Senior Backend Developer

Fifty Energy AB; Stockholms kommun; On-site; SEK 549,028 - 768,639; Full time

Job summary

A forward-thinking energy tech company is seeking a Senior Backend Developer to build high-impact systems and influence architecture. The successful candidate will develop scalable backend services in Python and work with cloud-native architectures. Applicants should have strong Python experience and a solid understanding of backend systems. This role offers the chance to be part of an ambitious engineering team focused on energy optimization and innovation. The position is based in Stockholm and requires a valid work permit for Sweden.

Qualifications

• Strong experience with Python in production-grade systems.
• Deep understanding of backend architecture, distributed systems, and API design.
• Experience with cloud environments (preferably Kubernetes-based).
• A structured, proactive way of working.

Responsibilities

• Building scalable backend services in Python for the MMO engine.
• Developing cloud-native systems running on Kubernetes.
• Designing and improving APIs for various applications.
• Working closely with cross-functional teams to deliver features.

Skills

Python

Backend architecture

Distributed systems

API design

Cloud environments (Kubernetes)

Tools

Kubernetes

TimescaleDB/Postgres

Job description

Senior Backend Developer – Join Fifty and Shape the Future of Energy Optimization

Company: Fifty AB

Fifty is building the next generation of energy optimization technology. As markets shift, electrification accelerates, and flexibility becomes critical, we are developing an intelligent platform that helps businesses unlock the full value of their energy assets — batteries, EV charging, solar, heat pumps, and more.

We are now looking for a Senior Backend Developer who wants to build high-impact systems, influence architecture, and be part of a fast-moving team pushing the boundaries of what’s possible in the energy space.

What you will be working on

Building scalable backend services in Python that power our Multi-Market Optimization (MMO) engine.

Developing cloud-native systems running on Kubernetes and TimescaleDB/Postgres.

Designing and improving APIs used for forecasting, optimization, real-time control, and data ingestion.

Working closely with data scientists, product, and platform engineers to ship features that matter.

Taking ownership of architecture decisions and guiding the evolution of our core platform.

Ensuring reliability, performance, and security as we scale.

What we are looking for

Strong experience with Python in production-grade systems.

Deep understanding of backend architecture, distributed systems, and API design.

Experience with cloud environments (preferably Kubernetes-based).

A structured, proactive way of working. You take responsibility and drive things forward.

Curiosity, pragmatism, and a willingness to challenge assumptions.

Experience in energy tech, optimization, forecasting, or numerical computing is a plus but not required.

Work on technology that contributes to a greener, more flexible energy system.

Be part of a tight, ambitious engineering team where your voice matters.

Freedom to influence architecture, tooling, and workflows.

Build something from the ground up

A culture of trust, ownership, and straightforward communication.

Requirements

Valid work permit for Sweden or EU citizenship.

Ability to work on-site regularly with the team (Åre or Stockholm).