Hemel Hempstead Solar and Battery: Do You Need G99?
Adding a battery to solar in Hemel Hempstead is likely to mean a G99 application to UK Power Networks. How G98, G99 and export limits work, and who submits.
Does a Solar and Battery System in Hemel Hempstead Need a G99 Application?
Planning permission is only half the paperwork for a home energy system. The other half is your connection to the electricity network, which is governed by two engineering standards with similar names: G98 and G99. This guide explains which one applies to a solar and battery system in Hemel Hempstead, why adding a battery so often changes the answer, and who is responsible for each step.
Short answer: once a battery is added to solar, MCS says the G99 route is likely, and that means the network operator is consulted before installation.
Who runs the electricity network in Hemel Hempstead?
Your distribution network operator (DNO) owns and runs the local cables and substations. In Hemel Hempstead the DNO is UK Power Networks, which distributes electricity across the East of England. In August 2020 it was replacing one transformer at its Warners End substation in Hemel Hempstead with two new 33/11 kV transformers, to boost the reliability of supplies for more than 8,000 customers.
If you are comparing quotes for solar panels in Hemel Hempstead, ask each installer how they will handle the UK Power Networks paperwork, because the answer can shape both your design and your timeline.
G98: install first, then notify
The Energy Networks Association's Engineering Recommendation G98 covers fully type-tested micro-generators up to and including 16 A per phase. G98 converts that limit into kilowatts as follows:
| Supply | G98 limit |
| Single-phase | 3.68 kW |
| Three-phase | 11.04 kW |
Three details catch people out:
- The limit is aggregate. G98 looks at the total aggregate capacity of the installation, not at each device on its own.
- Batteries count. G98 states that its definition of micro-generators includes electricity storage devices.
- Notification can come afterwards. Under G98 the installer must make sure the DNO is aware of the installation no later than 28 days after commissioning, counting the day of commissioning. MCS confirms the form used is G98 Form B.
A modest solar-only system on a single-phase supply, within 3.68 kW of capacity, can therefore usually be connected first and notified afterwards.
G99: apply first, then install
Once total aggregate capacity exceeds 16 A per phase, G98 says the G99 process applies and the DNO needs to be consulted before the installation is undertaken.
MCS guidance for installers is direct about batteries: where storage is installed alongside another technology such as solar PV, the total connected generation is likely to be above 16 amps per phase, so G99 will apply. MCS also notes that network operators treat batteries as generation when they export to the network and as demand when they import from it.
How battery inverter ratings interact with the thresholds
The thresholds are set in amps per phase, which is about output capacity, not how much energy a battery stores in kWh. That has some practical consequences:
- Storage size is not the trigger. A battery's kWh figure tells you how much it holds; the connection threshold looks at its rated output.
- One inverter can decide it. A Tesla Powerwall 3 is rated at 11.5 kW continuous, well above the 3.68 kW single-phase figure before any solar panels are counted.
- Ratings can sometimes be restricted. G98 allows a manufacturer to restrict a micro-generator's rating through software settings, provided those settings are not accessible to the customer. Whether that is possible depends on the product.
- Running during a power cut changes the route. G98 says installations designed to operate as an islanded installation should refer to G99. If you want a battery that keeps powering your home when the grid is down, raise it at design stage.
Our battery storage page covers the SigEnergy and Tesla Powerwall 3 systems we fit.
The fast-track option
G99 is not one single process. MCS describes a fast-track application for integrated solar and storage where the capacity is over 3.68 kW, the inverter is type-tested to G98 standards, and the storage will not operate when the grid fails. The form is Form A1-2, and MCS notes that it must be submitted before installation, although the connection time is reduced.
Separately, the ENA's draft connection guide for G98 and G99 says a power generating module of less than 17 kW single-phase or 50 kW three-phase can use a simplified application form. That is a different threshold from G98's 16 A per phase, not a replacement for it.
What export limitation does, and does not, change
An export limitation scheme caps how much power a system sends back to the grid. The ENA's technical guidance for customer export limiting schemes is Engineering Recommendation G100.
An export limit is not a way around the G98 threshold. G98 says that where new solar and a battery are installed through a G98 type-tested inverter together with an export limitation scheme that conforms to G100, reference should be made to G99, where the integrated micro-generation and storage procedure may be appropriate. That integrated procedure does not apply where the combined capacity is 16 A per phase or less, in which case G98 applies.
If export income matters to you, our SEG calculator lets you estimate what exported units could earn.
How long does it take?
Timescales depend on the route, so be wary of anyone who promises a fixed connection date before the paperwork is in.
| Route | When the DNO is told | What the guidance says |
| G98 | Before, or no later than 28 days after, commissioning | The installer makes the DNO aware using G98 Form B |
| G99 fast track (Form A1-2) | Before installation | MCS says the connection time is reduced |
| G99 with a connection offer | Before installation | Construction can begin once you have accepted the offer |
The ENA's draft guide also notes that a connection offer specifies the conditions for your connection and any connection charge you may be asked to pay, and that connection offers are time limited.
Who submits what?
| Step | Responsibility |
| G98 Form B, no later than 28 days after commissioning | Your installer |
| G99 application, including Form A1-2 where it applies | Your installer, on your behalf |
| Checking and accepting any connection offer | You, as the customer |
| Planning checks with Dacorum Borough Council | A separate process from the DNO |
The ENA's draft guide says your installer should submit the application form on your behalf.
What to do next
- Check your supply. A single-phase or three-phase supply changes the G98 figure from 3.68 kW to 11.04 kW. A surveyor can confirm which you have.
- Decide whether you want the battery to run during a power cut. It can affect which G99 route is open to you.
- Ask who handles the network paperwork. It should be your installer.
- Keep export payments in mind. Smart Export Guarantee payments require an MCS-certified installation.
SOLA UK is an MCS-certified installer based at Arquen House in St Albans, fitting AIKO 485W all-black panels, SigEnergy battery systems and the Tesla Powerwall 3. If you would like us to look at your roof and your grid connection, call 0800 470 0922.
