These phrases are mainly used in informal English. lots of sounds a bit more informal than a lot of. Both forms are used in singular and in plural sentences.

It is not the phrase a lot of or lots of which determines singular or plural, but the subject of the sentence (here water and computers).

A lot of water is wasted.
Lots of water is wasted.
singular
A lot of computers are needed at schools.
Lots of computers are needed at schools.
plural

In formal English plenty of or much and many is used for a lot of/lots of.

Plenty of water is wasted.
Much water is wasted.
singular
Plenty computers are needed at schools.
Many computers are needed at schools.
plural