Reliable supply means having electricity when it is needed. Summer surplus helps winter supply when energy can be stored or exchanged for delivery later.

We need a combination of production, stored energy, flexible demand and dependable trade.

Two seasons, different balances

In the 2024/25 electricity year, Switzerland exported more electricity than it imported. During that year’s winter, imports exceeded exports. Winter 2023/24 had net exports. The balance changes between years.

BFE: seasonal balances, table 5
Observed cross-border trade · five yearsA surplus over the year can hide a winter import balanceRead both seasons. Some winters also have net exports.
Net importsNet exports

TWh · bar length shows the size of each balance · common scale 0–15

2020/2021

Winter1.82imported
Summer3.30exported

2021/2022

Winter7.82imported
Summer2.22exported

2022/2023

Winter3.64imported
Summer6.98exported

2023/2024

Winter2.04exported
Summer12.15exported

2024/2025

Winter0.73imported
Summer4.61exported

Winter: October–March. Summer: April–September. Each value is imports minus exports, or the reverse. The figures record physical cross-border trade. Assessing supply dependence requires hourly conditions and alternative-supply assumptions.

BFE table 5 · Full balances (CSV)

Imports meet demand with electricity delivered across the border. To assess dependence on that supply, we also need hourly flows and a case with restricted imports.

Balance definitions and limits

What supplies the winter months?

Hydro and nuclear were the two largest sources of domestic generation in winter 2024/25. Solar also contributed. The chart includes electricity generated from previously pumped water; pumping uses electricity elsewhere in the balance.

BFE: generation categories and pumping, table 5
Where Swiss winter electricity came from
Domestic generation, October 2024–March 2025. TWh.
  • Hydro18.45
  • Nuclear12.596
  • Other thermal1.887
  • Solar1.773
  • Wind0.092

Generation before subtracting electricity used to pump water uphill. Hydro includes pumped-storage output. The totals cover winter 2024/25.

BFE: generation by technology, table 5

Our assessment: retain useful winter production where safety and economics support it. Assess replacement projects against the hours and seasons they must cover. The replacement must cover the timing of demand as well as its annual total.

Put a supply plan through a difficult week

The following case combines hypothetical new nuclear and solar capacity with hydro. It permits neither imports nor batteries. Pumped storage remains available, with finite storage limits.

Case definition and full-year result
Scenario · a deliberately difficult weekAn exporting system can still face shortfallsHypothetical 2050 demand and capacity, using 2011/12 weather. The model shows the shortfall under these stress assumptions.

The case assumes 8.8 GW of new nuclear, with one 1.1 GW unit unavailable for 90 winter days. Solar has 26 GWp of rated peak power. Hydro and pumped storage remain; imports and batteries are excluded.

DemandDemand metUnmet demand

Hourly power, GW · 6–12 February · scroll the chart on a small screen

Demand and served electricity in the selected modelled winter weekDemand exceeds supplied electricity in some hours. Total unmet demand is 118.4 GWh. A daily table and hourly download follow.0510152025306 Feb7 Feb8 Feb9 Feb10 Feb11 Feb12 Feb

118.4 GWh unmet in this week. It is the complete winter calendar week with the greatest unmet energy in this case. The same case has annual net exports.

Daily energy ledger, selection and model limits

Complete Monday–Sunday week with the greatest unserved energy among winter weeks in this solved case; 168 hourly intervals. Selected deliberately to illustrate stress, not typical operation.

National model electricity bus, including modelled network losses; excludes pumping input from the demand line. Served demand = bus demand − unserved demand. Generation and storage flows are separately provided in the ledger.

GWh per day, rounded. Hydro excludes pumped discharge in this table.
FebruaryDemandNuclearSolarHydroPumped outputPumping inputImportsExportsUnused generationUnmet
6513.2181.166.9211.435.313.300031.7
7498.1181.142.423133.112.900023.4
8455.1181.143.1216.724.525.400015.1
9451.3181.149.2211.524.825.20009.8
10432.7181.151.21992029.900011.3
11430.3181.136185.617.50.900011
12442.1181.163.6171.616.76.200.8016.2

The model knows future conditions, pools natural hydro and omits internal grid constraints and commercial dispatch. Project delivery, commercial feasibility and the likelihood of shortage require further analysis.

Storage, imports or different generation and demand could change the outcome. Their costs and availability need separate tests.

168-hour ledger (CSV) · Selection and definitions (JSON)

Full-year case and limitations · Reproducible week selection

This demanding domestic-supply test exposes a timing gap despite substantial annual exports. It tests the specified hypothetical mix. Comparing it with the official strategy requires matched scenarios and the same import assumptions.

Selection, assumptions and limits

Four ways to strengthen the balance

Produce when needed

Keep useful generation available

Assess continued safe nuclear operation, maintenance timing and credible replacement projects. Allow for prolonged outages.

Examine the nuclear record →
Move energy between seasons

Develop specific hydro projects

Check the water, storage space, permits and business case. Pay owners for the defined service and the costs it imposes.

See what the dams can add →
Change the timing of use

Reward voluntary flexibility

Test charging and heating schedules that preserve the required travel and comfort. State participation and operating limits.

Understand the demand assumptions →
Share supply, test dependence

Give trade an explicit role

Compare normal imports with restricted-import cases. Use the domestic-only case to test a loss of access to imports.

Compare the planning tests →

These measures need specific assets, contracts and participation commitments. The hourly supply test and the owner’s investment case address different parts of that decision. Demand assumptions · Owner economics and boundaries

Our proposal: publish a winter delivery budget

Our proposed action

Show what each project can deliver, and by when.

For each supply option, publish winter energy, available power, delivery milestones and a case where a major project is late.

Project owners should offer defined services under viable contracts. Buyers should pay for those services, with their costs visible in the comparison.

Apply the same dry-year, outage and import tests to a nuclear-inclusive plan and to the current strategy.

The official monitoring framework already covers winter imports, networks and supply security. Our critique should test its assumptions and delivery measures, with the same scrutiny for our proposed alternatives.

BFE: monitoring framework

References

  1. Swiss Electricity Statistics 2025
    Swiss Federal Office of Energy (BFE) · 2025
  2. Swiss seasonal balances and electricity examples
    WattInfo · Five hydrological years, 2020/21–2024/25
  3. Visual explanations: seasonal stress, demand and storage
    WattInfo · Visual chapter data and explicit teaching assumptions
  4. Hourly system screening: no-battery case
    Swiss Energy research project · Frozen research output
  5. Independent demand model v1
    Swiss Energy research project · Frozen research output
  6. Storage owner economics: historical-price benchmark
    Swiss Energy research project · Frozen research output
  7. Energy Strategy 2050: Monitoring Report 2025, full report
    Swiss Federal Office of Energy (BFE) · 2025 report, corrected March 2026

Find definitions, calculations and model inputs in the source register.