The seven ton K15 has a 700 kilometer range with a one ton warhead and 1,900 kilometers, with a 189 kg warhead. The latter weight is sufficient to handle a nuclear warhead if India has been successful in developing warhead technology to the same point the U.S. and Russia were over three decades ago.
The first SLBM was the U.S. Polaris A1, which began development 57 years ago and entered service in 1961. Like the K15 it was a two stage, solid fuel missile. The Polaris A1 weighed 13 tons, had a range of 2,200 kilometers and a one ton warhead.
It was in 2009 that India launched its first nuclear submarine, the INS Arihant (Destroyer of Enemies). This came after over a decade of planning and construction. What was not revealed at the times was that the Arihant was launched without its nuclear reactor, which was not ready until the following year.
The Arihant was launched when it was because work on the sub had been going on for more than a decade and it was becoming embarrassing to have nothing to show for all that effort. The first of six Arihants was supposed to enter service in 2008, as an SSBN. That was delayed and Arihant has only recently begun sea trials, which may take a while. The 6,000 ton Arihant has four vertical launch tubes, which can carry twelve (three per launch tune) smaller K15 missiles or four larger K-4 (based on the Agni 3) missiles, each with a range of 3,500 kilometers. The K-4 is still in development. Two more Arihants are under construction.
The Arihant is based on the Russian Charlie II sub, which it resembles. A leased Russian Akula II nuclear sub will basically serve as a training boat for India's new nuclear submarine force. Russia retired all its Charlie class subs in the early 1990s. India leased one from 1988-91, and gained a great deal of familiarity with it. The Charlie class had eight launch tubes, outside the pressure hull, for anti-ship missiles.