Survey, G99 Application, and Stage Invoicing Discipline for Battery Storage Installers

Battery storage installation has become one of the faster-growing segments in the UK renewables market, with residential retrofits, solar-paired residential systems, and commercial BESS projects all competing for the same pool of certified installers. The operational challenge for any battery storage business is that each job runs through a compressed but compliance-heavy sequence: site survey, DNO notification or G99 application, equipment procurement, installation, commissioning, MCS documentation, and handover. Get any one of those disciplines wrong and the downstream consequences are immediate - a G99 application submitted late means equipment sits in a warehouse, a poorly specified survey means the wrong battery cabinet gets ordered, and a missed MCS certificate means the customer cannot register for the Smart Export Guarantee (SEG). This resource covers the four disciplines that determine whether a battery storage business operates profitably and at scale.

Survey and System Sizing Discipline

The quality of a battery storage installation is determined at the survey stage. For residential jobs, the survey establishes the existing electrical installation, the proposed battery location, cable routing from the inverter, and the DNO connection point details. For commercial jobs, the scope is significantly wider: the surveyor needs to understand load profiles, peak demand periods, existing grid connection capacity, and whether the system will be solar-paired or grid-only.

A clear survey output needs to capture: battery capacity in kWh, power rating in kW, battery chemistry (LFP is now standard for most commercial BESS), whether the system will be AC-coupled or DC-coupled, inverter specification, and the proposed grid connection arrangement. For systems that will be added to an existing solar installation, the survey must also establish whether the total combined capacity will push the system above 3.68kW per phase - because that threshold determines the DNO notification route.

Getting the sizing decision wrong creates problems that cannot be easily unpicked. Undersized batteries fail to meet customer savings expectations. Over-specified batteries cost more than the business case supports. The two most common survey failures in battery storage are not capturing load profile data on commercial sites - which means the BMS set-up cannot be optimized at commissioning - and not confirming the DNO connection position before ordering equipment, which can leave the business exposed if a G99 application comes back with conditions.

The survey should also confirm battery location relative to fire safety requirements, particularly for larger commercial systems. Containment, ventilation, and clearances affect both the installation programme and the cost of the job.

G99 Application and DNO Management

DNO compliance is the single most common cause of battery storage project delays. Under the Engineering Recommendations set by the Energy Networks Association (ENA), DNOs classify battery storage as demand when importing from the network and as generation when exporting to it. This means battery storage is subject to the same G98 and G99 framework that governs solar PV.

For systems at or under 3.68kW per phase, G98 applies: the installation can proceed and the installer notifies the relevant DNO using Form B within 28 days of commissioning. For systems over 3.68kW - which covers the majority of battery storage installations, including most residential retrofits paired with a hybrid inverter - G99 applies and prior approval must be obtained before the installation takes place. The G99 Form A1-1 must be submitted and approved before work starts.

For combined solar PV and battery storage installations where the total capacity exceeds 3.68kW, a fast-track route is available using Form A1-2, but this form must still be submitted before installation begins. The fast-track process reduces the connection time compared to a standard G99 application, but it is not instantaneous and must be built into the project programme.

The practical implication for job management is that G99 approval status must be tracked as a project milestone, not as a background administrative task. A battery storage business running 15-20 active jobs at any one time needs a clear view of which jobs are in G99 submission, which have approval, and which are awaiting conditions to be resolved. Seven DNOs operate across 14 districts in the UK, and response timelines vary by network and by current application volume. Treating DNO management as an afterthought produces a backlog of jobs that cannot legally proceed.

Equipment Procurement and Lead Time Management

A battery storage installation typically draws on three separate procurement streams: the battery system itself (battery modules and battery management system), the inverter or hybrid inverter, and the balance of system components (DC and AC cables, isolators, protection equipment, and any additional earthing or fire detection requirements).

For residential jobs, procurement is usually straightforward - a single supplier delivers a pre-integrated battery cabinet with built-in BMS and the installer sources the inverter separately or as a bundle. For commercial jobs, the picture is more complex: battery modules, inverters, an energy management system (EMS), and switchgear may come from different suppliers, each with their own lead times and delivery terms.

