What most people won't tell you is that diversifying within the renewable sector is just as important as diversifying outside of it. An ETF that only targets solar panels is way riskier than one that mixes solar, wind, batteries, hydrogen, and other technologies. So before you pick one, look carefully at which companies are inside the fund, not just the pretty name it has.
The reality is these funds are going to have pretty strong ups and downs depending on whether the current government throws more money at renewables or if oil prices go up. That's clear in the answers above and it's true. But there's a trick that works: open a spreadsheet and simulate how much you would've made or lost if you'd invested 3, 5, or 10 years ago. You don't need exact numbers, just see how the fund performed through full cycles. If you see it dropped 40% but bounced back in 2 years, then you know what you're getting into. If you see it went south and never came back, you'd better think twice.
As for whether it's a good long-term investment, it depends on your time horizon. If you can leave the money there 10+ years without needing it, the sector has a lot of room to grow. But if you need that money in 2-3 years, it's probably not your best bet. And if you do decide to get in, the system of going little by little that someone mentioned above is the most sensible thing there is: start with a small amount each month or every two months. That way you avoid catching a historic peak and you also do less psychological damage to yourself when it drops.