I like wind power and solar power, I really do. I taught a course on alternative energy once and I worked for years in photovoltaics. I am all for using these technologies where they make sense. Home use, remote sites, and small additions to utility grids are great. But they run into serious problems when trying to fill more than perhaps 20% of the demand load on a power grid.
Any AC power grid has to balance supply (electrical generation in watts) against demand (the total load on the grid in watts) continuously, second by second in order to maintain the voltage and frequency of the grid. A grid consisting of soures and loads has almost no energy storage capacity. If the load exceeds the supply even for a fraction of a second, the voltage will dip, reducing the power to all loads, thereby dropping the total load down to match the instantaneous supply. If the generators push out too much current for the total load at any moment, the voltage will increase and the load will rise to compensate.
A grid with variable sources and varying loads is kept stable by having some control mechanisms, and by adding some storage elements to ride through instantaneous variations less than a second or so. Overall grid loads are somewhat predictable: typically peaking during the afternoon and reaching their minimum in the middle of the night. Weekends and weekdays provide other changes, as do high and low ambient temperature extremes. But any grid needs quick control mechanisms to handle sudden load changes, line faults, or sudden generator shutdowns.
Power generators have different control time frames. A nuclear power plant can be slowly adjusted on an hourly basis, but takes a day or so to properly shut down, and longer to start up. Thus nuclear plants usually provide a steady base load, perhaps up to 40% of the maximum load for the grid. Always on, and almost always the same power supplied into the grid.
Hydroelectric and coal/oil fired plants can be adjusted more quickly, but still take minutes to ramp up or down smoothly. Gas turbine generators are quicker, and are may be used to make minute-by-minute changes to the supply side, in response to other grid changes. Those supplies can therefore make up the variable parts, or peak loads, during the day, often being scheduled (dispatched) ahead of time for the expected changes.
Most grids now have some capacitors or batteries for sub-second control of small, fast changes. These are electronically switched in or out as needed. In some cases, certain loads can be placed under grid control to shut them down if the net supply cannot meet the instantaneous load demand. In this way, a large grid keeps the voltage and frequency within tight constraints as required by the government regulators.
Now add alternative energy into the mix. Solar power drops precipitously when clouds cover the sun, and of course falls to zero at night. Winds are notoriously variable, so wind power output is constantly changing in unpredictable ways. This is not a problem if the sources are a small part of the grid; the existing storage and control mechanisms can make up for the variability using other sources or shutting down loads as needed. However, as governments and people seek to use more "green energy", the percentage of the grid supplied by these alternatives goes up and starts causing major problems for the utility.
Some examples: California, in the summer of 2020 when rolling blackout had to be used because there wasn't enough power from the renewable supplies to meet the demand. Ontario has a similar problem: its contracts with wind and solar require it to pay top dollar for "green energy", whether it is needed or not. As a result, Ontario must often give away megawatts of power to other jurisdictions when the supply exceeds the demand, even as they pay for solar and wind energy, losing money overall in the exchange.
To maintain a stable grid in Ontario, the utiliy keeps gas turbine generators spinning with zero output, to kick in quickly when the wind or solar drops out suddenly. Clearly this is inefficient, and the resulting emissions detract from the supposed pollution-free wind and solar generation.
To grow the percentage of the grid beyond say, 20% wind/solar requires adding massive energy storage capacity. This isn't just a few batteries here and there. If the grid was 60% renewable, for example, it would need many hours of backup storage for those cloudy days, or weeks with little wind, to meet the peak load demand. Worse, when winter comes, there isn't much sunlight: in Ontario, for example, November averages two hours of direct sun per day, compared with perhaps 6 plus hours in June. Trying to balance supply and load over the seasons would require ridiculous amounts of energy storage.
This raises another issue. If you want a 100% renewable energy grid, you need to size the peak supply to more than 200% of the peak load. That is needed to average out the day-night cysles on the solar and the calm/windy weather for wind turbines. That becomes rather expensive, of course. Even if a watt of rated solar power is cheaper than a watt of rated nuclear power, the nuclear can run 80% of the time or more, year round, while the solar can barely operate 30% of the time at that level, at best, and much less in the winter.
For a 100% renewable energy grid in Ontario, the grid would have to provide Terawatt Hours (TWH) of backup energy storage to ride through the ups and downs of the solar and wind generators, while meeting the load demand and matching the load variations. Doing that with batteries would be insanely expensive, even if Ontario could find that much capacity to purchase.
There are other energy storage technologies. Flywheels, compressed gas and hydrogen storage have all been touted, but they too are pricey or the round-trip efficiency is poor. The best current method is pumped water storage: pump large volumes of water up a high hill to a big reservoir when power is plentiful, then let it run downhill through a hydroelectric turbine when peak power is needed. This can be fairly efficient, and can store lots of energy if you invest in huge pumps, reservoirs and infrastructure. But even so, there are not that many suitable locations.
Fortunately, for places like Ontario, the situation is not quite so dire. Nuclear power plants provide the base supply around 50%. Hydro power provides about 40% of Ontario's supply, and hydro-power sites are often good candidates for pumped storage: you already have the volume, the turbines, and gravitational head, at least in some places. Mind you, you cannot back up or turn off a river very far or for long.
Most utilities in North America are not so fortunate as Ontario, depending as they do on fossil fuel (coal, oil, gas) for most of their electricity generation. Using nuclear and hydro as your principal sources, and adding solar and wind, with significant storage of some sort, is probably the most practical way to go to get up over 60% non-fossil-fuel supply mix in most locations. Even so, pushing wind and solar beyond perhaps 20% of the total will require massive additional storage of some kind.
Perhaps at some future time a more steady renewable energy source will be developed, or maybe the cost of large-scale energy storage will drop significantly. Something like that would be essential to get to a fully renewable, reliable utility grid without nuclear and hydro power. Until then, we can make marginal improvements, supply mix adjustments, and storage additions as feasible and financially practical. But until then most utilities will depend on various forms of fossil fuel power generation to keep our electric grids running reliably.