Hydro projects can change how quickly electricity arrives and how much water is kept for winter. Those are different services. We prioritise Grimsel 4 for power, Grimselsee for storage and a viable Mattmark shift for winter production.
Water, turbines and owner rights limit delivery. We support these projects with voluntary, viable contracts; the current hourly cases give future projects no extra supply credit.
Project priorities and conditionsThree different ways to improve hydro
River plants use flowing water. Reservoirs can retain natural inflows. Pumped storage uses electricity to lift water for later generation. A scheme can combine these functions. Hydro principles · Pumped storage
More room for water
A larger reservoir can retain more inflow for later use. The annual energy gain depends on the project.
More power
A larger turbine can deliver more electricity at once. It does not create more water.
Pumping back uphill
Where a pump and connection exist, electricity can lift water for reuse. The cycle has energy losses.
This diagram explains the functions. Actual lake connections vary by project; pumping requires an installed pump and a physical route uphill.
Reservoir history: a full lake is an assumption
Swiss reservoirs held 6.38 TWh on 19 October 2026, or 71.7% of reported capacity. That is below every earlier year in our 2000–2025 nearest-week comparison. It is a dated stock comparison, not a forecast of a shortage.
Stored electrical energy, TWh
Secure the starting water as well as the turbines. Our supply programme already tests lower lake levels; a national stock total does not give it rights to an owner’s water. See the lake-level supply cases.
Dates, capacities and comparison method
For each year, we select the weekly observation closest to 19 October. Reported capacity changes over the series. This comparison does not measure water that is contractually available for a national reserve. The supply model starts on 1 October; this chart compares a later date.
| Year | Observation date | Stored energy, TWh | Reported capacity, TWh |
|---|---|---|---|
| 2000 | 16.10.2000 | 8.231 | 8.56 |
| 2001 | 22.10.2001 | 7.648 | 8.56 |
| 2002 | 21.10.2002 | 7.482 | 8.56 |
| 2003 | 20.10.2003 | 6.877 | 8.56 |
| 2004 | 18.10.2004 | 7.373 | 8.54 |
| 2005 | 17.10.2005 | 6.491 | 8.54 |
| 2006 | 16.10.2006 | 7.786 | 8.505 |
| 2007 | 22.10.2007 | 7.103 | 8.52 |
| 2008 | 20.10.2008 | 7.19 | 8.505 |
| 2009 | 19.10.2009 | 7.075 | 8.765 |
| 2010 | 18.10.2010 | 7.369 | 8.765 |
| 2011 | 17.10.2011 | 7.023 | 8.78 |
| 2012 | 22.10.2012 | 7.195 | 8.77 |
| 2013 | 21.10.2013 | 7.219 | 8.795 |
| 2014 | 20.10.2014 | 7.671 | 8.8 |
| 2015 | 19.10.2015 | 7.199 | 8.82 |
| 2016 | 17.10.2016 | 6.906 | 8.825 |
| 2017 | 16.10.2017 | 7.487 | 8.84 |
| 2018 | 22.10.2018 | 7.174 | 8.85 |
| 2019 | 21.10.2019 | 8.124 | 8.85 |
| 2020 | 19.10.2020 | 7.723 | 8.87 |
| 2021 | 18.10.2021 | 7.397 | 8.865 |
| 2022 | 17.10.2022 | 7.415 | 8.885 |
| 2023 | 16.10.2023 | 7.545 | 8.895 |
| 2024 | 21.10.2024 | 7.646 | 8.9 |
| 2025 | 20.10.2025 | 7.093 | 8.895 |
| 2026 | 19.10.2026 | 6.379 | 8.895 |
More turbine power helps meet a peak. More stored water can extend delivery or move it into winter. Neither automatically increases the water flowing into the system. This is why the projects below have different jobs.
Project definitions and reviewed distinctionsReal projects, different benefits
The projects below would add different capabilities. Their current development stages show what remains before those benefits can be delivered.
Grimselsee
More stored energy for winter
Additional storage in the proposed enlargement. The building-permit phase comes next.
Additional stored energy. The effect on annual generation is a separate project question.
Stage supported by evidence dated 2026-09-09.
Mattmark
Move production into winter
The operator estimates CHF 83 million. Annual production would stay unchanged.
Potential seasonal transfer. The project status is supported by the dated proposal below.
Stage supported by evidence dated 2025-01-16.
Grimsel 4
More power between existing lakes
Two 84 MW machines; a CHF 300 million investment decision. Works were scheduled to finish in 2032.
Adds power and flexibility. Additional reservoir volume and natural generation are outside the stated contribution.
Stage supported by evidence dated 2026-07-07.
GWh measures stored or generated energy. MW measures power. These project figures describe different services and cannot be added together.
Grimsel 4 is under construction. Grimselsee has its concession but still needs a building permit. Mattmark’s proposal has open concession, environmental and economic conditions. Our current hourly cases credit none of these additions.
