Electricity, heat and air conditioning · 2015–2026
What cooling a home costs the grid: the megawatts in every degree of Romanian heat
At 35 °C outside, a Romanian summer afternoon demands about 1,170 MW more than a 26 °C one: more than one reactor at Cernavodă, the country's nuclear power station, and less than both. The price of a degree has not changed in ten years. The hour has: since 2023, summer's heaviest hour falls in the evening, at 20:00 or 21:00.
#The butterfly: load falls, then heat lifts it back up
Plotted against the temperature outside, Romania's load draws two shoulders: cold lifts it in winter, and from about 26 °C upwards, cooling lifts it in summer. On June–August weekdays, each degree of the daily maximum above 26 °C added on average about 130 MW to the afternoon peak. The slope has not moved in a decade: in each of four three-year periods it came out between 126 and 134 MW a degree, with no direction. The level has: in recent years the whole butterfly sits lower, most of all at midday, when rooftop panels cover part of the demand.
So on a 35 °C day the afternoon demands roughly 1,170 MW more than on a 26 °C one. One reactor at Cernavodă, Romania's nuclear power station on the Danube, delivers about 706 MW; the two together, about 1,411 MW. In the evening, between 18:00 and 23:00, a degree of heat costs a little less, about 100 MW, again with no rise from year to year.
#The summer peak has moved from midday to evening
From 2015 to 2022, each summer's record day peaked at 13:00. Since 2023, at 20:00 or 21:00. The shape of a July weekday has flipped, but not because evenings grew: at 21:00 the mean load stayed almost the same, 7,226 MW in 2022 and 7,195 MW in 2023. Midday fell: at 13:00, from 7,470 MW to 6,887 MW, in a single year.
The 2023 jump did not come from reported solar generation, which rose by only 64 MW at midday that year. The likeliest explanation for most of it is rooftop panels (at night, load fell by only 132 MW): power that homes and firms generate and use on the spot never reaches the measured load, so midday looks ever lighter to the grid. Reported solar has grown too, from a mean of 608 MW at midday in July 2015 to 2,091 MW in July 2026. In the evening, as the sun sets, the grid carries everything again.
#Summer has not beaten winter yet
In every one of the twelve years, the annual peak stayed a winter one. The gap has narrowed, though: in 2024, the summer peak, 8,530 MW on 17 July 2024 at 20:00, came within 230 MW of the winter peak, 8,760 MW on 23 January 2024. In winter, each degree the day's mean temperature falls below 5 °C added between 61 and 89 MW to the evening peak, with no clear upward trend.
| Year | Summer peak (MW) | Winter peak (MW) | Gap (MW) |
|---|---|---|---|
| 2015 | 7,945 | 9,334 | 1,389 |
| 2016 | 8,023 | 9,575 | 1,552 |
| 2017 | 8,169 | 9,728 | 1,559 |
| 2018 | 8,058 | 9,695 | 1,637 |
| 2019 | 8,334 | 9,443 | 1,109 |
| 2020 | 8,211 | 9,320 | 1,109 |
| 2021 | 8,756 | 9,614 | 858 |
| 2022 | 8,032 | 9,211 | 1,179 |
| 2023 | 7,793 | 8,623 | 830 |
| 2024 | 8,530 | 8,760 | 230 |
| 2025 | 8,127 | 8,882 | 755 |
| 2026 | 7,970 | 9,208 | 1,238 |
Each calendar year's peaks: the heaviest day in June–August and the heaviest day in January, February and December of the same year. For 2026 the data stop on 5 November, so December is missing.
#Your city: how much cooling a summer asks for
Cooling degree days add up, day by day, how far the day's mean temperature passed 24 °C. Between the two decades below, the sum rose in every city on the list. Pick a city, set the cooled area and your own price per kWh.
