System requirements

Item	Requirement
Operating system	Windows Server 2019 or newer (64-bit); Windows 10/11 for test setups. The installer requires x64 and administrator rights
.NET runtime	none — the package is published self-contained and brings the runtime with it
Database	none — an embedded SQLite file in the data directory (WAL mode)
Memory	4 GB minimum, 8 GB recommended (more when using OCR)
Disk space	2 GB for the application, plus room for the invoice archive and the usage log
Network	one free TCP port (default 5060). No internet access required
Mode of operation	the Windows service Invoiceflair, autostart, automatic restart after a crash

No database, no runtime, no internet access: that is the shortest summary of the requirements.

Optional for two additional functions

A licence key for the PDF engine and the Tesseract language data — needed for:

• the OCR of scanned PDFs in the “PDF to e-invoice” tool
• creating invoices from your own Word templates

Everything else runs without either

PDF invoices with a text layer are read in full even then. The auto-layout PDF and the Factur-X embedding are licence-free in any case.

Linux: libicu is mandatory

The application runs platform-neutrally, but invariant globalisation is deliberately off — otherwise all cultures collapse to invariant and the nine language resources no longer apply.

Without ICU the process does not start

libicu must be present on a Linux server. That is not a recommendation but a condition for starting.

A standalone Linux installation package does not currently exist (an open point); until then a Linux installation is delivered as a service engagement. The deployment mode is set there as a systemd drop-in:

[Service]
Environment=InvoiceApi__Deployment__Mode=OnPremise
Environment=InvoiceApi__PublicBaseUrl=https://rechnung.firma.de

Reverse proxy

The application speaks plain HTTP when no certificate is stored. For operation behind an upstream proxy, see Running behind a reverse proxy — it sets out which headers must be forwarded so that addresses and the language switch work correctly.