a few key concepts: capacity factor, instantaneous supply/demand balance, baseload, dispatch, grid inertia.
solar and wind power is given as instantaneous power. a 50 kW rated solar panel WILL produce 50 kW when the sun is bright and overhead. But that's not its average power. The average power is given by the
capacity factor C = (real average power / nominal instantaneous power).
Nuclear capacity factor is ~90%. Fossil fuel is ~60%. Hydro is ~40%. Wind is ~30%. Solar is ~10-20%.
So this is not just a storage issue, it is one of outright power production. 1 GW of nuclear is 900 MW. 1 GW of solar is actually 100 MW. But it also means that nuclear is very slow to change. It is always on.
In general, the power produced by the grid is vast and completely outstrips the capability of storage medium to accept (with two very important exceptions). It must have
instantaneous supply/demand balance. A massive battery bank can store maybe a few MWH? A few spins of a single wind turbine produces MWH. But supply/demand fluctuates throughout the day and may not follow production of i.e. solar or wind. So how do you make up the difference? Well, you typically use slow changing, high capacity factor power generation as
baseload, the minimum load that occurs throughout the day i.e. at 3 AM. When demand is at a minimum, your grid runs solely on baseload. This is usually coal or nuclear which are slow to turn on/off but have high capacity factors.
When you have increased demand as the day goes on, you start turning on or
dispatching other power sources that are fast reacting. You can turn hydro on/off at any time - just open the gate valve. You can turn gas turbines on/off too. You can turn solar and wind on/off by connecting or disconnecting their output. The marginal price you can get for this electricity is higher because you're selling into the grid as demand increases. When demand falls, prices fall, and you can just quit the market.
Now what about storage? Batteries are tiny compared to the storage of 2 major sources: water reservoirs and
grid inertia. Power comes from, in almost all cases, rotating turbines (water, steam, gas). These turbines generate electromotive force (EMF) through a rotating magnetic field creating current as well known in the Maxwell's equations. These turbines are very heavy, spin very fast and thus have substantial angular momentum stored, so small fluctations in load don't do much, because it just slows or speeds the turbine a microscopic amount. This phenomena is called "grid inertia" and is extremely important because this serves as instantaneous energy storage to balance out all the grid fluctuations.
Why does wind have higher capacity factor than solar? One reason is that wind blows quite a bit throughout the day, but also because it has grid inertia and solar doesn't. Wind turbines keep spinning after the wind stops and so produce a smooth power curve. But since this is unpowered, it eventually stops and no longer contributes to grid inertia.
In general you need tons of grid inertia to smooth out the fluctuations. This means you cannot escape high capacity factor, turbine based power sources - coal, nuclear and hydro.
