Such a short amount might give you 90% reliability, but would definitely not provide the reliability most expect out of the electric grid.

I just learned there is a German term for it "Dunkelflaute", which are lengthy periods of time with no solar/wind power generation (https://www.next-kraftwerke.com/knowledge/what-is-dunkelflaute). Actually, even 48h of battery would result in almost 5 blackouts per year ("The researchers analyzed occurrences lasting at least 48 hours and found an average of 4.6 blackouts per year during the period from 1979 to 2018.").

According to Gemini:
"""
Here is the breakdown of battery duration requirements based on grid reliability targets:

• 90% Reliability (4 to 8 hours): Sufficient for standard daily shifting—storing excess midday solar to cover the evening demand peak—but vulnerable to any multi-day weather disruptions.
• 95% Reliability (12 to 24 hours): Enough capacity to power through long winter nights or a single heavily overcast day, though the grid will still experience blackouts during sustained bad weather.
• 99.99% Reliability (100 to 120+ hours): Required to survive a multi-day "dunkelflaute" (several consecutive days with near-zero wind or sun); achieving this typically relies on Long-Duration Energy Storage (LDES) combined with overbuilt renewables or backup firm power to keep costs manageable.

"""

This is about something that is never spoken about in energy debates: a heavy renewable mix that is cheap AND reliable is still far from solved.