Choose a green host
When choosing a host for your website or application, check the provider's green credentials. Have they made any kind of pledge or statement about their sustainability policies? Do they use only renewable energy suppliers themselves? If not, it might be better for you to take your business elsewhere.
Switching to a greener hosting provider does not necessarily mean that your costs will go up. At time of writing, renewable energy is actually undercutting fossil fuels in terms of price and that trend is forecast to continue. Not moving to a green host now may in fact end up costing you more in the long run.
The Green Web Foundation maintains a database of green hosts and provide a tool that can be used to check whether any website is hosted green. Ensure that you read the status description given by GWF tool carefully - just because a host does not evaluate as "green" does not necessarily mean that it isn't hosted green, it could just be that a database record doesn't exist for it yet.
You should try to always ensure that your application and its infrastructure is as portable as possible and try to minimize vendor lock-in. This will allow you to swap hosts easily and with minimal disruption, making the decision to migrate to a greener provider trivial. High levels of portability can be achieved through the use of containerization and abstraction through libraries.
This site is currently hosted on Netlify. Netlify have published a Sustainability Pledge publically and use AWS under the hood. Not only are AWS committed to being 100% renewable by 2025, they have alerady achieved this in some European sites such as Frankfurt.
Being able to say that your applications are hosted by green providers is one of the most basic yet powerful statements digital companies can make in terms of its own sustainability policies. Finding out whether your provider has a sustainability policy is just a google search away. If they do not mention sustainability in any of their literature, contact customer support to ask them about it. If the host has no green policies, you should seriously consider switching to a different host.