Is it possible to set a schedule for just the heating element of a water heater (WaterHeater:Mixed or WaterHeater:HeatPump), while allowing the tank to discharge at any time? Modelling this using PlantEquipmentOperationSchemes makes the tank unavailable to meet the load so is there another way?

What I'm trying to do is model a residential Economy7 storage system which charges up at night and then provides heat throughout the day. I've had a look through the example files and can't see any that fit the bill. I think I must be missing something obvious as this seems like it would be a common requirement. How do people normally model a storage heater if not as I've been trying?