The share of intermittent renewable electricity (IRE) in the future urban electricity system is expected to increase significantly. Sufficient back-up capacity is needed in the period when IRE output is low. Bioenergy is both dispatchable and carbon-neutral, and can hence be a promising option to back up IRE. The objective of this study is to explore the potential of urban waste biomass in backing up IRE in an urban electricity system. An urban electricity system model is developed to project future electricity generation configurations. Given the projected electricity generation configuration, the potential demand for bioenergy as back-up capacity is estimated by simulating hourly electricity demand and the supply of IRE for a whole year. The estimated potential demand for bioenergy is then compared with the potential supply of bioenergy from the urban waste stream. We apply our model using data for the city of Amsterdam, the Netherlands. The complementarity of wind and solar energy is found to reduce the demand for back-up capacity from bioenergy. An extreme weather day with hardly any wind and solar energy supply requires about 2800 tonne waste biomass per day in an emission reduction scenario and 1300 tonne waste biomass per day in a renewable energy quota scenario, respectively. The average daily waste biomass generated in the city is about 1400 tonne. Bioenergy storage as a buffer is found to be necessary due to the monthly fluctuations in both the supply and demand of waste biomass.