The procurement risk on battery storage jobs is tied directly to the sizing decision. Equipment ordered to the wrong specification - wrong kWh capacity, wrong power rating, wrong AC or DC coupling arrangement - cannot be returned to stock and used on another job. The cost sits as write-down until it finds a matching project. The way to manage this is to make equipment procurement conditional on confirmed survey output and, for G99 jobs, on submission of the DNO application.

Battery and inverter lead times from UK distributors vary. During periods of high market demand, hybrid inverters and popular battery cabinet configurations can carry lead times of four to eight weeks from order to delivery. For commercial projects with a confirmed installation date, procurement needs to be sequenced against that date with enough buffer for delivery confirmation and goods inspection on arrival.

MCS Documentation, Commissioning, and Handover

Battery storage installations carried out by MCS-certified installers must meet the MCS installation standard and produce a complete documentation package for the customer. MCS certification is the quality mark that enables the customer to register for the Smart Export Guarantee - the government scheme that pays eligible households for electricity exported to the grid. An installation without MCS certification removes a meaningful financial benefit from the customer and weakens the installer's competitive position.

The commissioning process for a battery storage installation covers: IET 18th Edition inspection and testing of the electrical installation, BMS commissioning and parameter configuration, EMS setup for the agreed operating strategy (peak shaving, load shifting, solar optimization, or combined), and a full functional test of charge, discharge, and any backup power mode.

For G99 installations, the combined installation document (Form A3-1/A3-2) must be completed after commissioning to confirm the installation meets G99 standards. This form confirms to the DNO that what was installed matches what was approved.

The handover pack is both a compliance deliverable and a customer service discipline. Customers who receive complete documentation on the day of completion are more likely to provide referrals and less likely to raise warranty queries months later. For installers running a high volume of residential jobs, using a digital eForm to capture commissioning data on site - rather than relying on paper records to be completed in the office - reduces the number of incomplete handover packs significantly.

Stage Invoicing and Payment Structure

Battery storage jobs, like most renewable energy installations, have a clear cash flow structure that should be reflected in the invoicing model. The primary cost exposure is equipment: a residential battery and hybrid inverter combination represents a significant portion of the total job value, and on a commercial project the equipment cost can be 60-70% of the contract price. Installers who issue a single invoice at job completion are carrying that exposure for the full duration of the project.

A standard stage invoicing structure for battery storage installations works as follows. A deposit invoice at contract acceptance - typically 30-50% of the contract value - covers the cost of equipment procurement and confirms the customer's commitment. A second stage payment is raised on equipment delivery, once the battery and inverter are on site. The final invoice is raised on completion, after the MCS certificate and DNO documentation have been issued.

For commercial BESS projects, a more granular milestone structure is appropriate: contract acceptance deposit, G99 approval confirmation, equipment delivery, installation complete, and final account following commissioning sign-off and handover pack delivery. Each milestone is a natural payment trigger that matches the installer's cost position at that stage.

The risk in not using stage invoicing becomes clear when a job is delayed. A G99 application that takes longer than expected, a delivery that misses its window, or a commissioning issue that requires a return visit all extend the gap between cost and revenue. Without stage payments, the installer absorbs that extension entirely.

How Zigaflow Supports Battery Storage Operations

Battery storage businesses running 20 or more active jobs at different stages need a system that can track each job's position - survey complete, G99 submitted, G99 approved, equipment ordered, installation booked, commissioned, MCS issued - without relying on spreadsheets or email folders. Zigaflow manages the job lifecycle from quote through to final invoice, with purchase orders raised against specific jobs so equipment costs are matched to the correct project.

Stage invoices are created directly from the job record and linked to the project milestones agreed at quote stage. For businesses using Xero or QuickBooks, invoices sync automatically, removing the double-entry that typically creates accounts reconciliation problems at month end. The eForms App allows commissioning data to be captured on site and linked to the job record in real time, so the MCS documentation pack can be compiled immediately after handover rather than chased up three days later.

Battery storage businesses that manage survey, DNO, procurement, and invoicing as separate manual processes will find that operational discipline breaks down as job volume increases. The companies that scale without increasing administration overhead are the ones that connect these stages into a single tracked workflow before the business outgrows its current system.

Keeping survey data, G99 status, purchase orders, and stage invoices connected to the same job record is not complicated to set up. It is, however, a discipline that needs to be built into how the business operates from the first day it manages more than a handful of live jobs simultaneously.