The deal sees SolarCity offering new customers a four-day window, in the first instance, to spend $1 on the equivalent of $400 of future payments.

SolarCity installs PV systems for homeowners at no upfront cost in many cases, and then collects monthly lease payments over several decades – with the lease payments typically lower than the savings they see on their electricity bills.

SolarCity calls the deal with Groupon, which has 51 million active customers, “the first online offer of its kind for solar power systems”.

Customer acquisition is one of the most important fronts in the residential PV battle playing out across the US.

SolarCity relies on its own on-the-ground sales force to drum up new customers, and last year it acquired Paramount Solar in a $120m (mostly stock) deal, for its virtual sales expertise and footprint.

Additionally, California-based SolarCity has forged a growing network of “sales channel” partners, ranging from big-box retailers, like Home Depot and Best Buy, to car dealerships, like BMW, Honda and Tesla, to homebuilders like Taylor Morrison – where new customers are exposed to SolarCity’s products and offered deals.

SolarCity believes its early push towards vertical integration - meaning that it is able to generate new customers, finance systems and install them - gives it a key advantage over its rivals.

SolarCity currently installs roughly one in every four PV systems going onto residential rooftops in the US.