Daily Halacha: Baking, Cooking, and Heating Water on Shabbat

Can food that was baked before Shabbat be cooked on Shabbat?

Just like there is a halachic principle that there’s no cooking after something has already been cooked, the same applies to baking. There is no cooking after baking, and no baking after baking.

What does that mean practically?

For example, it is permissible to pour hot water from a kli rishon (the original pot or kettle the water was cooked in) onto instant coffee, because the coffee is considered baked and not raw.

It is also allowed to add soup croutons which are baked to a bowl of hot soup, even if that soup was served directly from the pot it was cooked in (kli rishon). This is because the croutons are baked, and adding them does not count as cooking.
These rulings are based on Shulchan Aruch.

Similarly, there is no problem of “baking after baking,” so it is permitted to place a slice of bread on a hot plate (plata), even if it will become toast. The change in texture is not considered a new form of baking.

Can you wash dishes on Shabbat with water heated by a solar water heater?

Yes. It is permitted to use water heated by a solar heater (dud shemesh) for things like washing dishes or other similar needs. However, you should not bathe in water that was heated by the sun, due to a rabbinic concern known as the "decree of the bathhouse attendants" (gezeirat balanei merchatzaot).
This is discussed in Yabia Omer.

Can you use hot water from a boiler that was turned on by a Shabbat timer?

In this case, you should not use hot water that came from a boiler heated by a Shabbat timer, even for something like washing dishes.
This ruling is found in Yabia Omer.

From “Ach Tov VaChesed – Daily Halacha” by Hidabroot