From other power sources. Let me break it down differently. Suppose you have
Day demand: 100-120 GW
PV: 40 GW
Nuclear: 60 GW
Fossil: 0–20 GW
Night demand: 60–70 GW
Nuclear: 60 GW
Fossil: 0-10 GW
Now store some of the solar and replace it with nuclear power.
Day demand: 100-120 GW
PV: 30–40 GW
Nuclear: 70–70 GW
Fossil: 0–10 GW
Storage: 10–0 GW
Night demand: 60–80 GW
Nuclear: 70–70 GW
Fossil: 0–0 GW
Storage: -10–10 GW
Notice that the fossil fuel load dropped in half during the day and to zero at night, the nuclear base load increased, and the only difference is the availability of storage.
Now I suppose one can argue over how much of this is actually storing nuclear power versus PV power, but that's kind of splitting hairs.