der Saft
juice
The drinkable liquid that is pressed from fruit or vegetables.
Möchtest du ein Glas frisch gepressten Orangensaft?
Would you like a glass of freshly squeezed orange juice?

juice / power
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juice
The drinkable liquid that is pressed from fruit or vegetables.
Möchtest du ein Glas frisch gepressten Orangensaft?
Would you like a glass of freshly squeezed orange juice?

The meaning of 'Saft' (juice/power) is immediately clear from context. Words like 'trinken' (to drink) or types of fruit point to a beverage, while 'Akku' (battery) or 'leer' (empty/dead) refer to electricity.
Ich trinke morgens gerne ein Glas Apfelsaft.
I like drinking a glass of apple juice in the morning.
To specify the type of drink, 'Saft' (juice) is very often used as the second part of a compound word. The first part is usually a type of fruit or vegetable.
Im Supermarkt gibt es Organgensaft, Apfelsaft und Karottensaft.
At the supermarket there is orange juice, apple juice, and carrot juice.
The use of 'Saft' (power/juice) for electrical energy is very colloquial. You often hear and read it in informal conversations, but not in formal texts.
Gib mir mal kurz Saft, mein Handy ist gleich aus.
Give me some power for a second; my phone is about to die.
The idiom 'jemanden im eigenen Saft schmoren lassen' literally means to let someone stew in their own juices and figuratively means to leave someone alone with their problems without helping them. It comes from cooking.
Nach dem Streit hat sie ihn einfach im eigenen Saft schmoren lassen.
After the argument, she simply left him to stew in his own juices.
For the colloquial meaning 'Strom' (electricity/power), there are typical formulations such as 'der Saft ist alle' (the power is gone) or 'keinen Saft mehr haben' (to have no power left).
Ohne Strom hat die ganze Wohnung keinen Saft.
Without electricity, the whole apartment has no power.
electricity / river
'Strom' and 'Saft' can both mean electrical energy. 'Strom' is the normal and neutral word, for example for devices, outlets, or bills. In this meaning, 'Saft' is colloquial and sounds casual. You are more likely to use it in everyday speech when a device has no power or is working again. Learners may confuse the words because 'Saft' usually means a drink. When talking about electricity, 'Strom' is always safer and more neutral; 'Saft' only fits in informal situations.

Der Akku ist leer; das Handy hat keinen Saft mehr.
The battery is dead; the phone has no power left.
Im Büro ist der Strom ausgefallen, deshalb funktionieren die Computer nicht.
The electricity has gone out in the office, so the computers are not working.