The 30-second summary
G99 is the grid connection process. Every commercial PV install needs G99 approval from the local DNO (Western Power, UKPN, Scottish Power, etc.). G100 is the additional certification that your inverter is type-tested to limit export below a specific kW figure. If you don't want to export at all, or you want a smaller export than your inverter capacity, you need G100 — adds 4–8 weeks to the application and ~£3–8k to the project. If you would rather hand the paperwork over, our grid connection consultancy prepares and submits it for you.
Why DNOs care about export
Local 11kV substations have finite capacity to absorb export. If 20 commercial sites in a postcode all export at peak, you can overload the transformer. G100 lets the DNO approve installs that would otherwise be rejected for export capacity. The DNO knows the inverter physically cannot export above the limited level. For the application itself, step by step, see our G99 application guide.
When G99-only is enough
G99-only works when: (1) Your DNO has capacity for full export (rural, low-density areas often do). (2) Your project size is well within the local feeder capacity. (3) You don't plan to add battery later (battery firmware can change export profile). Rural sites like these can often get full-export G99 approval in a couple of months.
When you need G100
G100 becomes necessary when: (1) DNO flags capacity constraint at application (most urban / industrial estates). (2) You're at an oversubscribed substation. (3) You want a high DC-AC ratio (i.e., 240 kWp panels with 200 kVA inverter — needs export limited to 200). (4) You're planning future battery installation that might change export. Food processors and cold stores on busy industrial estates commonly end up on G100.
The cost of G100 — actually
Inverter cost uplift: typically £200–400 per inverter for the type-tested G100 firmware activation (G100-compliant inverters: Sungrow CX, SolarEdge SE, Fronius SymoGen24). Application cost: nil if filed alongside G99. Time cost: 4–8 weeks added to the G99 timeline because DNO testing/verification takes longer. Total project cost: typically £3,000–£8,000 vs G99-only.
Picking the right export limit
Three approaches: (1) Match your DC capacity (no export limit needed — go G99 only). (2) Match your historical max import (export = 0 — fully self-consumption oriented). (3) Match local feeder capacity (DNO will tell you in their G99 response). On the food processor case, we sized 240 kWp DC, 200 kVA AC inverter, 0 kW export — feeding 100% of generation into on-site demand.
The battery + G100 interaction
If you add battery, the export profile changes. G100 firmware needs to know whether battery is on the DC or AC side, and whether it's discharging. Most modern hybrid inverters handle this automatically — Sigenergy, Fox, Sungrow, GivEnergy. Older string inverters with retrofit batteries may need re-certification. Plan for battery from day 1 if you might add it later.
What to do if your DNO rejects
Three options: (1) Resubmit at a lower capacity (e.g., reduce from 500 kVA to 300 kVA AC). (2) Apply for a constraint connection (you accept curtailment during DNO-defined peak windows). (3) Pay for grid reinforcement (DNO will quote — typically £15k–£500k+ depending on what needs upgrading). Sites with a large, steady daytime load — cooling, refrigeration, process lines — can often run at zero export and still use nearly all of their generation.
