DNO letter: what it is and when a solar install needs one
The written decision from a local electricity Distribution Network Operator confirming whether a proposed solar panel, battery, or heat pump connection can proceed, on what terms, and at what cost.
A DNO letter is the written decision issued by a local Distribution Network Operator (DNO) - the regulated company responsible for maintaining the electricity cables and substations in a given area - confirming whether a proposed generation or load connection can proceed, on what terms, and at what cost. For renewables installers, it is the document that determines whether a job can be commissioned: without it, a G99 solar PV or battery system cannot legally connect to the grid, and a heat pump installation that triggers the "Apply to Connect" threshold cannot go ahead until the DNO has assessed supply capacity and responded in writing.
What a DNO letter contains
The letter is the DNO's formal response to a connection application. It typically states one of three outcomes: full approval to connect, conditional approval setting out export limits or required equipment, or a request for network reinforcement before the connection is possible. A conditional letter might cap an export at a specific level even if the installed system is larger, and will specify the inverter settings or export limitation device that must be in place before commissioning. The letter also carries reference numbers that tie directly to the Smart Export Guarantee (SEG) registration process, making it essential documentation for customers who want to receive payment for electricity they export to the grid.
When an install needs one before work starts
Not every job requires a DNO letter before the installer arrives on site. Under Engineering Recommendation G98, which covers single-phase systems at or below 3.68kW, the installer can commission the system first and notify the DNO within 28 days afterward. No pre-approval letter is issued in advance for G98 jobs. Under G99, which applies to systems above 16A per phase and to most combined solar and battery installations, a formal application must be submitted and approved before the system connects to the grid. Approval can take between 6 and 12 weeks depending on the DNO and the complexity of the connection. For heat pumps, a similar split applies: smaller installations fall under "Connect and Notify," while those increasing Maximum Demand above 60A require an "Apply to Connect" submission and a written DNO response before the unit can be energized.
Do not book the install date before the letter arrives
For G99 solar jobs and Apply to Connect heat pump installations, the DNO letter gates the commission date. Committing to a start date without approval in hand risks a rescheduled installation and delays the customer's SEG registration.
How installers manage DNO letters in practice
Keeping track of where each job sits in the DNO process is one of the more operationally demanding parts of running a renewables business. A team running 10 to 20 live jobs at a time may be waiting on DNO decisions from multiple network operators simultaneously, each with its own response timescale and application format. Attaching DNO correspondence directly to the job record - alongside the survey report, the signed quote, and the MCS documentation - prevents the letter from sitting in someone's inbox where the rest of the team cannot act on it. Standardizing the application pack with document templates reduces the risk of an incomplete submission that restarts the approval clock.