Data centres need low carbon electricity, but they cannot compromise backup power. This paper presents a sizing method for photovoltaic generation, battery storage, grid supply, and an existing diesel generator. The diesel generator remains in the system and supplies only the outage energy that the photovoltaic and battery system cannot cover. The method tests 49 designs with the same Monte Carlo outage events. For a five megawatt data centre in Abu Dhabi, the lowest cost design that meets the illustrative outage autonomy target uses twenty megawatts of photovoltaic capacity and forty megawatt hours of battery capacity. It covers 45.8 percent of sampled outages without diesel and reduces diesel served outage energy by 68.2 percent. Sensitivity checks show that the results depend on weather, outage mix, battery power, tariffs, and battery life.