software testing jobs in London

Job title: Principal Software Engineer

Location: London

Contract Type: Permanent, Full-Time

About you

You should have a long-term commitment to advancing the energy transition and be driven to build robust, scalable systems that power our grid modelling platform. Passionate about creating highly performant back-end services, you understand the technical challenges associated with building niche B2B products that handle complex computational workloads. In addition to seeking out people with a proactive, problem-solving attitude and an eye for detail, as a growing organisation, collegiality and empathy for our coworkers are a requisite. While we are a rapidly growing tech startup, we are also a non-profit, so a high level of adaptability and willingness to approach challenges with creativity and curiosity is essential.

About the role

We are looking for an experienced Lead Engineer (back-end) to lead the team building and scaling the infrastructure behind Scenario Builder, our electricity grid modelling platform. Scenario Builder is an in-browser platform that currently provides capacity expansion and dispatch modelling through an accessible no-code interface. The vision for Scenario Builder is to expand into power flow modelling to create a fully integrated grid modelling platform with the features and functionality needed to enable users to develop bankable scenarios that support energy investment decisions.

You will line-manage a small team of engineers while remaining a highly active individual contributor. You will own the technical shape of engineering work on the back-end team — leading planning, architecture, and code quality. You will work closely with the wider product and engineering team to design, develop, and maintain the back-end services that power our platform.

This hybrid role is based out of our London office near Waterloo Station and reports to the Head of Platform.

Responsibilities

Back-end development & architecture

• Design, develop, and maintain scalable back-end services and APIs using Python and modern frameworks (FastAPI).
• Design and build high-performance API endpoints to handle complex hierarchical data queries and multi-dimensional aggregations for grid modelling computations
• Ensure system reliability, performance, and security through best practices in code quality, testing, and monitoring.
• Collaborate with front-end engineers to design and implement RESTful APIs that support our web application.
• Collaborate with data engineers and domain experts on the evolving core data model

Infrastructure & DevOps

• Manage and optimise cloud infrastructure (GCP) to support our computational workloads.
• Implement and maintain CI/CD pipelines to enable rapid, reliable deployments.
• Monitor system health and performance, proactively identifying and resolving issues.
• Implement and maintain infrastructure-as-code

Technical leadership & collaboration

• Participate and lead in code reviews, architectural discussions, and technical planning sessions.
• Own technical planning: break down roadmap items into well-scoped engineering work, lead estimation, and drive architectural decision records (ADRs).
• Collaborate with cross-functional teams, including product, data engineering, and modelling, to deliver impactful features.
• Set and uphold the team's technical standards — code quality, testing strategy, API design — through direct example and code review.

People & line management

• Line-manage a team of 3–5 engineers: run regular 1:1s, support career development, and provide timely, constructive feedback.
• Partner with the Head of Platform on performance reviews and personal development plans.
• Act as the first point of escalation for day-to-day technical blockers

Skills and experience

Essential

• 7+ years of experience in back-end software development, with strong proficiency in Python.
• Experience designing and building RESTful APIs and microservices architectures.
• Strong understanding of database technologies (PostgreSQL, Redis, Firestore), ORMs and data modelling principles.
• Experience with cloud platforms (preferably GCP) and containerisation technologies (Docker).
• Solid understanding of software engineering best practices, including testing, code review, and documentation.
• Excellent problem-solving skills and ability to work independently on complex technical challenges.
• Experience line managing or formally mentoring engineers, with confidence running 1:1s and supporting career development.
• Understanding of OAuth and web security best practices.
• Experience with AGILE/Scrum based ways-of-working

Desirable

• Experience in the energy sector or with computational modelling/simulation software.
• Familiarity with scientific computing libraries (NumPy, Pandas) and optimisation frameworks.
• Experience with generative AI/LLMs in applied engineering contexts
• Experience with infrastructure-as-code tools (Terraform, Terragrunt) and GitOps practices.

About us

We are a climate-tech startup established in 2021. We provide electricity grid modelling software to accelerate energy planning in emerging markets and developing economies. Today’s tools are costly, closed, and hard to use, with modelling studies taking months or years when they should take weeks or days. We designed Scenario Builder to meet this challenge, so energy planners and investors can make better decisions, faster.

Our funding currently comes from a range of US and EU foundations, including the Quadrature Climate Foundation, Google.org, the Sequoia Climate Foundation, Bloomberg Philanthropies, and the European Climate Foundation.

At TransitionZero, we understand that diversity is an essential component of a successful team, whether that be diverse ways of thinking, personal or professional backgrounds, or skills. We aim to be the most talented non-profit in our vertical through our culture of ownership and adaptability.

Benefits

• Enhanced competitive pension scheme - auto-enrollment from day 1
• 25 days annual leave (excluding UK public holidays) and additional discretionary days off such as over the festive period
• 20 days of annual allowance to work from anywhere in the world
• Hybrid working and core working hours model
• Allowance to set up your home office
• Annual budget and dedicated leave time for relevant training courses
• Enhanced gender-neutral parental leave (16 weeks fully paid)
• Private healthcare following the successful completion of the probation period
• Yearly team offsites

Our commitment to diversity, equity, and inclusion

Studies have shown that some people from marginalised or underrepresented groups are less likely to apply to a role unless they meet all of the hiring guidelines or qualifications. Whoever you are, even if you don't meet all of the criteria, if you can demonstrate a variety of skills and experience relevant to this role, we encourage you to apply as you might just be the candidate we hire! At TransitionZero, we're looking for people who are genuinely passionate about what they do, and we welcome all people, regardless of their background.

If you would like to discuss any reasonable adjustments to the application or hiring process that may better facilitate your participation, please contact our People Team (people@transitionzero.org) for an informal chat. We will make every effort to respond to your request for assistance as soon as possible.

Pay: £87,000.00-£95,000.00 per year

Benefits:

• Private medical insurance

Work Location: Hybrid remote in Waterloo (Greater London, Greater London Region)