We were recently asked to review a proposed solar PV and battery storage system for a large residential property with significant electrical demand. The proposal consisted of 60 panels (405 W each) and a 50 kWh battery. On paper, the system looked reasonable — but our assessment found scope for optimisation both technically and financially.

Step One: Understanding the Load

We modelled three usage scenarios — Low (37,000 kWh), Medium (42,000 kWh), and High (50,000+ kWh) — to reflect possible variations in how the building might operate over time. This formed the basis for sizing the system and understanding self-consumption potential.

Simulation Method

All options were modelled using software that dynamically simulates site-specific solar generation and load interaction at sub-hourly intervals, using historic weather data for the Isle of Man. This allows us to assess how the system behaves on a realistic daily and seasonal cycle — capturing the impact of variable solar irradiance, consumption peaks, and battery charge/discharge cycles with a high degree of resolution.

This approach provides much more reliable output than basic spreadsheet methods or tools that rely on monthly averages.

Key Refinements

The use of higher-output 445 W panels allowed the system to increase from 24.3 kWp to 26.7 kWp within the same footprint.
Battery capacities between 20 kWh and 40 kWh were modelled to identify the most effective configuration in terms of return on investment.
A particular concern was the 5-year battery warranty in the original proposal, which raises questions about long-term performance and replacement risk in the context of a 25-year system lifespan.

📊 Self-Consumption vs Battery Size

Self-consumption increases with battery capacity, as expected. However, beyond a certain point, the additional capital cost outweighs the marginal benefit.

📊 Financial Summary (Medium Usage Scenario – 42,419 kWh/year)

The ~20 kWh option provides the best overall outcome — delivering the highest net profit with a strong payback period and a good balance between performance, warranty life, and cost.

🌱 CO₂ Offset Over 25 Years

While larger batteries deliver more operational carbon savings, the data reveals a clear point of diminishing returns. The initial jump from a PV-only system to one with a ~20 kWh battery provides the most significant impact, boosting the annual CO₂ offset by nearly 70%.

Beyond this, the gains become less pronounced. This is critical because every battery has an upfront embodied carbon cost from manufacturing (not shown). For each step up in size, the smaller operational savings must justify a larger carbon footprint. In this analysis, the ~20 kWh system hits the sweet spot, delivering the most effective carbon reduction when weighed against its environmental cost.

Final Thoughts

This analysis shows that the biggest system is rarely the most effective. While the original proposal was broadly suitable, our modelling identified the true sustainable sweet spot: a configuration that delivers a stronger long-term outcome by properly balancing financial returns with environmental benefits. A modest number of changes led to a significantly improved result.

If you have received a proposal for a solar PV system and would like a second opinion focused on lifetime value, I invite you to get in touch via direct message.

Solar PV & Battery Storage Facts:

Fact 1: The levelised cost of utility-scale solar PV has fallen over 85% in the last decade, making it the cheapest source of new-build electricity in history.

Fact 2: The solar energy striking the Earth's surface in 90 minutes is sufficient to meet total global energy demand for an entire year.

Fact 3: Lithium-ion battery prices have declined by approximately 90% since 2010, unlocking the economic viability of grid-scale energy storage.

Fact 4: Battery storage is now displacing traditional gas peaker plants for grid balancing services, offering response times in milliseconds versus minutes.

Fact 5: Global solar deployment continues to accelerate at an exponential rate. The amount of new solar capacity the world is expected to install in 2025 alone will be greater than the entire planet's total installed capacity from just a decade ago, in 2015.

When it was decided that The Buchan School would relocate to the King William’s College campus, the design team faced a common choice: demolish and rebuild, or retain and adapt.

Two existing buildings - Jackson House and Stenning - were structurally sound, underused, and well-placed. Rather than clear the slate, the team opted to reuse and reconfigure. A full new-build equivalent would have embodied around 680 tonnes of CO₂e. The refurbishment, by comparison, is estimated at 190 tonnes[1]; an avoidance of 490 tonnes.

That saving comes from materials that weren’t demolished, concrete that wasn’t poured, steel that wasn’t fabricated, and envelope systems that were never required. In everyday terms, it’s equivalent to:

The annual emissions of 105 petrol cars[2]
The carbon captured by 800+ mature trees over 25 years[3]
The energy generated by a 2500 m2 solar PV system over more than a decade[4]

Adaptive Reuse in Practice

The retained fabric included loadbearing masonry, existing slabs, roof trusses, and selected windows and doors. Interventions were targeted and minimal: localised steel insertions, new partitions and linings, updated ceilings, and bespoke joinery to suit new educational layouts.

MEP Design In Support of Reuse

At March Consultants Limited, our role was to ensure that building services supported, rather than disrupted, the logic of reuse. Penetrations were kept minimal, distribution routes were adapted to the existing fabric, and services were carefully coordinated to preserve structural integrity. While many systems are new, they were selected for their compact footprint and high operational efficiency; ensuring their inclusion contributed meaningfully to long-term performance and energy savings.

The project features hybrid ventilation units, each carbon neutral in manufacture[5], across teaching spaces. These units help reduce heat loss by an estimated 47,800 kWh per year, avoiding approximately 10.3 tonnes of CO₂e annually[6]. A further 5,100 kWh/year is saved through CO₂ sensor-controlled demand ventilation, avoiding an additional 2.2 tonnes of CO₂e[7].

These may not be headline-grabbing numbers, but they represent deliberate, cumulative design decisions that enhance the building’s efficiency without compromising its retained structure.

Reuse and Responsibility

While the structural choices drove the bulk of the carbon saving, the MEP design ensured those savings weren’t undermined. In refurbishment, the greatest contribution is often knowing where not to intervene.

Footnotes

Estimates based on lifecycle modelling using RICS WLC methodology and structural engineer’s baseline material schedules.
UK BEIS 2023 emissions factor of 4.66 tonnes CO₂e/year per average petrol car.
Based on an average sequestration rate of 21 kg CO₂/year per mature broadleaf tree (Forestry Commission UK, 2022).
Assuming 250 kWh/m²/year generation for roof-mounted PV in Isle of Man climate.
Manufacturer declaration; embodied carbon offset at point of sale (EPD available).
Based on IOM gas carbon factor of 0.216 kg CO₂e/kWh.
Based on IOM electricity carbon factor of 0.430 kg CO₂e/kWh.

Project Info:

Architect: Wilson Mason Architects

Structural: BB Consulting Engineers

PQS: Bell Burton Associates

Contractor: Excel Group

Client: King William’s College