Terawatt (TW) deployment of renewable energy is critical for the world to achieve net-zero emissions. Solar power is one of the most promising technologies for renewable electricity generation and has the largest available resource for exploitation. To boost solar electricity to TW levels, we must accelerate the development of new technologies enabling ever higher efficiencies. At present, the dominant silicon technology is close to reaching its practical efficiency limit. For higher performance to be unlocked, other semiconductor absorbers must be adopted in what is known as a tandem architecture: where two or more light absorbers are integrated on top of each other to make better use of high energy visible photons, reduce thermalisation losses and convert a higher fraction of the solar energy into electrical energy. Among such new absorbers, mixed organic-inorganic metal halide perovskite semiconductors have recently witnessed unprecedented progress and are the most promising technology to integrate into a tandem device. Significant advances have already been made integrating perovskites with silicon to make high efficiency tandems, but efforts so far have almost ubiquitously employed high-end silicon heterojunction rear cells, which do not represent the main-stream mass-produced Si PV technology. In this project, we will tackle the development of perovskite-on-silicon tandem solar cells based on the lowest cost "PERC" and "TOPCon" silicon cells. Our goal is to deliver a novel tandem technology with the potential to scale up to TW levels, due to moving away from the use of rare materials, and employing fully-scalable manufacturing methodologies, for both the silicon and perovskite cells. Enabling the vast installed capacity for silicon cell production to "upgrade" to perovskite tandem technology will accelerate deployment of perovskite-on-silicon tandems in a way that it is not yet possible with current designs. Most importantly, a shift towards scalable tandems will produce a step change in energy capture per metre square as high as 45%rel (from 24% to 35%abs), at a marginal extra cost. Because half the CO2 emissions of PV manufacturing come from silicon production, tandem higher efficiencies greatly reduce the carbon footprint per unit energy generated, potentially to the lowest level of any electricity generating technology to date.