Principal Software Engineer

Location: London

Salary band: £88,000

Role: Permanent, full-time

About yo

uYou 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 rol

e We are looking for an experienced Principal Back-End Engineer to lead the team building and scaling the infrastructure behind Scenario Builder, our electricity grid modelling platfor

m.
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 decisio

ns.
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 platf

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

form.
Responsibil

ities
Back-end development & archi

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

model.
Infrastructure &

• ; DevOpsManage and optimise cloud infrastructure (GCP) to support our computational wo
• rkloads.Implement and maintain CI/CD pipelines to enable rapid, reliable depl
• oyments.Monitor system health and performance, proactively identifying and resolving
• issues.Implement and maintain infrastructure

-as-code
Technical leadership & coll

• aborationParticipate 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 record
• s (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 cod

e review.
People & line

• managementLine-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 developm
• ent plans.Act as the first point of escalation for day-to-day technica

l blockers
Skills and

experienc

• e
  Essential7+ years of experience in back-end software development, with strong proficienc
• y in Python.Experience designing and building RESTful APIs and microservices ar
• chitectures.Strong understanding of database technologies (PostgreSQL, Redis, Firestore), ORMs and data modelling
• principles.Experience with cloud platforms (preferably GCP) and containerisation technologi
• es (Docker).Solid understanding of software engineering best practices, including testing, code review, and do
• cumentation.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 bes
• t practices.Experience with AGILE/Scrum based way

s-of-work

• ing
  
  DesirableExperience in the energy sector or with computational modelling/simula
• tion software.Familiarity with scientific computing libraries (NumPy, Pandas) and optimisati
• on frameworks.Experience with generative AI/LLMs in applied engine
• ering contextsExperience with infrastructure-as-code tools (Terraform, Terragrunt) and Git

Ops prac

tices.
About usWe 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 dec

isions, 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 Cli

mate 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 adap

• tability.
  BenefitsEnhanced competitive pension scheme - auto-enr
• ollment from day 125 days annual leave (excluding UK public holidays) and additional discretionary days off such as over
• the festive period20 days of annual allowance to work from any
• where in the worldHybrid working and core w
• orking hours modelAllowance to set u
• p your home officeAnnual budget and dedicated leave time for relevan
• t training coursesEnhanced gender-neutral parental leave (16
• weeks fully paid)Private healthcare following the successful completion of th
• e probation periodYe

arly team offsites
Our commitment to diversity, eq

uity, and inclusionStudies 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 o

f 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