The biggest differences between electricity and water is that water is that water is really hard to transport (cause heavy) and droughts can last for years and effect giant areas. With solar and wind on the other hand, their supply is fairly predictable and variation is mostly local (i.e. it sometimes is cloudy in Germany, but Germany and Britain have almost completely uncorrelated weather). Also it's easy to send electricity 2-4 thousand miles with only minor (10%) losses using high voltage DC. As such you can build a grid with 60-80% renewables with minimal storage. You just make it large to remove local variation in weather and use a mix of wind and solar for your renewables (which are anti-correlated which gives you better reliability). You then can make up any renewable shortages with peaker plants that burn fossil fuels, but if you have a little extra renewable capacity you can keep them from running most of the time.
Edit: Also hydro makes a really good battery for the several week timespan. It can't meet 100% of power needed but (especially if you bank water) can provide a decent percent of total demand for a while.
Edit: Also hydro makes a really good battery for the several week timespan. It can't meet 100% of power needed but (especially if you bank water) can provide a decent percent of total demand for a while.