The risks of cooling water shortages to thermo-electric power plants are increasingly studied as an important climate risk to the energy sector. Whilst electricity transmission networks reduce the risks during disruptions, more costly plants must provide alternative supplies. Here, we investigate the electricity price impacts of cooling water shortages on Britain’s power supplies using a probabilistic spatial risk model of regional climate, hydrological droughts and cooling water shortages, coupled with an economic model of electricity supply, demand and prices. We find that on extreme days (p99), almost 50% (7GW_e) of freshwater thermal capacity is unavailable. Annualized cumulative costs on electricity prices range from £29–66m.yr^-1 GBP2018, whilst in 20% of cases from £66-95m.yr^-1. With climate change, the median annualized impact exceeds £100m.yr^-1. The single year impacts of a 1-in-25 year event exceed >£200m, indicating the additional investments justifiable to mitigate the 1^st-order economic risks of cooling water shortage during droughts.