20kW Solar System UK: The Complete Commercial Buyer’s Guide (2026)

At this system size, the difference between a strong investment and an underperforming one rarely comes down to hardware — it comes down to how well the system is matched to your building’s electricity usage, your supply type, and the constraints imposed by your local grid. A 20kW system sits at an important threshold in the commercial market: large enough to make a visible impact on operating costs, but still straightforward enough to install on most commercial roofs without structural upgrades or complex planning.

For the right building, it is one of the most reliable capital investments a UK business can make in 2026. For the wrong building, the same system can deliver a fraction of the expected return. The difference is in the preparation. This guide focuses on that. For a broader comparison across commercial system sizes, see our commercial solar cost guide.

Quick Facts: Cost, Output and Specs at a Glance

These are the baseline figures for a 20kW system, consistent across most UK installations and providing a useful reference point. What determines your return sits in the sections below.

The Question Every Business Should Answer Before Sizing a 20kW System

Before system size, before cost, and before installer selection, there is one question that determines whether a 20kW system will perform well on your building: when does your building consume electricity?

A 20kW solar system generates most of its output between approximately 9am and 4pm, with a peak around midday. This production pattern is consistent across installations. What differs from one building to another is how closely electricity demand aligns with those hours. Where demand and generation overlap, most of the energy is used on-site, offsetting electricity at commercial rates — which is where the financial return is strongest. Where they do not align, surplus electricity is exported to the grid at a significantly lower rate, reducing the overall value of the system.

Two buildings can both consume 40,000 kWh per year and produce very different results from the same 20kW system because their demand profiles are different. In practice, that difference usually comes down to whether operations are concentrated during working hours or extend into evenings, how much demand exists during weekends and low-occupancy periods, and whether energy use is steady throughout the day or peaks at specific times. This is where half-hourly consumption data becomes essential. Most commercial properties already have this data recorded through smart or AMR meters. Any commercial solar proposal that does not incorporate this analysis is not tailored to your building — it is based on generalised assumptions.

Which Businesses Does a 20kW System Actually Suit?

A 20kW system works best where there is consistent daytime demand that aligns with solar generation hours. The difference becomes clearer when you look at typical building profiles.

Where alignment is strong, self-consumption typically sits between 70% and 80%, which is where the financial case becomes compelling. Suitability at 20kW is not about business size — it is about how demand aligns with generation. See our overview of 10 things UK businesses need to know about commercial solar for further context on sizing and suitability.

Single-Phase vs Three-Phase: Why Your Electricity Supply Changes Everything at This Size

At smaller system sizes, electricity supply type is easy to overlook. At 20kW, it becomes one of the first real technical and commercial constraints in the project. When an installer prepares a G99 application for a system at this size, one of the first questions the DNO will care about is whether the building is connected by single-phase or three-phase supply. That answer has a direct effect on how the project is assessed and how likely it is to receive restrictive export conditions.

Most smaller commercial premises and older units operate on single-phase supply. At this size, that makes the grid connection more sensitive to additional generation. A 20kW system sits well above the point where the DNO becomes cautious about export on a single-phase connection. That does not make the project impossible, but it does make it much more likely that the connection offer will include an export limit.

If your building is on single-phase supply, there are usually three practical routes: reduce the system size to create less export pressure; include export limitation within the design; or investigate a three-phase upgrade. That upgrade route involves additional cost and lead time, but in some cases it unlocks a significantly better project over the long term. Supply type should be confirmed early — not discovered after the quote has already created unrealistic expectations.

The G99 Process: What Happens, How Long It Takes, and What the Approval Letter Actually Says

For a 20kW commercial rooftop system, G99 approval is mandatory before installation can proceed. The G99 process is not just administrative paperwork — it is the process that determines how the system is allowed to behave once installed.

The process typically starts with a pre-application stage where the installer contacts the local DNO to identify whether there are any obvious constraints at the connection point. Once the design is finalised, the installer submits a formal G99 application pack including inverter specifications, electrical diagrams, site layout, protection settings and the proposed connection arrangement. The DNO then assesses how your proposed system will affect the local network. The central concern is voltage rise: if your system exporting electricity could push local voltage above the statutory limit, the DNO will impose conditions on the connection. For a 20kW system, this assessment usually takes between 10 and 45 working days.

The outcome is a formal connection offer — and this is the document buyers should pay much more attention to, because it defines the operating terms of the project. A connection offer will confirm the maximum export capacity the DNO will allow, any technical conditions that must be met, and any fees attached to the connection. From submission to approval, the overall timeline is usually six to twelve weeks. That is why a well-managed commercial project pushes G99 forward as early as possible. Our DNO solar application guide explains the full process in detail.

Export Limitation: The Outcome Nobody Warns You About

Export limitation is one of the least understood parts of a 20kW commercial solar project, and often one of the most important. When your solar system generates more electricity than the building is using at that moment, the surplus flows out to the grid — until the DNO decides the local network cannot safely absorb that level of export. When that happens, the connection offer comes back with an export limit.

A 20kW system with a 5kW export limit can still generate 20kW and use that electricity on-site. The issue appears when building demand falls. On a bright weekend or closure period, the building may be taking very little electricity while the system is producing strongly — and surplus generation above the export cap has nowhere to go.

In practical terms, there are three main ways this is handled: export limitation controls, which instruct the inverter to reduce output when the cap is reached; battery storage, which absorbs surplus generation instead of losing it; and Active Network Management (ANM), where offered by the DNO, allowing more flexible export control. Battery storage is often the most commercially useful response because it converts what would otherwise be curtailed energy into usable electricity later in the day.

Finance Options: Outright Purchase, Asset Finance and the Honest Truth About PPAs

At 20kW, the funding route you choose changes ownership, tax treatment and how much long-term value the business actually keeps. There are three main routes.