In București, a summer asked for a mean of 45.8 cooling degree days in 1991–2000 and 133.4 in 2016–2025. For 50 m² cooled, the simple model set out in Method gives about 55 kWh per summer then and 160 kWh now; at 1 leu/kWh, about 55 lei then and 160 lei now. Nights with a minimum of at least 20 °C: 10.5 per year then, 31.5 now.
| City | Cooling degree days | Days of at least 35 °C | Nights of at least 20 °C |
|---|---|---|---|
| București | 45.8 → 133.4 | 2.0 → 6.7 | 10.5 → 31.5 |
| Cluj-Napoca | 1.7 → 10.0 | 0.0 → 0.0 | 0.1 → 1.3 |
| Iași | 24.7 → 68.6 | 0.5 → 1.2 | 6.7 → 21.2 |
| Constanța | 21.4 → 62.3 | 0.1 → 0.0 | 22.7 → 48.0 |
| Timișoara | 50.3 → 109.1 | 2.0 → 6.0 | 9.9 → 24.4 |
| Brașov | 0.4 → 3.1 | 0.0 → 0.0 | 0.0 → 0.6 |
| Craiova | 87.6 → 181.0 | 4.9 → 10.1 | 21.8 → 42.2 |
| Galați | 55.0 → 133.5 | 1.4 → 3.6 | 20.0 → 42.7 |
| Oradea | 31.1 → 67.0 | 1.0 → 1.6 | 7.4 → 18.3 |
| Ploiești | 14.6 → 61.6 | 0.2 → 1.6 | 2.7 → 14.1 |
| Sibiu | 1.1 → 5.8 | 0.0 → 0.0 | 0.1 → 0.7 |
| Arad | 44.5 → 95.2 | 1.5 → 4.3 | 10.8 → 24.3 |
| Pitești | 4.9 → 21.1 | 0.2 → 0.2 | 0.7 → 4.6 |
| Bacău | 8.6 → 35.1 | 0.2 → 0.5 | 2.1 → 9.5 |
| Târgu Mureș | 3.0 → 22.7 | 0.0 → 0.2 | 0.4 → 4.2 |
| Baia Mare | 5.1 → 16.7 | 0.0 → 0.0 | 1.4 → 5.0 |
| Buzău | 35.6 → 114.6 | 1.3 → 4.2 | 8.0 → 26.8 |
| Suceava | 2.3 → 11.9 | 0.0 → 0.1 | 0.3 → 1.9 |
| Drobeta-Turnu Severin | 37.9 → 94.7 | 1.9 → 4.4 | 7.9 → 21.5 |
| Giurgiu | 68.7 → 165.2 | 2.9 → 8.4 | 17.6 → 41.0 |
Cooling degree days (24 °C base, daily mean), days with a maximum of at least 35 °C and nights with a minimum of at least 20 °C, ERA5-Land yearly means. (1991–2000 → 2016–2025).
#Method, limits and related pages
Romania's hourly load is the ENTSO-E series republished by Energy-Charts (Fraunhofer ISE), 1 January 2015 to 5 November 2026, averaged into whole hours, Romanian time. Hourly temperature is ERA5-Land, the 9 km version of Copernicus's reanalysis, read from the Open-Meteo open archive (to 31 October 2026) and averaged with 2021 population weights for the ten largest cities. The per-degree slope comes from regressing the daily afternoon peak (12:00–18:00, weekdays, June–August) on the part of the daily maximum above 26 °C, with a separate level for each year and each month. Without those levels, the 2024–2026 slope would have come out at 157 MW a degree: load fell from one of those summers to the next, and the hottest of them, 2024, also had the highest level. The 26 °C threshold is a choice: with a threshold anywhere from 24 to 28 °C, the gap between a 35 °C day and a 26 °C one comes out between 990 and 1,170 MW.
The limits are in the measurement. The published series is the load the grid sees: self-consumption from rooftop panels does not appear in it, which is largely why a model without yearly levels, fitted on 2015–2023, overstates 2024 afternoons by 773 MW on average. The slopes mix home cooling, offices, shops and heat-sensitive industry; this page does not separate them. For each city, ERA5-Land is read at the nearest land cell, with the temperature brought to the city's altitude (6.5 °C per kilometre). Consumption is a simple model: cooling degree days times area, with a loss coefficient of 3.0 W per square metre per degree and a cooling efficiency of 3.0. The ratio between decades comes from the data; the absolute level rests on those two assumptions.
Related: the hourly price of power, in Power is no longer cheap at night; life without nuclear output, in The days without Cernavodă; what rooftop solar earns, in What a rooftop kWh is worth.
Sources
Energy-Charts (Fraunhofer ISE), Romania's load and solar generation from ENTSO-E · ERA5 and ERA5-Land, Copernicus, via Open-Meteo · Nuclearelectrica, the Cernavodă units' output (706.5 and 704.8 MW) · National Institute of Statistics, city populations at the 2021 census.