Electricity supply has to exactly match electricity demand at all times, which is why (in my opinion) 100% renewable energy doesn't make sense. There has to be /some/ source of electricity that a country can turn to if the wind stops blowing, the sun goes behind a cloud, or - indeed - demand for electricity suddenly spikes.
The only way you could do this with renewable energy is if you had a way of 'storing' electricity, which could be used to meet demand if there was a shortfall in supply -- and this becomes incredibly expensive very quickly.
The most efficient approach to the electricty supply is to have a base-load of electricity being supplied by something which is 'always on', e.g. fossil fuels or nuclear energy, and then having renewables top up the rest.
[All this is just my opinion, by the way, but didn't want to litter the whole post with "in my opinion..."! You get the idea!]
I agree that there is a need for reliably energy producers in standby to satisfy energy demand at every point in time.
Using those for the base-load, as you suggest, however does not make sense. The base load has to be supplied by renewables and nuclear energy. On top of that a quick-to-regulate energy source is needed. Nuclear only really makes sense for the base load as it best runs full power all day. About 50% Nuclear plus coal plus a bit gas is a good idea for a stable grid but not so good CO2-wise. Mixing a high percentage of nuclear with wind and solar does not make too much sense. Then you either need to throttle nuclear power or add another source many times. That leaves coal, oil, gas or hydropower for the grid security. The cleanest here is hydropower where additional turbines are installed but only activated when the demand is high. Using excess energy to pump water back up in the storage lake is also a possibility but more expensive than using what's already there.
An alternative to alleviate the strain on the grid is to regulate consumption. This is best done by having some large consumers getting cheap electricity but having to shut off when the grid demands it. Another idea is to use price regulation and the "smart grid". E.g. electric cars shifting their overnight charging time to adjust the demand with the supply. We'll have to see how far the "smart grid" takes us. When a large percentage of cars are electric cars with batteries it might make sense.
That's why all the articles about countries doing so well on renewables are always on countries with lots of hydro and/or geothermal. There really should one word for those sources, which provide energy pretty much on demand but aren't available everywhere, and a different word for wind/solar which have the opposite characteristics. Using the same word for both tends to make people overoptimistic about wind/solar.
Some renewables are "baseload" sources of power. Things both proven, like geothermal and hydro, or more experimental like OTEC. They can work 24/7/365.
Hydro is sort of wasted as "baseload" source, because it's so readily adjustable, i.e. it is good as "dispatchable" source.
Nuclear is the typical baseload source, as adjusting output is not that fast.
But of course, if your hydro is really plentiful, then it doesn't matter.
Take the case of Norway and Denmark: Denmark has wind energy, which is very erratic. Norway has lots of hydro power which is perfectly dispatchable. When there's wind and not much consumption, Norway can buy wind electricity cheaply from Denmark (in fact the price may be negative because the excess production has to be sunk somewhere). The hydro plants are stopped. When there's no wind and there's a lot of consumption, Norway can sell electricity at a very good price to Denmark.
So, capacity is not the only thing that matters, dispatchability is important, and Norway clearly has the upper hand here because its wattage capacity is of a better kind.
The only way you could do this with renewable energy is if you had a way of 'storing' electricity, which could be used to meet demand if there was a shortfall in supply -- and this becomes incredibly expensive very quickly.
The most efficient approach to the electricty supply is to have a base-load of electricity being supplied by something which is 'always on', e.g. fossil fuels or nuclear energy, and then having renewables top up the rest.
[All this is just my opinion, by the way, but didn't want to litter the whole post with "in my opinion..."! You get the idea!]