Mobile Network Operators (MNOs) are facing huge operational costs, due to the staggering increase of mobile traffic and to substantial bandwidth reliability requirements needed to enable the services of Smart Urban Ecosystems. With the purpose of reducing the cost due to power supply, dynamic load adaptation techniques are often implemented in Mobile Networks, in order to save energy when the traffic demand is low. Moreover, renewable energy (RE) sources are commonly introduced to power base stations, further contributing to decrease the operational expenditures. Finally, in a Demand Response context, the Smart Grid (SG) may actively ask its customers to dynamically adapt their consumption, by means of monetary incentives. The MNO is interested in improving the interaction with the SG, since mutual benefits can be obtained: cost reduction for the MNO and ancillary service provisioning from the SG side. We investigate via simulation a mobile access network where WiFi offloading techniques are combined with a properly designed energy management strategy, in order to reduce the load and better satisfy the SG requests. The impact of WiFi offloading is analyzed in different scenarios, including those envisioning the use of RE to power base stations (BSs) and/or the application of Resource on Demand (RoD) strategies, that activate or deactivate BSs based on traffic demand. Real data about traffic, RE production and SG requests are adopted. WiFi offloading results effective both in improving the probability of providing ancillary services and in reducing operational costs in any scenario, even when no RE is available. Furthermore, its impact is even more significant than the application of RoD strategies. Positive revenues are also possible for the MNO when RE are used, even when photovoltaic panels with relatively small capacity are installed.