A critical component of a solar power system is the power controller which regulates the flow from the solar panel to charge the battery at the proper rate and stop charging when full, much like the regulator in a car. Many charge controllers are electrically noisy and can cause the read range to drop and in some cases cause the temperature of the reader electronics to increase. Controllers use DC-DC conversion circuits which have oscillators that control the power level. These oscillators can generate electrical noise on the power line.
To determine if there is a noise problem, measure the read range with just a battery (which has no noise) and then attach the solar controller. If there is no difference, there is no problem.
Solar controllers can have quiet and noisy periods as it charges the battery with a different behavior when the battery is full. Evaluate a controller for noise across a full battery charge cycle. A good test is to measure the read range on a sunny day at noon with a fully charged battery and one that needs charging.