Why is this even a comment that people make about solar power? Every single generating plant connected to the grid fluctuates its output in response to factors that may be far less predictable than a simple day-night cycle, such as consumer demand, fuel price, unexpected failure/efficiency elsewhere in the grid… no giant solar plant is going to operate in isolation from that system, so the day/night cycle is simply another logistical fluctuation. In fact, demand for electricity at night drops off considerably, so Apple might just opt to run on grid overnight. Or they could generate excess power during the day to feed into the grid for power credit to use at night, or do esoterica such as pump water uphill, spin up flywheels, pump air into tanks, charge batteries- but massive local storage in any form is probably less cost effective/power effective than simply using the give-take of the grid.
Remember, the power grid itself is just as important to our use of electricity as a society as any generating source. Yet it is also largely outdated, haphazard, insecure, and non-redundant, an issue that has largely been ignored in energy politics. Spending money at the Federal level on modernization, upgrades, and maintenance is a surefire way to increase energy efficiency and create jobs- a solution should not be overlooked in the controversies about building different generation sources.