* Adds 30,000 customers in Q2, books orders 218 MW
* Shares up 5 pct in extended trade (Adds CEO, COO comments, details from conference call, updates share))
By Shubhankar Chakravorty
Aug 7 (Reuters) - SolarCity Corp, the largest installer of residential solar panels in the United States, reported a smaller-than-expected quarterly loss, citing "unprecedented demand."
The company, backed by Tesla Motors Inc founder Elon Musk, said it added more than 30,000 customers in the second quarter ended June 30, compared with 8,559 a year earlier.
SolarCity's shares were up 5.3 percent at $79.63 in extended trading on Thursday.
The San Mateo, California-based company said it deployed 107 megawatts (MW) of solar panels in the quarter, in line with its forecast of 105-110 MW. The company also booked orders for 218 megawatts.
The company said it expected to deploy 135-150 MW in the third quarter.
"We are well on our way to achieve our million customer goal in 2018," SolarCity Chief Executive Lyndon Rive said on earnings conference call.
The company had installed solar panels for 141,034 customers so far.
SolarCity also said it expected to reach 1 gigawatt of capacity by the end of 2016.
The company has become a hit with Wall Street due to its business model that allows homeowners to spread payments for solar panels over 20 years, instead of a one-time hefty down payment.
"Our installation costs have dropped from $3.16 per watt at the time of IPO to $2.29 in Q2 2014," SolarCity Chief Operating Officer Tanguy Serra said on the call.
SolarCity bought solar panel maker Silevo to drive down costs in June.
With the Silevo acquisition, which is expected to close by the end of August, the company expects to cut installation costs to $1.90 per watt by 2017.
The company appointed a new chief financial officer on Monday to help steer back to profitability.
SolarCity's stock had soared almost ten-fold since it went public in December 2012 at $8.00.
The company signed a deal with Groupon Inc in May to offer deals to customers on the Groupon site.
At the end of the second quarter, SolarCity had $3.3 billion of contracted payments remaining, up 32 percent from the first quarter.
SolarCity, which is expected to face competition from First Solar and SunPower Corp in the solar installation market, said it was on track to deploy 500-550 MW this year and 900-1,000 MW in 2015.
Excluding one-time items, SolarCity posted a loss of 96 cents per share, smaller than the average analyst estimate of 99 cents per share, according to Thomson Reuters I/B/E/S.
The company's revenue rose 62 percent to $61.3 million.
The net loss attributable to stockholders widened to $47.7 million from $39.5 million as operating expenses jumped 123 percent.
The company said it expects operating expenses to rise to $115-$125 million from $97 million in the second quarter.
SolarCity last reported a quarterly profit in the fourth quarter of 2013. Prior to that, the company had not reported a profit since the first quarter of 2012.
(Reporting By Shubhankar Chakravorty in Bangalore; Editing by Maju Samuel)