Project priorities and their remaining delivery conditions
Why we prioritise these projects
Grimsel 4 is the delivery priority among these examples. Construction has started, with completion scheduled for 2032 in the reviewed evidence. It adds 168 MW between existing lakes. We value that power and flexibility, while crediting no new reservoir energy.
Construction milestone and schedule Power and investment decisionGrimselsee is the next storage-development priority. The proposed enlargement adds 240 GWh of storage. Its concession is a step towards delivery; a building permit and further work remain. We support advancing that work without counting the storage as already available.
Proposed storage contribution Concession and next stageMattmark is a concrete candidate for more winter production. Its proposed 60 GWh shift uses existing inflow. We recommend pursuing it where concession, environmental and commercial terms make it viable. Annual generation remains unchanged in the operator’s proposal.
Seasonal shift and unresolved conditionsThese priorities reflect the documented services and development stages. A final investment choice also depends on the owner’s complete project economics.
Basis and limits of the prioritiesOther hydro projects and their delivery conditions
A larger pipeline, with very different delivery prospects
Gornerli deserves development alongside Grimselsee: the operator projects 435 GWh of winter storage around 2040 and a CHF 510 million investment. The eventual 550 GWh potential depends on later glacier retreat. These are staged projections, not electricity already secured for 2050.
Chlus would add generation; lake-raising projects mostly change when water can be used. Marmorera depends on lengthy road works. Curnera–Nalps is not currently being pursued. A place on a priority list does not set a commissioning date.
Dated project records and operator sources
All 18 reviewed projects and their development stages
| Project | Development stage |
|---|---|
| Chummensee | Feasibility studies; design and funding open |
| Curnera-Nalps | Not currently pursued by the operator |
| Gorner | Design and preparation for permits |
| Gougra | Moiry variant selected; preparing permits |
| Griessee | Development; current delivery date unresolved |
| Grimselsee | Concession granted; building permit still needed |
| Lac d’Emosson | Preliminary study under a binational framework |
| Lac des Toules | Planning designation; investment stage unresolved |
| Lago del Sambuco | Design contract awarded |
| Lai da Marmorera | Road relocation and concession dependencies |
| Mattmarksee | Dam-raising variant under study |
| Oberaarsee | Early feasibility study |
| Oberaletsch klein | Current project identity and stage unresolved |
| Reusskaskade | Next design phase started |
| Trift | Alternative access under study |
| Chlus | Concession granted; project approval and investment pending |
| Grimsel 4 | Construction started; adds power between existing lakes |
| Dix+ | Technical feasibility reported; regulatory questions open |
- Without support24 GWh/year
- With current support792 GWh/year
- Unprofitable even with support431 GWh/year
Surveyed opportunities only; these are annual energy quantities, not winter delivery or the total national hydro potential. Profitability with support does not establish a final investment decision.
Water rights and changing inflows belong in the contract
Moiry’s concession expires in 2039 and the Reuss scheme’s in 2043. Renewal terms and compensation for remaining asset value affect whether owners invest. A national supply model cannot assign their water or turbine capacity for free. Concessions and owner conditions
Residual-flow rules preserve water in rivers below abstractions. Climate change also shifts snowmelt and river flows, while shrinking glaciers change the resource over time. Test each project against its lawful water allocation and seasonal inflows, rather than assuming today’s pattern continues unchanged. FOEN: residual flows · FOEN: climate and water
Why holding water has a cost
Keeping water for winter can mean giving up an earlier sale. The later electricity must earn enough to justify that choice. Pumped storage also buys charging electricity and loses some energy during the cycle. Hydro seasonal-shift economics · Storage losses
The BFE-commissioned hydro revenue study also accounts for this trade-off: reserving capacity for grid services can reduce income from electricity sales. An owner needs the combined result after those competing uses are reconciled.
Hydro revenue study: reserve capacity and foregone sales, page 6Our position: offer voluntary contracts with clear delivery obligations. State who pays for availability, who pays for delivered electricity, and who carries the risk. Price availability and delivered energy separately in the agreement.
Concessions, partner plants and who carries market risk
Axpo’s concession paper describes partner plants whose shareholders pay annual costs and receive their share of output. Those electricity-taking shareholders bear market risk. Changing ownership leaves maintenance, financing and operating obligations to be funded.
Axpo: partner-plant costs, rights and risks, pages 23–25Under current rules, eligible hydro owners must provide a compensated reserve. Our proposed voluntary winter-supply contracts must respect those existing obligations.
ElCom: existing reserve duty and compensationMatch the delivery contract to the water right and its renewal date. Identify who controls the electricity and who funds investment, maintenance and future renewal. Compare competing owners and service offers on the same obligations. Axpo’s preferred partnership is a commercial position, rather than a binding plant offer.
Concession renewal, ownership choices and capital riskThe Grimselsee owner-cost example
The owner gives a CHF 307 million budget in 2025 prices. With capital recovery, assumed upkeep and summer sales forgone, our example needs around CHF 108 per shifted MWh. That assumes one 240 GWh shift each year, a 60-year life, 5% return and CHF 30/MWh summer opportunity cost.
