Intro

This is a tricky question, primarily because solar sails benefit from a low mass spacecraft and with the addition of an electric propulsion system, you are increasing the mass decreasing the effective of acceleration and total delta V capabilities.

System Architecture and Supporting Systems

Solar Sails – Solar sails require photons to gain acceleration and as a solar sail propelled spacecraft moves further away from the sun, the sails effectiveness at propelling the spacecraft forward is greatly dimensioned. The supporting systems required to deploy a sail are relatively mass-light compared to chemical or electric propulsion systems. For the solar sail propulsion system, you have the mass of the deployable booms, Mylar sail, deployment mechanism, and sail control system.

Solar Sail ISP Metric:

Where:

• a0 = Sail characteristic acceleration
• T = Mission duration (usually mission acceleration duration)
• R = Payload Mass Fraction (mass_1/mass 2), where mass_1 is mass of sail and mass_2 is mass of payload or spacecraft.
• G = Gravity (-9.80665 m/s^2)

This quickly shows that as you increase the spacecraft mass (combining a solar sail + electric propulsion system together) decreases the overall systems ISP metric

Electric Propulsion System – These are extremely complex and elaborate systems as their performance is a function of several variables including power, spacecraft mass, engine IPS, etc… While I am by no means an expert in EP systems they do require a lot of supporting systems when compared to a chemical or solar propulsion system.

• Large power requirements – On average (depending EP system selected) 150W - 2kW of power – Aerojet’s Electric Propulsion Catalog. There are two main options of supplying power. First option is using solar arrays, however they’re power generation drops off at r^2. As an example, you generate 1/27 the power at Jupiter as you do at Earth. The second option is using a radioisotope thermal generator (RTG) which are heavy, extremely expensive, and only produce around 80-120 W and with NASA’s next generation RTG (MMRTG) only capable of producing around 110W, it’s unlikely that an ION RTG powered spacecraft will occur.
• Requires the addition of propellant tanks and propellant management systems (this includes heaters because fuel must remain at operational temperatures).
• Additional hardware and electronics to manage the complex electric propulsion system.

Conclusion

This brings me to my last point, combing these two systems together brings up new problems of how to manage them independently of each other. When you combine a solar sail and electric propulsion spacecraft, together you need a way to support the solar sails systems as well as the electric propulsion systems (structurally, hardware wise, electrically) and doing this greatly increases the total spacecraft mass and complexity involved. Ultimately it comes down to design complexity and does the added mass of each system increase or reduce the spacecrafts total detla V capabilities.

Python Code for ISP Relationships:

You can play around a view the relationships between the two systems.

'''
Requirements: 
- Python: 2.7 or 3.6
- Python Numpy
'''


def solarSailISP(characteristic_acceleration, mission_duration_days, mass_ratio):
    import numpy as np
    gravity = -9.80665
    delta_v = characteristic_acceleration*mission_duration_days*86400.0
    isp = (delta_v/gravity)/(np.log(mass_ratio))
    return isp

def engineISP(delta_V, mass_ratio):
    import numpy as np
    gravity = -9.80665
    isp = (delta_V/gravity)/(np.log(mass_ratio))
    return isp


solarsail_isp = solarSailISP(0.001, 500, 0.1)
electric_isp = engineISP(3200, 0.3)

print("Solar Sail Propulsion System ISP: {} sec").format(solarsail_isp)
print("Electric Propulsion System ISP: {} sec").format(electric_isp)