Not sure where you're getting "at best 15x", the best panels from 2013 were found to have a 30x EROI: https://www.sciencedirect.com/science/article/abs/pii/S13640...

And from a more recent study:

"This work has shown that the EROI of fossil fuels drops considerably when moving from a final stage (approximately 8.5) to a useful stage analysis (approximately 3.5). The low overall EROI value at the useful stage, however, hides large differences across fossil fuel groups and end uses, with average useful stage energy returns being much higher for heating compared with mechanical end uses. In addition, we find that fossil fuel useful-stage energy returns have remained fairly constant on average over time (except for fossil gas) and may even have slightly increased. Such findings contradict the conventional narrative according to which fossil fuels present very high, although rapidly decreasing, energy returns.

Next, we find that the EROI equivalent value for which electricity-yielding renewable energy systems deliver the same net useful energy as fossil fuels is as low as 4.6, due to the substantially higher final-to-useful efficiency of electricity compared to those of fossil fuel-based energy carriers. This value is, however, highly variable across the fossil fuels and end uses considered. We also find that most literature-sourced EROI values for electricity-yielding renewable energy technologies are higher than the EROI equivalent we have calculated, even when adjusting the values for the implications of intermittency using a wide range of energy transition scenarios. This result suggests that renewable energy may deliver more net useful energy than their fossil fuel counterparts for the same amount of final energy invested."

https://www.sciencedirect.com/science/article/abs/pii/S13640...