Powering a 90m² Passive House Annex & Meeting Space with a Pre-Built Victron Energy & Pytes Lithium Battery Storage Solution

Constructing an off-grid building requires careful integration of energy generation, battery storage, and high-efficiency building design. When a client set out to build a 90m² two-bedroom annex and corporate meeting space entirely detached from the electrical grid, achieving total power independence without compromising comfort was the core objective. To meet this demand, Callidus Trade supplied a pre-built, pre-programmed off-grid solar and battery power system designed for straightforward self-installation.

Engineered around Passive House thermal insulation principles, the annex combines low structural heating demand with advanced Mechanical Ventilation with Heat Recovery (MVHR) and biomass pellet boiler heating. By pairing ultra-efficient building architecture with a heavy-duty Victron Energy inverter charger and 20.48 kWh of Pytes lithium battery storage, this case study details how a complete self-contained energy system delivers seamless, reliable power to a high-specification off-grid property.

Project Background: Designing a 90m² Off-Grid Annex

The original client enquiry specified the creation of a 90m² two-bedroom building designed to function as both an independent residential annex and a professional meeting venue. Situated in a location without a grid connection, the building required an autonomous power source capable of running domestic appliances, lighting, office electronics, and climate control equipment year-round.

To ensure low base-load power consumption, the structure was insulated to rigorous Passive House standards. Structural heating and domestic hot water are supplied primarily by a pellet boiler, while clean indoor air quality is maintained continuously via a Mechanical Ventilation with Heat Recovery (MVHR) system. Key architectural features included:

• Large Flat Roof Space: Designed specifically to accommodate a substantial solar photovoltaic (PV) array without overshadowing or structural complex pitch requirements.
• Dedicated Plant Room: A generous internal plant room provided ample space for housing electrical control boards, inverter equipment, and lithium battery cabinets safely.
• Continuous Base Loads: Powering MVHR air exchange, pellet boiler pumps and ignition systems, lighting, and workspace electronics 24 hours a day.
• Self-Installation Requirement: The client needed a modular, pre-wired system that could be safely and rapidly set up on site without requiring extensive bespoke electrical programming during installation.
  

System Architecture: Pre-Built Victron & Pytes Power Solution

To eliminate complex multi-component assembly on site, Callidus engineered and supplied a pre-built power board package. Every core component, including inverter charger, solar charge controller, monitoring equipment, and protective switchgear, was mounted, pre-wired, and rigorously tested in our workshop prior to dispatch.

The complete off-grid system combines high-efficiency solar generation, heavy-duty power inversion, and fire-suppression-equipped lithium storage:

• 7.2 kWp Solar Photovoltaic Array: Comprising 16 high-efficiency JA Solar 450W photovoltaic modules mounted on flat-roof mounting structures. This array captures clean solar energy to power daily loads and replenish battery capacity.
• Victron Energy MPPT 450/100 Charge Controller (SCC145110410): An advanced high-voltage Maximum Power Point Tracking controller that efficiently converts solar energy from the roof array into DC charging power for the battery bank.
• Victron Energy MultiPlus-II 15kVA Inverter Charger (PMP483150000): The central power hub providing 15kVA of continuous AC power output. It effortlessly handles high-surge inductive loads from kitchen appliances and workshop tools while managing battery charging.
• 20.48 kWh Pytes Lithium Battery Storage Bank: High-density lithium iron phosphate (LFP) chemistry providing generous energy reserve. Built specifically for safety and temperature resilience, the modules incorporate internal fire suppression and automated heating elements for low-temperature operating environments.
• Victron Cerbo GX & Touch 50 Display: Provides total system oversight via a high-resolution 5-inch touchscreen interface, allowing real-time tracking of solar generation, battery state-of-charge, and consumption metrics.
• Automated Generator Integration: Features built-in automatic start/stop control logic to manage a backup generator during extended winter months when solar irradiance is minimal.
  

Installation, Commissioning & Client Testimonial

A key requirement of the project was straightforward installation. Because the Victron equipment, protective breakers, and Cerbo GX controls were fully mounted and pre-wired on the Callidus pre-built board, the client successfully energised the complete system on Tuesday shortly after delivery.

Following successful commissioning, the client provided the following feedback regarding the installation process and system performance:

“I just want to say that I got the system up & running on Tuesday and i am blown away with it. I can't stop talking about it. It's amazing. SO easy to set up & get going. I've just done a little insta post about it & given you a small plug - lots more to follow and I can't wait to show Kevin McCloud when he comes back in the Spring for a Grand Designs revisit.”

— Self-Install Client, 90m² Off-Grid Annex Build

Key Benefits for Off-Grid Residential Annexes

The successful energisation of this 90m² two-bedroom annex demonstrates the practical advantages of combining high-efficiency building envelopes with pre-assembled off-grid power infrastructure:

• Simplified Self-Installation: Pre-built boards eliminate workshop-level wiring on site, allowing builders and self-installers to connect solar and battery inputs quickly and safely.
• High-Capacity Energy Storage: The 20.48 kWh Pytes battery bank provides ample reserve capacity to sustain overnight loads, MVHR ventilation, and heating controls without grid connection.
• Cold-Weather Reliability: Integrated heating modules within the Pytes lithium batteries ensure reliable charging and discharge performance even during cold winter periods in plant rooms.
• Passive House Integration: Ultra-low heating demands from Passive House insulation standards complement solar PV generation, creating an economical year-round energy balance.
  

About Callidus Trade

Callidus Trade specialises in designing, assembling, and supplying custom off-grid power systems, solar PV packages, and commercial battery storage solutions. By supplying fully pre-built and bench-tested inverter panels featuring leading brands such as Victron Energy and Pytes Lithium, Callidus Trade empowers self-builders, electrical contractors, and commercial clients across the UK to deploy reliable off-grid energy infrastructure.