Any refunds will be handled by Airbnb, not you.
Have you contacted Airbnb yet? If so, what did they say? Relate what they said to the guests; message to them on the platform. I would call them 15 minutes or so after your message to them to follow up.
Include this in your message to the guests. Will they have Wi-Fi? I assume not. Able to cook in kitchen (gas)? What won’t they have? Tell them.
I would think/hope that Airbnb under AirCover will try to find them alternative housing. If so or if not, write them what Airbnb is doing.
Depending on what Airbnb says I wonder whether you might proactively find a nearby similar Airbnb, find out if they’ve been affected, if it’s available. If so, provide that info to Airbnb and see what next steps Airbnb recommends.
Don’t you cancel. It’s possible that Airbnb will treat this as a penalty-free Host cancellation, but wait for them to say that to you and ideally Airbnb not you cancels the reservation if it comes to that.
At this point focus on working with Airbnb to help the guests find alternative housing; and if that cannot be found communicating what you’re able to provide and not provide until the power is restored.
My message to your guest might be something like (edit appropriately):
I am so sorry to tell you that due to a storm our property has lost power.
I have contacted Airbnb to see if it can find alternative housing for you. They’ve told me [case number?] . . .
At the same time I am proactively looking for nearby Airbnbs that might be available (not all properties in my area have been affected).
I will keep you apprised as I learn more.
Meanwhile, if somehow we cannot find alternative housing you should know what we can provide/not provide until power is restored.
We can provide . . .
We cannot provide . . .
In full disclosure my utility is [name] at [phone number] and [URL] with my address/neighborhood being [fill]. I have been in contact with them and will contact them proactively and let you know what I learn as soon as I learn it. At this time they do not have an estimate of when the power might be restored.
I will call you in 15 minutes or so to discuss this.
Please know that I am doing everything I can to make sure that you have suitable housing.