How to input the available electric heater power?

I am studying the total energy consumption of a small building after applying energy efficiency measures. One of the measures is installing a solar water heater for domestic hot water, instead of the standard electrical heater. How can I model in openstudio, a solar heater with solar tubes that is provided by electrical resistance heating that can be used at night or during some winter days. please I need the modeling instructions in details because I am beginner with openstudio modeling.