FRANKFURT (Reuters) - The German unit of British retail electricity startup Octopus Energy on Monday launched a service providing top-up power supply to households owning rooftop solar panels and a Tesla-branded Powerwall storage battery. Under the terms of the scheme, customers can use grid power supply from Octopus as a back-up to their home-grown power production and the Powerwall unit when the solar output used to charge it tails off. The back-up supply will also be 100% renewable, Octopus said. Typical customers will pay direct energy procurement and network costs and a flat fee of 3 euros ($3.48) a month for two years guaranteed, but not the all-in prices charged by other distributors for their overheads and profits, it added. “We want to show regulators what is already possible, so that the German power market gets moving,” said Octopus Chief Executive Andrew Mack in a statement, referring to his company’s digital pricing structures. Surging energy prices, driven by factors ranging from Asia’s economic recovery to European carbon allowances policy and lighter winds this year, have hit consumers worldwide and are squeezing out retail companies, with Britain the hardest hit. In Germany fallout has been limited, with just a handful of operators withdrawing from the market, but the crisis has shone a spotlight on the market design, which shows a high level of competition but has still produced the highest prices in Europe. End customer prices are also being inflated by expensive renewable energy support, taxes and network charges collected by the state and by grid operators. “The prices guarantee protects customers in the Tesla tariff from further price increases,” said Octopus of its new scheme. The offer is not dependent on ownership of a Tesla car. Reporting by Vera Eckert; Editing by Miranda Murray and Jan Harvey Our Standards: The Thomson Reuters Trust Principles.