If the specific model of heat pump you are using has the ability to run in heat mode at your city's heating design day temp (x degrees), then you would size the electric heater for the BTUh difference between the heating load calc at x degrees outside and the heat pumps output at that same x degrees. In other words, the electric heater just makes up the differece on what the heat pump is not sized to handle. However, if your heat pump has a hard cut-off and won't run at all below a certain outdoor temp (say 10 degrees), then you need to size your electric heat to handle 100% of the heating load on your design day (x degrees). BTW - don't use this type of heat pump on an all-electric system in a cold climate for this exact reason.
How do you calculate the size of an electric heater for RTU..
If the specific model of heat pump you are using has the ability to run in heat mode at your city's heating design day temp (x degrees), then you would size the electric heater for the BTUh difference between the heating load calc at x degrees outside and the heat pumps output at that same x degrees. In other words, the electric heater just makes up the differece on what the heat pump is not sized to handle.
However, if your heat pump has a hard cut-off and won't run at all below a certain outdoor temp (say 10 degrees), then you need to size your electric heat to handle 100% of the heating load on your design day (x degrees). BTW - don't use this type of heat pump on an all-electric system in a cold climate for this exact reason.