Guide · KSeF and e-invoices
Updated: · 7 min read · By Agnieszka Saladra, Certified Accountant
From 2026, invoices between businesses are issued in the National e-Invoicing System (KSeF) in a structured form. In this guide we show, step by step, how to prepare, sign and send such an invoice - from granting permissions, through choosing a tool, to the KSeF number and emergency modes. Luno-Group clients issue and receive KSeF invoices free of charge in the client panel.
In short
A structured invoice is not an ordinary PDF, but an XML file with a strictly defined structure, compliant with the official schema. From 1 February 2026 the template labelled FA(3), published by the Ministry of Finance, applies. Thanks to the uniform structure, the system can automatically read every field of the invoice - from the seller's and buyer's details to the line items and VAT rates.
The obligation to issue invoices in KSeF came into force in stages. From 1 February 2026 it covered companies whose sales value including the amount of tax exceeded PLN 200 million in 2024, and from 1 April 2026 - all other entrepreneurs. Until the end of 2026 certain transitional easements apply, including the option to issue invoices outside KSeF when their monthly value does not exceed PLN 10,000 gross. The details depend on the specific situation, so if you have any doubts about the deadline, we will clarify them against your data.
The FA(3) file is not created by hand - a program does it for you. You have several routes to choose from, and each leads to the same result: an invoice accepted by the system and given a KSeF number.
For individual invoices, the free Taxpayer App is entirely sufficient. If you issue more of them or you want invoices to go straight into accounting, integrated software or a client panel is more convenient.
Before you issue your first invoice, the system needs to know who acts on behalf of the company. In a sole proprietorship the owner has owner permissions automatically and logs in, for example, with a Trusted Profile (Profil Zaufany). In companies and larger firms, permissions must be deliberately granted.
Permissions are granted in the KSeF app or by submitting a ZAW-FA form, when access is to be obtained by a specific individual. This is a one-time step - once configured, it works until you change it. We help Luno-Group clients set up permissions so that invoices reach us automatically.
With a tool and permissions in place, you can create an invoice. In the app you fill in a form that, in the background, converts the data into a correct FA(3) file. Some fields are mandatory, and it is they that decide whether the system accepts the document.
It is worth noting that a structured invoice no longer includes certain elements known from paper, such as a signature or a logo - what counts is the data in the structure's fields. If a required field is omitted, the system will reject the file, so we will clarify the completeness of the data in advance.
You send the finished invoice to KSeF with a single click. The system checks that the file is correct and - if everything matches - accepts it and assigns a KSeF number. This is a unique identifier that unambiguously confirms the invoice has been effectively issued and is in the system.
In online mode an invoice is treated as issued on the day it is sent to KSeF (Article 106na(1) of the VAT Act), so send it on the same day as the one you enter in field P_1. If you entered the invoice on Friday but sent it on Monday, the system treats it as an invoice issued in offline24 mode. The date from field P_1 is the issue date precisely in the special modes: offline24, system unavailability and emergency mode. The buyer can receive the invoice in the system as soon as it is accepted - there is no longer any need to send it to anyone by email.
When an invoice is rejected (for example due to an error in the data), it is treated as not issued. You then correct the file and send it again - only acceptance and a KSeF number complete the process.
The regulations anticipate that the system may be temporarily unavailable, so alongside the online mode there are special modes. They share one feature: you issue the invoice outside the system, mark it with a QR code, and send it to KSeF within a set deadline. They differ in the reason for use and the time allowed to send the file.
Such an invoice receives a KSeF number only after the file has been successfully sent to the system. This matters for corrections - a correction to an invoice from offline or emergency mode is issued after the original invoice has been assigned a number.
Mistakes happen to everyone, and a structured invoice cannot simply be deleted from the system. An error is corrected with a correcting invoice, which is also an FA(3) document sent to KSeF and also receives its own KSeF number.
In the correction you indicate the KSeF number of the original invoice, which is how the system links the two documents. You can correct both formal details (e.g. a typo in a name) and values - the price, quantity or VAT rate. Because the rules for recognising a correction in the settlement can be intricate, we will clarify the exact timing and method of accounting in relation to the specific situation.
At Luno-Group we run online accounting, but behind the settlements stand experienced accountants, not the app alone. For many companies the rollout of KSeF is a significant change, which is why we will guide you through it step by step - from setting up permissions to your first issued invoices.
Our clients issue and receive KSeF invoices free of charge in the client panel, without logging in to government apps. Sales and purchase invoices go straight into accounting, so you do not have to copy or send anything separately. We provide the entire service in Polish and English.
A KSeF invoice is an XML file compliant with the FA(3) schema, not a PDF. It is created by a program that then sends it to the system. Once accepted, the invoice receives a KSeF number and is available to the buyer without being sent by email. A PDF can at most serve as a visualisation of such an invoice.
For individual invoices, the free KSeF Taxpayer App or e-mikrofirma for the smallest companies is enough. With a larger number of documents, accounting software integrated with KSeF is more convenient. Luno-Group clients issue and receive invoices free of charge in the client panel.
The KSeF number is a unique identifier that the system assigns to an invoice once it is accepted. It confirms that the invoice has been effectively issued. In online mode you receive it almost immediately, while in offline or emergency mode only after the file has been sent to the system.
In online mode the issue date is the day the invoice is sent to KSeF, not the date entered in field P_1. So if you prepare the invoice one day and send it the next, the system treats it as issued in offline24 mode - and only then is the issue date the one from field P_1. This matters for tax settlement.
Then you use a special mode: offline24 for internet problems, unavailability mode during maintenance work, or emergency mode when a failure is announced. You issue the invoice outside the system and send it in within the statutory deadline - with offline24, no later than the next business day.
A KSeF invoice cannot be deleted - you correct an error with a correcting invoice, also in the FA(3) format. In the correction you indicate the KSeF number of the original invoice, which is how the system links the two documents. You can correct both formal details and values, for example the price or VAT rate.