, in 2021 it was 1,12, in 2023 it was 2,91, in 2024 it was 3,37 and in 2025 already 4,03. The residual factor rose over six years by 76,59 %, the generation-mix factor fell over the same period by 42,40 %; the starting values of 2019 sit on the oldest published annual page.
The denominator behind each of these figures is one kilowatt-hour of the mix in question for a calendar year. The register states the volume in TWh for the supplier mix (23,44 TWh) and for the generation mix (26,58 TWh); for the residual mix it states none at all — weighted calculations with the residual mix therefore have no basis in these pages.
The third factor, the one of the supplier mix, is often left out of this comparison, although it moves over time differently from both outer ones. In 2019 it held 187,10 gCO₂/kWh, in 2022 it held 147,35 and in 2025 it held 259,19; against 2019 that is a rise of 38,53 %, movement in the same direction as the residual factor but half as steep.
Conclusions
The residual mix for 2025 carries an emission factor of 350,70 gCO₂/kWh. The supplier mix carries 259,19 and the generation mix 87,04 gCO₂/kWh; the ratio between the outer values is 4,03-fold and all three figures come from the same annual page of the register.
More than half of the residual mix is fossil. The fossil share is 52,39 %, of which gaseous fuel 29,06 % and hard coal 18,20 %. In the generation mix the fossil group holds 14,44 %, that is 37,95 percentage points less.
The gap between the mixes is growing. The ratio of the residual factor to the generation-mix factor was 1,31 in 2019 and 4,03 in 2025. The residual factor rose by 76,59 %, the generation factor fell by 42,40 %; the values for 2024 sit on the annual mix page for 2024.
Every factor needs its denominator stated. It is one kilowatt-hour of a specific mix for a calendar year, not a country average without further definition. For the residual mix the register states no volume in TWh, so any weighted comparison with it has no support in the published data.
Analysis, not personalised advice. Before a transaction decision, verify the tariff value or the wording of the regulation directly with the network operator or the regulator.