Project budget Capital, upkeep and owner compensationMajor maintenance at Bieudron and operating flexibility trials
Keep capacity available through major maintenance
Hydro also needs long periods of maintenance. Bieudron’s July 2026 update describes an overhaul of about 9 months per generating unit. Its staged programme keeps the other units operating and reports CHF 54.4 million of investment. That is a programme amount while work continues, rather than a final audited cost.
Operator: overhaul scope, investment and schedulePreserve owner funding for renewal and plan around unavailable machines. Our national hourly calculation pools hydro capacity; a dependable delivery contract also needs each station’s maintenance schedule. A reservoir full of water cannot compensate for every unavailable turbine or constrained connection.
Hydro representation and operational limitsGet more flexibility from existing plants
The SmallFLEX Goms trials at Mörel demonstrated additional power adjustment of approximately ±3 MW. Controlled use of water already inside the plant’s waterways can provide short-term flexibility. Safe water levels, inflow and the risk of air entering the waterways limit how it can operate.
Mörel trials and hydraulic limitsHydroLEAP’s full-scale tests also show why rapid response has operating limits: pump trips affect turbine output and pressure, while recirculation heats the water. Its fatigue and laboratory studies identify wear questions, rather than audited lifetime savings for every station.
Full-scale tests, wear models and refurbishment evidenceOur proposal: assess these operating improvements alongside larger construction projects. Buy a defined power response for a stated duration, with maintenance costs and foregone sales included.
What the hydro trial establishes
The reported result concerns short-term power adjustment under tested conditions. A winter-energy contribution needs its own duration and water balance.
The report also models business cases. Its imbalance-cost analysis uses annualised forecast data that are not fully aligned with market-price dates, and describes its stochastic model as unfinished. Realised owner returns require further evidence.
Summary, page 4; trials, pages 128–136; economic limits, page 162
Storage explains where batteries and electric cars can complement hydro.
References
- WattInfo archive-to-model checks: demand, thirteen weather years and heat storage
WattInfo · Additive model evidence review, 18 September 2026 - Bieudron overhaul programme July 2026
Grande Dixence · Operator works update, 15 July 2026 - HydroLEAP final report
Swiss Confederation / ARAMIS · 2025-07-22 - WattInfo proposed supply path: decisions and supporting calculations
WattInfo · Proposed priorities and fixed-charging supply screen, 18 September 2026 - Hydropower explained
U.S. Energy Information Administration (EIA) · Web edition retrieved 2026-09-16 - Energy storage for electricity generation
U.S. Energy Information Administration (EIA) · Web edition retrieved 2026-09-16 - Grimselsee enlargement: project design
Kraftwerke Oberhasli AG (KWO) · Undated project page; design includes older milestones - Grimselsee concession and Grimsel 4 construction start
Kraftwerke Oberhasli AG (KWO) · Published 2026-09-09 - Mattmark: proposed dam raising
Axpo · Published 2025-01-16 - Grimsel 4: investment decision and technical specification
Kraftwerke Oberhasli AG (KWO), mirrored by VSE · Published 2026-03-27 - Storage owner economics: historical-price benchmark
Swiss Energy research project · Frozen research output - Visual explanations: seasonal stress, demand and storage
WattInfo · Visual chapter data and explicit teaching assumptions - SmallFLEX Goms: flexibility from existing hydro
HES-SO and project partners / BFE · Final report dated 22 May 2026 - Hydropower revenue opportunities and marketing costs
Swiss Economics / ETH Zürich for BFE · Final report dated 30 September 2023 - KWO Grimselsee expansion
Kraftwerke Oberhasli AG · Archived edition reviewed 22 September 2026 - Winter solar: capacity, seasonal costs and paid storage
WattInfo · Conditional component comparison, 22 September 2026 - Gewässer im Klimawandel
BAFU · Undated page; checked 2 October 2026 - Ausbaupotenzial bei bestehenden Wasserkraftanlagen
BFE · 2025-06-13 - Hydro development pipeline and operating constraints
WattInfo · 2026-10-02 - Restwasser
BAFU · Undated page; checked 2 October 2026 - Renewal of hydropower concessions: opportunities and challenges
Axpo · 2026-10-08 - Reservoir energy and filling levels, official machine-readable history
BFE · 2026-10-23 - Reservoir weekly statistics: official factsheet
BFE · 2025-05-30 - ElCom directive 1/2026: hydro reserve and owner compensation
ElCom · Directive 1/2026, winter 2026/27 - ElComJuly2026newsletter:final hydro reserve holding-payment rate
ElCom · 2026-07-06 - ElComJuly2025newsletter:correction of hydro reserve holding-payment rate
ElCom · 2025-07-23 - Delivery budgets and paid flexibility: calculations and boundaries
WattInfo · 2026-11-06
Find definitions, calculations and model inputs in the source register.