Most batteries are removable for safety, anti-theft, and maintenance purposes. Pick a bike that requires a key to remove battery (they can account for ~40% of the cost of the bike) then take it upstairs/into work to keep it charged.

In addition to mekhos excellent comment, there is also bike insurance and some bike lock manufacturers offer “anti-theft protection” provided certain conditions are filled.

Disclaimer: I cannot vouch for any of these options, I am just informing you of their existence.

Since ebikes generally are pretty expensive bikes you have to treat them like that. Outdoor storage is difficult for expensive bikes, since they make nice targets for theft. Generally, 10% of the price should be spent for a lock, maybe some less for the very expensive bikes.

Also, just because chances are high that it’s going to get stolen nonetheless, you should get insurance for the bike. If it’s properly locked to a sturdy structure with a lock that’s a special kind (known brand more than 50€ or sth like that) and gets stolen, then they give you money to buy a new one. Insurance is pretty cheap, considering the whole price point of the bike.

edit: typo

You can get foldable ebikes (e.g. a Brompton), they might be easier to store inside.

Some have removable battery packs you can bring in to charge and reattach when they’re ready.

Also I have 100% already seen asshole with extension cords run across the sidewalks for their electric cars