One and a half hours. One hour and a half.
Yes! 1.5 hours is grammatically plural in English.
Here both घंटा and घंटे are accepted. Does it mean that डेढ़ can be both singular and plural?
Depending on the conversation, it can used in both forms.
It may be that in Hindi, one and a half is singular, but in English it is plural.
Why do you use डेढ़ when it's not half past one o'clock? Why not use साढ़े since that is half past?
It is a specific word that means 'one and a half'. So using it is more precise. Half past is only how we say it in English. Theoretically we could equally say "The time is One and a half o'clock." But by convention we don't.