Total force produced by all of stretched strings of the piano is 7 tons (7,000 kg). This huge amount of potential energy, hidden inside the piano, can lift up a small truck. If you change the tune of piano just a semitone down, the force will reduced by 1-2 tonnes.
Unlike the piano, the force produced by the strings of a guitar much less. It as few as 30-50 kg. The strength of the strings, also depends on the type of guitar and how people make tune adjustment. If you increase in tuning the guitar a tone down, the force will increase by 26%.