Renewable energy is expected to constitute a significant proportion of electricity production. Further, the global population is increasingly concentrated in cities. We investigate whether it is possible to cost-effectively employ 100% renewable energy sources (RES)-including battery energy storage systems (BESS)-for producing electricity to meet cities' loads. We further analyze the potential to use only RES to meet partial loads, e.g., by meeting load demands only for certain fractions of the time. We present a novel flexible-load methodology and investigate the cost reduction achieved by shifting fractions of load across time. We use it to evaluate the impacts of exploiting flexibility on making a 100% RES scenario cost effective. For instance, in a case study for Kortrijk, a typical Belgian city with around 75,000 inhabitants, we find that from a purely economic viewpoint, RES-BESS systems are not cost effective even with flexible loads: RES-BESS system costs must decrease to around 40% and 7% (around 0.044 (sic)/kWh and 0.038 (sic)/kWh), respectively, of the reference levelized costs of electricity to cost-effectively supply the city's load demand. These results suggest that electricity alone may not lead to high RES penetration, and integration between electricity, heat, transport, and other sectors is crucial. (C) 2018 Elsevier Ltd. All rights reserved.