Outright purchase gives the business full ownership from day one: full access to tax relief, full retention of electricity savings, and full control over the asset. Asset finance spreads the cost over a repayment term, usually between three and seven years, and can often keep the project close to cash-flow neutral while the finance is being repaid. A PPA removes the upfront capital requirement because a third party owns and installs the system, and the business agrees to buy the electricity generated at a discounted rate. The trade-off is that most of the ownership upside stays with the provider — there is no AIA benefit to the occupier and no ownership of the asset. For most profitable UK businesses, outright purchase or asset finance delivers better lifetime economics than a PPA. PPAs make the most sense where a business has no capital available, no taxable profits to offset, or no appetite to own the asset on its balance sheet.

Tax Reliefs and Financial Returns

The financial case for a 20kW system becomes materially stronger when tax treatment is understood properly. The Annual Investment Allowance allows 100% of the qualifying installation cost to be deducted from taxable profits in year one. For profitable businesses paying corporation tax at the standard rate, that can significantly reduce the effective cost of the project.

For a typical 20kW system, the headline numbers look like this:

The practical point is straightforward: payback should be assessed against the effective cost after tax relief, not just the headline installation figure. There is also an important property-related point: new rooftop solar installations are exempt from business rates increases through to March 2035. For more on tax relief mechanisms and available funding routes, see our commercial solar grants guide.

Battery Storage With a 20kW System: When It’s Essential and When It’s Optional

Battery storage is often presented as the automatic next step in commercial solar. At some 20kW projects it is critical. At others it is optional. The right answer depends on how the building operates and what the grid connection will allow.

The first situation where battery storage becomes highly relevant is where the G99 approval includes an export limit. In that case, surplus generation that cannot be exported has to be curtailed unless there is storage available to absorb it. Batteries can also make strong sense where the business is closed for part of the week, where demand drops sharply during strong solar periods, or where there is meaningful late-afternoon demand that extends beyond the solar production peak.

Battery storage becomes much more optional where the building already consumes 80% or more of the generation during operating hours, the export cap is generous, and there is limited value in shifting energy into later periods. Batteries should be specified where they solve a clear mismatch between generation, demand and export conditions — not sold as a default upsell. For a full breakdown of storage options and costs, see our battery storage page.

ESG and Sustainability Reporting: What the System Provides Beyond the Bill Saving

For many businesses, a commercial solar installation is no longer only about lowering electricity costs. It also contributes to sustainability reporting, procurement positioning and wider ESG commitments.

A 20kW system generating around 17,000 kWh per year typically reduces grid-related electricity emissions by approximately 3,400–3,800 kg of CO₂ annually. That gives the business a practical and reportable Scope 2 reduction, which is increasingly useful in both internal and external reporting. This becomes relevant for businesses dealing with supply chain sustainability reporting, B Corp and broader ESG commitments, ISO 14001 environmental management, and public sector contract and framework expectations.

What matters here is not just the installation itself, but the data the system produces after handover. A well-specified commercial project should provide the monitoring, reporting visibility and baseline documentation the business needs to evidence the benefit over time. Solar4Good systems include monitoring as standard.

How Solar4Good Approaches 20kW Commercial Installations

Most 20kW commercial projects follow a structured process. At this size, that process matters because the strongest-performing systems are usually the ones that were designed carefully before any equipment reached site.

Step 1 — Initial consultation and consumption data review

The project should begin with half-hourly consumption data and recent electricity bills, not with a rough annual estimate. That allows system sizing to be based on how the building actually behaves rather than on broad assumptions.

Step 2 — Commercial site survey

The survey assesses roof suitability, usable space, shading, orientation, structural condition and electrical infrastructure. At this stage, supply type should be confirmed and the practical groundwork for the G99 strategy put in place.

Step 3 — System design and financial proposal

The proposed layout, inverter strategy and connection arrangement are developed. The proposal should reflect expected self-consumption, likely export, project payback, AIA impact, and whether batteries or export limitation need to be included.

Step 4 — G99 application

Once the design is ready, the G99 application should be submitted promptly. This is often the longest-moving element of the project and one of the biggest sources of delay if handled too late.

Step 5 — Planning confirmation

Most rooftop systems of this scale fall within permitted development under commercial solar regulations, but planning status still needs to be checked properly before installation is programmed.

Step 6 — Equipment procurement and scheduling

Once approvals are in place, the project moves into procurement and scheduling. Panels, inverters, mounting systems and any battery equipment are secured for the installation window.

Step 7 — Installation

A 20kW commercial rooftop installation is usually completed over two to three days, depending on access, roof configuration and electrical complexity. If battery storage is included, the programme may extend slightly.

Step 8 — Commissioning, certification and handover

The system is tested, configured, connected and handed over with monitoring access and certification. At that point, the business receives both an operational energy asset and the reporting visibility needed to track performance over time.

Conclusion

A 20kW solar system is one of the strongest capital investments available to many UK small businesses in 2026 — but only when it is sized around how the building actually uses electricity, approved with a clear understanding of grid constraints, and designed to handle whatever export conditions the DNO imposes. The businesses that see the best returns are not necessarily the ones that bought the cheapest system. They are the ones whose project was designed around reality rather than averages.

The financial case for a 20kW commercial solar system is robust, but the detail sits in the preparation. Consumption data, supply type, DNO constraints, export conditions and finance structure all have a direct effect on how the project performs. Those are not side issues — they are the factors that separate a well-performing system from one that looks good in a quote and underdelivers in practice. If you are considering a 20kW solar system, the next step is not to ask for a generic quote. It is to review the building’s real consumption profile and design the system around it.

Frequently Asked Questions