Pylon Solar Design Software is the best SolarPlus alternative.
While SolarPlus will charge you costly monthly fees with a cap on how many users are on your team, Pylon gives you premium solar design software with $0 Monthly Fees and unlimited users.
SolarPlus did not previously provide you with high resolution imagery, further increasing your overheads as a solar installer, but with the release of SolarPlus V4, imagery is included at an additional cost for some locations.
Pylon has always included high resolution aerial imagery with resolution up to 5cm per pixel so your solar quotes are precise.
Regarding feature-set, SolarPlus lacks a trial of the software as well as e-signatures and a payment gateway, which are popular with Pylon users.
Additionally, a setup and training fee from $500 ($400 on an annual plan) applies to all SolarPlus plans. With Pylon, you only pay $4 per project and get world-class support and training for free.