A country like Germany could expand its current electricity production by about 50% while simultaneously phasing out nuclear and fossil fuels at a whole sale price for electricity below current market rates and with current technology.

In case of Germany, a 2015 study compared various scenarios for how electricity demand will develop by 2050. The most expansive scenario assumes a yearly demand of 800 TWh, compared to roughly 500 TWh in 2021. [1] The additional demand comes from electrifying cars and heating, etc.

Demand does of course vary by time of day, but is virtually stable from month to month. Variability of renewable production is quite high, so let's assume that you'd need long-term grid-scale electricity buffers for about 30% of yearly demand. The most basic approach to this would be the transformation of electricity and water into Hydrogen when an excess of electricity is available and the reverse when demand outstrips supply. The combined efficiency of that process (electricity->Hydrogen->electricity) is roughly 50%. I'm actually pretty sure that by 2050 we'll have much better solutions available, but this approach is doable with current tech and has the added bonus of being able to be combined with a hydrogen infrastructure similar to how natural gas is currently used, i.e. for industrial processes and heating.

That would put total yearly demand at 1,040 TWh, which based on the efficiency of currently available PV and wind turbines [2] would require a total nominal installed capacity of about 1.040 GW (based on real world data from Germany, 1 GW in installed renewable capacity is roughly equal to a yearly production of 1 TWh. This will vary based on local circumstances, but Germany is not particularly bountiful in its renewable potential compared to other countries).

Currently, Germany has a total installed capacity of solar, wind and water power generation of about 125GW but I will assume that all these installations will reach their end of life before 2050 and will need to be replaced as part of this project.

At current prices for the installation of new PV (700,000 €/MW) and wind onshore/offshore (3,000,000 €/MW) and assuming a 50/50 split in installed capacity, the total investment for the required 1,040 MW would come to roughly 2 Trillion Euros over 30 years, or about 66 Billion Euros per year.

Divided by 800 TWh of yearly demand, this comes out to 8.3 cents (Euro) per KWh. This is comfortably below the 2021 average whole sale price of roughly 10 cents/KWh and in Germany would come out to about 20 cents/KWh for private consumers, compared to the current market average of roughly 30 cents/KWh.

Of course current energy production is quite heavily subsidized by the German government. If subsidies continue (or taxes on electricity are lowered), price per KWh could be reduced substantially.

This is of course only a very rough and simplified calculation. But it is straightforward and based on real world data, a pessimistic energy consumption scenario and current technologies. It ignores the potential for geothermal energy, biomass, imported hydrogen, long-distance imports of solar energy from the Sahara, or hydro power from Northern Europe, as well as likely technological improvement for generating and storing renewable energy. Looking at this, I find it hard to believe that absent a complete revolution of reactor design and cost-effectiveness, nuclear power will have any serious role to play in this, given its regulatory challenges, capital expenditure requirements, construction times and unsolved waste management issues.

[1] https://www.agora-energiewende.de/fileadmin/Projekte/2015/St...
[2] https://www.smard.de/home/marktdaten