From 1 February 2026, a major shift in invoicing becomes reality: mandatory use of the National e-Invoicing System, known as KSeF. This is not just a technical update. It is a full transformation of how businesses will document sales. The changes will affect not only the invoices issued in day-to-day operations, but also the entire process of sending, approving, and storing them.
For many companies, this means redesigning procedures, integrating systems, and adapting to a new model of real-time reporting. In this article, we explain what the new obligations for businesses in 2025 actually mean, how to avoid errors and delays, and how the Altera app can help you prepare for the KSeF changes step by step.
1. New obligations for businesses in 2025. Check what is changing
What changes does the National e-Invoicing System introduce from 1 February 2026?
From 1 February 2026, issuing invoices through the National e-Invoicing System, or KSeF, will become mandatory on a broad scale. Paper invoices and PDF documents will no longer be the default. The standard will be structured invoices only. The system is designed to standardise invoice issue dates, increase transparency, and tighten the tax system. The new rules will apply to almost all taxpayers, with the exception of certain special procedures.
The move to mandatory KSeF is the result of a multi-stage process led by the Ministry of Finance, which has been announcing changes to e-invoicing for quite some time. Although the original plan was to launch the system in 2024, the final date was pushed back in order to refine the platform and reduce the risk of system failures.
The changes will include:
the transition to XML invoices compliant with a defined logical structure,
the requirement to use a KSeF number and the system-generated identification number,
no option to bypass KSeF, even if the transactions involve small amounts,
the requirement to use issuer certificates or trusted signatures,
full digitalisation in commercial transactions, also for other entrepreneurs.
During the mandatory KSeF period, it will be especially important to prepare systems to work online, including in situations where KSeF is temporarily unavailable, for example during a system outage. That is exactly why it is so important to plan the implementation early, taking into account your industry, the scale of your operations, and the software you use.
2. Why it is not worth waiting to prepare for KSeF
What risks come with failing to implement the National System on time?
Failing to adapt to the obligation to issue invoices through the National e-Invoicing System can lead to serious financial and operational consequences. If you are unable to issue a structured invoice or send it through KSeF, your sales process may effectively be blocked. On top of that, financial penalties from the Ministry of Finance are a very real risk.
Although the draft legislation gives businesses until 1 February 2026, it is worth starting the testing and implementation process now. Here are the main reasons:
you need time to train your team in issuing electronic invoices and using the new system,
you need integration with commercial software that can issue invoices in line with KSeF requirements,
there is a risk of system overload or data transmission errors during outages,
internal processes need to be adapted to the new rules, including control over invoice content and KSeF numbers.
The technical risk should not be underestimated either. Even a short period of KSeF unavailability can disrupt sales. That is why KSeF implementation should be treated as a strategic project, not just an IT task.
3. Structured invoices in KSeF. What will change in day-to-day work
What are structured invoices, and how will they change the way documents are issued?
Structured invoices are electronic documents issued and sent only through the National e-Invoicing System, using one standard XML format. Their purpose is to simplify document flow, standardise data, and fully digitise the invoicing process. From 2026, they will become the norm. Sending invoices as PDFs or printing them on paper will no longer be an option.
The standardisation of the invoice issue date means that the date on which the document is sent to the system will also become its official issue date, regardless of when it was actually prepared. That means businesses will need to pay close attention to the accuracy and completeness of data already at the invoice creation stage.
What else will change?
The invoice will need to pass system validation, otherwise it will not be recognised as issued.
Documents will enter circulation only after approval and assignment of a KSeF number.
It will be necessary to use a logical structure compliant with the template provided by the KSeF taxpayer application.
Each invoice must be made available to the buyer through ePUAP, an official inbox, or directly from the taxpayer application.
The rules around split payment and the issuance of consumer invoices are also changing.
This transformation requires understanding the new rules and testing systems in real-life conditions, especially in the context of KSeF downtime, when a company must still be able to operate under emergency procedures and ensure continuity in documenting sales.
4. Step by step. How to prepare your company for mandatory e-invoicing
How can you prepare your company for the move to KSeF in several stages?
Preparing for mandatory KSeF requires a well-thought-out plan that covers changes in commercial transactions, technical requirements, and new obligations. This is not only about compliance, but also about operational efficiency. Breaking the process into stages helps avoid chaos and invoicing errors.
A recommended plan of action looks like this:
Analyse your current invoicing system
Identify the tools you currently use. Check whether they allow you to issue structured invoices, support XML format, and integrate with KSeF.
Choose the right software
Make sure you are using commercial software that meets the technical requirements and supports login via issuer certificates.
Create your account and integrate with KSeF
Register your company through ePUAP or in the KSeF taxpayer application, configure user roles, and prepare for login requirements.
Test and simulate
Carry out test submissions through KSeF. Verify the KSeF number, the system’s response to errors, and what happens when KSeF is unavailable.
Train your team
Train employees on the new rules, invoice content, KSeF handling, and the logical structures required by the system.
Go live and monitor the process
Keep track of invoice issuance on an ongoing basis, respond to KSeF downtime, and monitor the correctness of data such as issue date, sales values, and tax amounts.
For many companies, the KSeF mobile application will also be important, because it makes it possible to manage documents from anywhere and react quickly to possible outages.
5. The most frequently asked questions about KSeF and how it works
How does the National e-System work, and what does KSeF unavailability actually mean?
The National e-System, meaning KSeF, operates continuously online, but technical interruptions and overloads can happen. If KSeF becomes unavailable, emergency procedures apply, and once the issue is resolved, the overdue invoices must be submitted. It is worth documenting the outage, and the electronic invoice issued during that time should include the correct date and a note explaining the delay. Information about such incidents is to be communicated by the Ministry of Finance.
Can you issue an invoice outside KSeF?
After 1 February 2026, the obligation to issue invoices through KSeF will apply to almost all businesses. Invoices issued outside the system, for example as PDFs or on paper, will not be recognised. Exceptions concern consumer invoices, special procedures, or certain transactions involving small amounts.
How does invoice numbering work in the system?
Once accepted by the system, each invoice automatically receives a KSeF number. Internal numbering used in your ERP remains optional. Documents can also be searched using the identification number generated by KSeF.
Can you keep using your own software?
Yes. Commercial software can work with KSeF as long as it complies with the Ministry of Finance specifications. It must support document submission through KSeF in XML format and according to the required logical structure.
Does KSeF also apply to sole traders?
Yes. Other entrepreneurs, including sole traders, are also covered by the new system. Exceptions apply only to selected types of sales, for example certain transactions with buyers where issuing an invoice is not required.
What happens in the case of data errors or duplicate invoices?
The system automatically detects duplicates and errors. Without successful validation, an invoice will not be issued. In the event of a mistake, it is possible to issue a correction, also as a structured invoice, while keeping the reference to the original document.
Summary
The National e-Invoicing System will change the way companies operate, both technically and organisationally. Mandatory issuance of electronic invoices in a standardised format, full system integration, and new emergency procedures for outages all require deliberate preparation.
That is why it makes sense to choose tools such as Altera, which support compliance, automate processes, and provide full control over documents. The earlier you prepare, the smoother the launch will be, without stress, delays, or operational downtime.
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