The short answer
When billing for consumption, your guest pays exactly what electricity he uses: he buys kWh in an app and sees his consumption rise in real time, until his balance reaches zero. The most common mistake is almost never in that measurement itself, but in the layout that comes before it: landlords don't know exactly which circuit groups in the meter cabinet fall under the guest tariff and which ones don't. A circuit group is a separate electrical circuit in the meter cabinet, recognizable by its own switch; think of one group for the outlets in the living room and another for the kitchen.
The result of that unclear layout is twofold, and both versions lead to disputes. Either the guest pays for electricity that isn't his, such as outdoor lighting that stays on all night across the property. Or the guest uses electricity that falls outside the measurement, such as a loose outlet by the driveway that's still connected to the main meter. In both cases, the bill doesn't match what actually happened in the vacation home, and you only notice when a guest emails you about it.
Why this mistake occurs so often
Most meter cabinets in vacation homes were never designed with the idea that a guest would ever settle a bill. Circuit groups were added over the years as was convenient: an outlet here, a group for the heat pump there. When someone then has a module installed to measure guest consumption, they often think that module automatically "knows" what belongs to the guest and what belongs to the property. That's not how it works; the module measures exactly the circuit groups connected to it, nothing more and nothing less.
Three variants of the same mistake
The first variant: a shared circuit group. A washing machine in an outbuilding or a charger for the landlord's maintenance materials is on the same circuit group as the guest accommodation, so the guest pays for it without ever using that device.
The second variant: a forgotten circuit group. Outdoor lighting, a garden pump, or a carport outlet stays on the main meter while the guest does use it, so that electricity doesn't show up in the app and therefore isn't billed anywhere.
The third variant is subtler: the tariff itself. Landlords set a price per kWh when they start the subscription, but never adjust it afterwards when their own energy contract changes. A tariff that made sense a year ago may now be well below the actual purchase price.
How you prevent this
The solution isn't in the app, but in the meter cabinet, and before the module is even connected. Have an electrician physically separate the circuit groups that belong to the guest accommodation from the groups that belong to the landlord or the building. That's exactly the conversation you have with the installer beforehand: which outlets, which lighting, and which outdoor facilities count as "the guest", and which don't.
Then check the tariff at least a few times a year, especially when your own energy contract changes. A brief overview of what happens in your meter cabinet and how the measurement connects to it can be found in the explanation about meter cabinet and measurement in the knowledge base. Anyone who still needs to choose between the three billing methods and wants to see which fits their situation can do so by answering a few questions with immediate guidance.
For the electrician: the technical side
The module is connected after the main connection and before the groups assigned to the guest, with the exception of the refrigerator group which always remains separate and is never disconnected at a zero balance. In an existing distribution board with mixed wiring, it is common for the electrician to redistribute groups that are currently combined on a single circuit breaker across separate groups, each with its own residual current device. That is the device that automatically trips in case of a fault or leakage current, and that operates separately per group. With a three-phase connection, it is furthermore determined on which phase each group is connected, so that the measurement of the guest groups does not accidentally also include part of another phase.
Where you can have this checked
Whether the layout of your distribution board matches what you want to charge to your guest can usually be seen in one visit by an electrician. If you are in doubt whether your situation actually calls for this approach, then the overview with frequently asked questions provides initial guidance, and the page on how the measurement works shows exactly where the data comes from that appears in your guest's app.
Do you want to know if your own distribution board is ready for billing per consumption, or are you still considering switching from a fixed amount? Do the free quick-scan or leisurely browse through the other topics; nothing is asked of you to read what you need.