In cases where my software is only ever rented with a dedicated PC, is it permissable to compile Qt statically, as I would still technically own the program and it`s not being sold? Seems to be a bit of a grey area.
The right to lend or rent a work out is, at least in the EU, covered by copyright law. This means that if you rent out a work that is covered by copyrights without permission from the copyright holders, then you are liable for a copyright infringement.
This ties back to the following definitions in the GPL license
To “propagate” a work means to do anything with it that, without permission, would make you directly or secondarily liable for infringement under applicable copyright law, except executing it on a computer or modifying a private copy. Propagation includes copying, distribution (with or without modification), making available to the public, and in some countries other activities as well.
To “convey” a work means any kind of propagation that enables other parties to make or receive copies. Mere interaction with a user through a computer network, with no transfer of a copy, is not conveying.
As the renters of your PC receive a copy of the Qt software, the conditions that the (L)GPL license places on conveying a copy must be met, which means they must be told about their rights under the license and given the possibility to exercise those rights.
The fact that they receive their copy on a medium that does not become their property is also catered for in the GPL license:
If you convey an object code work under this section in, or with, or specifically for use in, a User Product, and the conveying occurs as part of a transaction in which the right of possession and use of the User Product is transferred to the recipient in perpetuity or for a fixed term (regardless of how the transaction is characterized), the Corresponding Source conveyed under this section must be accompanied by the Installation Information.
By renting out the PC, you are giving the renters a temporary right to have the PC in their possession and to use it.
You are still subject to (L)GPL compliance. You always retain ownership over your software, and also have to comply if you issue a gratis license to your software. So that doesn't help you determine what your obligations are.
Instead, the L/GPLv3 defines:
To “convey” a work means any kind of propagation that enables other parties to make or receive copies.
Additionally, your device may count as a “User Product” in the sense of the GPL.
Renting your computers with the installed software is clearly a kind of conveying the software because the recipient could make copies of the software. They are not allowed to actually make these copies because it's your proprietary software, but you still need to respect the LGPL license.
The spirit of the LGPL license is that end users can modify the LGPL-covered parts. This is typically done by dynamically linking the LGPL components and providing the necessary source code so that end users are able to modify the component. The LGPL does allow other approaches than dynamic linking (such as including the source code for the complete software, even if it's not under a free license). However, all other approaches will likely be more complicated than the standard dynamic linking technique.
If you make it clear that you are renting only the hardware and the lessee is only allowed to use that device as supplied (i.e. to run, but not to make copies of, any software contained), then the associated copyright licenses and the associated licensing requirements of the software residing on that device are not relevant to you and are not relevant to the lessee.
Why? This is because if you let someone use a physical medium in which software is contained (such as a PC, or some hardware device which contains ROMs with software written on them), copyright law only protects the action of making copies of (or modifying) the software. If the end user only runs the software, or uses it in some normal way, which does not otherwise require a copyright license, then the copyright holder has no right to dictate how a user may use (i.e. run, execute) a piece of software. I.e. copyright law protects only making copies of a work or modifying a work. The GPL goes so far as to point this out explicitly:
- Acceptance Not Required for Having Copies.
You are not required to accept this License in order to receive or run a copy of the Program. Ancillary propagation of a covered work occurring solely as a consequence of using peer-to-peer transmission to receive a copy likewise does not require acceptance. However, nothing other than this License grants you permission to propagate or modify any covered work. These actions infringe copyright if you do not accept this License. Therefore, by modifying or propagating a covered work, you indicate your acceptance of this License to do so.
Suppose you are a renter of a PC which includes some (L)GPL software. And suppose you do not agree to the terms of the (L)GPL license. According to section 9 of the (L)GPL license itself, you are under no obligation to accept the license requirements in order to use the software in question.
The GPL goes so far as to make the above explicit, but this notice is not actually specific to the GPL. Even if the software were under some other license (e.g. a proprietary license), if no copy is being made (i.e. no propagation is occcuring, and no modification is occuring), then no license is actually required to simply use hardware that happens to contain software which is under copyright.