Power is how fast. Energy is how much.
Power tells you how quickly a plant supplies electricity or a device uses it. Energy is the amount supplied or used over time.
Power
Measured in watts. Large plants use megawatts (MW) or gigawatts (GW).
Energy
Measured in watt-hours. Your bill uses kilowatt-hours (kWh). National totals often use terawatt-hours (TWh).
One TWh is one billion kWh. A large power rating alone does not tell us how much electricity a plant supplies in a winter.
EIA: power and energy unitsUnit conversions and capacity factor
One GW is 1,000 MW. One TWh is 1,000 GWh, and one GWh is 1,000 MWh.
Capacity factor compares actual generation with the energy a plant would produce at its rated power throughout the same period.
It is not simply the share of time a plant runs. Operation below full power also affects the result.
Calculate actual MWh ÷ (capacity in MW × hours in the period). Keep the measurement boundary consistent.
Capacity-factor method and Swiss reactor examplesA yearly surplus can hide a winter deficit.
Switzerland exported more electricity than it imported over the year from October 2024 to September 2025. In winter, the balance went the other way.
Observed physical electricity trade, in terawatt-hours (TWh).
Winter
October 2024–March 2025
0.73 TWh
net imports
Summer
April–September 2025
4.61 TWh
net exports
Full year
October 2024–September 2025
3.88 TWh
net exports
Net imports mean imports exceed exports over the period. Net exports mean the reverse. Neither figure measures power cuts.
BFE: seasonal electricity balances, table 5Compare five winters and check the calculation
| Winter | Net imports (TWh) | Direction of balance |
|---|---|---|
| 2020/2021 | 1.82 | Net imports |
| 2021/2022 | 7.82 | Net imports |
| 2022/2023 | 3.64 | Net imports |
| 2023/2024 | -2.04 | Net exports |
| 2024/2025 | 0.73 | Net imports |
Subtract physical exports from physical imports to get net imports. Divide GWh by 1,000 to get TWh.
The full-year export balance equals summer net exports minus winter net imports. Use the unrounded data for exact calculations.
These flows show what happened across the border. They do not establish how much trade was unavoidable or what was available in each hour.
Download all seasonal balances (CSV) · Definitions and data (JSON)
Calculation, original data and definitionsImports are not a blackout. They are electricity supplied from abroad. A winter import balance and unmet demand are different things.
Winter imports are not inevitable every year: Switzerland had net exports in winter 2023–24.
BFE: five winter trade balances, table 5For a future plan, ask what happens during difficult hours, including outages. An annual balance alone cannot answer that question.
Hydro and nuclear supply much of the winter electricity.
The measured winter mix also includes solar, wind and other thermal plants. Winter solar generation is lower than summer generation, but it is not zero.
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. This is one winter, not a forecast.
BFE: generation by technology, table 5What this generation mix includes
BFE records electricity used to pump water uphill separately. Subtract that input before calculating the national net electricity balance.
Electricity lost in the grid must also be allowed for when comparing supply with the amount delivered to customers.
“Other thermal” is BFE’s thermal generation category excluding nuclear. These totals describe domestic production, not the origin of a household’s purchased electricity.
Seasonal solar totals reflect both changing weather and additions to the installed fleet. They are not a comparison of an unchanged set of panels.
All generation, pumping, trade, loss and consumption fieldsExisting nuclear output is evidence of its contribution. It does not guarantee future availability or establish the cost of a new plant.
See ten years of reactor output, safety conditions and the case for nuclear →
Storage moves energy to a different time.
A battery must first be charged. Pumped storage uses electricity to lift water, then releases it through turbines. Both return less electricity than charging consumes.
EIA: storage, charging and energy lossesA simple example
An imaginary store holds 4 GWh of usable energy and supplies 1 GW continuously. It lasts 4 hours.
Assumed full store, no recharge and no further discharge losses or reserve. This is a calculation example, not a Swiss project.
Duration equals usable energy divided by power. The same unit rule applies to a battery or a water reservoir.
Example inputs, formula and limitationsHydro needs an extra distinction. Rivers supply flowing water. Reservoirs can hold natural inflows for later use. Pumping uses electricity to move water uphill.
EIA: river, reservoir and pumped-storage hydroOur planning rule: specify the hours to cover, the energy to store and how it will be replenished. Then test costs and operating income.
Do not assume that a hydro owner can provide a service without covering its costs. Apply the same test to batteries and new generation.
Replacing fuel changes electricity demand.
Electrification means using electricity for a task previously powered by another energy source. Replacing an oil boiler with an electric heat pump is one example.
A heat pump uses electricity to move heat from air, ground or water into a building. Part of the delivered heat comes from the environment.
SwissEnergy: how a heat pump worksSo we cannot replace each unit of heating oil with one unit of electricity in a demand calculation. We must account for the equipment’s performance.
Our demand scenarios model these changes. Their results depend on assumptions about transport, heating, industry and efficiency. They are not forecasts.
Demand model: inputs, efficiency and scopeAsk the same questions of every plan.
Our proposed test applies to a nuclear-inclusive plan and to the current strategy:
- Does it cover demand in difficult hours, as well as over the year?
- What must be built, connected and operating by each date?
- Which imports, fuels, materials and suppliers does it depend on?
- Who pays, and can the owners earn enough to maintain and replace their assets?
- What happens if costs rise, projects arrive late or a major plant stops?
We support safe existing nuclear and a credible path to new nuclear. That position must survive these tests.