This thesis investigates the opportunities and challenges of using repurposed electric car batteries, so called Second-life Battery (SlB), in a residential building as an energy storage. The performance of SlBs is compared to a First-life Battery (FlB) by identifying two potential scenarios and using a battery degradation model. The first scenario involved the batteries providing ancillary services to the grid, while in the second scenario the batteries were used for peak shaving. The battery degradation model is based on typical usage from the scenarios. The thesis findings indicate that the SlB degrades at a slower rate than the FlB, but can perform fewer cycles. Economic performance varies based on the application and initial assumptions. Both batteries proved to be profitable in the two scenarios studied, with some of the SlB configurations outperforming FlB configurations and vice versa. The yearly compensation received, when providing ancillary services with a 1000 kWh battery, ranged between 3-8 MSEK, while the yearly compensation received, when peak shaving with a 200-300 kWh battery, ranged between 20-35 thousand SEK. The main challenges identified included reduced lifespan, security risks, potential price increases, and space constraints. On the other hand, the main opportunities identified included potential price decreases and sustainability benefits such as carbon footprint reduction and grid stability. These results may provide valuable insights for informed decision-making regarding investments in FlBs and SlBs on the Swedish market.