On.Energy · Software Engineer
Updated · 2026-10-02

On.Energy Software Engineer
Interview Guide

THE 60-SECOND BRIEF

As a Software Engineer at On.Energy, you play a pivotal role in designing, developing, and optimizing the digital infrastructure that powers advanced energy storage and management systems. Your work directly enables the integration of renewable energy sources, battery energy storage systems (BESS), and high-demand grid applications. You will build software solutions that monitor, control, and optimize complex physical energy assets, translating raw data into actionable grid reliability. The impact of this role extends across mission-critical energy infrastructure projects, influencing everything from real-time operational technology (OT) to automated energy management systems (EMS). You will collaborate closely with power systems engineers, controls teams, and field deployment specialists to ensure our software reliably governs hardware operating in high-stakes environments.

This guide is scoped to a Software Engineer candidate at On.Energy.

On.Energy candidates report 3 rounds over 3-5 weeks. The stages below are what candidates describe, not a published process.

RHEL (Red Hat Enterprise Linux)Energy Storage Systems (BESS)Energy Management Systems (EMS)

25 min read

Browse Software Engineer questions

See the practice prompts

27Practice promptsAcross five skill areas

As a Software Engineer at On.Energy, you play a pivotal role in designing, developing, and optimizing the digital infrastructure that powers advanced energy storage and management systems. Your work directly enables the integration of renewable energy sources, battery energy storage systems (BESS), and high-demand grid applications. You will build software solutions that monitor, control, and optimize complex physical energy assets, translating raw data into actionable grid reliability. The impact of this role extends across mission-critical energy infrastructure projects, influencing everything from real-time operational technology (OT) to automated energy management systems (EMS). You will collaborate closely with power systems engineers, controls teams, and field deployment specialists to ensure our software reliably governs hardware operating in high-stakes environments. The complexity lies in bridging low-level hardware interactions with high-level software scalability, demanding both rigorous engineering discipline and a strong grasp of energy domains. Working at On.Energy means tackling unique challenges at the intersection of software engineering and the modern energy transition. You can expect a fast-paced environment where your code directly impacts physical assets deployed globally. Success in this role requires a blend of resilient system design, robust programming practices, and a passion for sustainable technology solutions.

01

Recruiter Alignment

reported

Initial discussion with the recruiter to align on role expectations and candidate fit.

What to demonstrate

  • Initial discussion with the recruiter to align on role expectations and candidate fit
  • Depth in RHEL (Red Hat Enterprise Linux)

How to prepare

  • Be able to walk your CV end to end in two minutes, and say why this company specifically.
  • Have your salary expectations, notice period and location constraints ready, and ask for the rest of the loop in writing.
On.Energy Software Engineer candidate reports ↗
02

Technical Screenings

reported

Assessment of technical skills through coding challenges and problem-solving exercises.

What to demonstrate

  • Assessment of technical skills through coding challenges and problem-solving exercises
  • Depth in RHEL (Red Hat Enterprise Linux)

How to prepare

  • Answer aloud and timed: Explain how you handle thread safety and concurrency when managing multiple hardware control loops.
  • Answer aloud and timed: What strategies do you use to ensure fault tolerance in distributed control systems?
On.Energy Software Engineer candidate reports ↗
03

Onsite Interview

reported

Comprehensive interview rounds that evaluate software craftsmanship, system architecture, and collaboration.

What to demonstrate

  • Comprehensive interview rounds that evaluate software craftsmanship, system architecture, and collaboration
  • Depth in RHEL (Red Hat Enterprise Linux)

How to prepare

  • Answer aloud and timed: How do you approach debugging intermittent communication failures between edge devices and centralized cloud services?
  • Answer aloud and timed: Walk through an implementation of a state machine for managing operational modes in an energy storage asset.
On.Energy Software Engineer candidate reports ↗

PracHub editorial advice for the preparation topics above.

01

Emphasize reliability and edge cases

Because our software interacts with physical energy assets, always proactively discuss how your designs handle network failures, latency, and hardware exceptions.

02

Structure your system design answers

Start by clarifying requirements and constraints, outline high-level components, dive deep into data flows, and conclude by discussing bottlenecks and scaling strategies.

03

Connect software to impact

Frame your technical achievements around how they improve system uptime, data accuracy, or operational efficiency for renewable energy deployments.

04

Be collaborative in technical discussions

Treat the interviewers as future engineering peers; invite feedback, talk through alternative approaches, and demonstrate that you take constructive criticism well.

05

Prepare STAR-format behavioral examples

Have concrete stories ready that highlight your ownership, cross-functional collaboration, and how you navigated technical disagreements or project ambiguity.

Choose a category, try a prompt, then open its approach, worked solution or follow-up when you need it.

21 technical prompts0 include a worked solution

Explain how you handle thread safety and concurrency when managing multiple hardware control loops.

medium
Software & Controls Fundamentals

Explain how you handle thread safety and concurrency when managing multiple hardware control loops.

Approach
  1. Say what the runtime actually does before reasoning about the code.
  2. Name what is shared across threads and what owns each piece of state.
  3. Identify the window where an invariant is briefly untrue.
  4. Distinguish a value from a reference to it, and say which one you handed out.
Follow-up
  • What happens if two callers reach this at the same time?
  • Where could this allocate more than you expect?

Merge partitioned event streams into one ordered feed with bounded lateness

hard
k-way mergewatermarksout-of-order streams

The read-model service consumes 64 log partitions carrying about 4,000 events per second in total. Each partition is ordered within itself, but partitions drift by up to 30 seconds, and the activity feed must present a tenant's events in occurred_at order. Produce the merge. State its complexity, the buffer it requires in events and in bytes, what happens when one partition is idle, and what you do with an event that arrives after you have already emitted its position. Payloads average 1 KB.

Approach
  1. Merge with a min-heap over the 64 partition heads keyed on (occurred_at, event_id): O(log P) per event and O(n log P) overall. The tie-break on event_id is what makes the output deterministic when two partitions carry the same millisecond, which matters because the feed is paginated and a non-deterministic order reorders pages under the reader.
  2. Emitting the heap head is only correct once every partition has produced everything up to that timestamp, so the emit condition is a watermark: the minimum across partitions of the highest occurred_at seen, less the allowed lateness. Events are held until the watermark passes them, which is what turns individually ordered streams into a jointly ordered one.
  3. Size the buffer from the lateness rather than guessing: 4,000 events per second times 30 seconds is 120,000 buffered events, and at 1 KB each about 120 MB of heap. That number is the real price of the ordering guarantee and belongs in front of whoever asked for it.
  4. Handle the idle partition explicitly, because it fails the feed rather than corrupting it: a partition with no traffic never advances its own maximum, so the watermark freezes and output stops entirely. Either every partition emits a periodic idle marker carrying the broker's current time, or the watermark falls back to wall clock for a partition silent beyond a threshold.
Follow-up
  • The lateness budget is raised to five minutes. What is the new buffer, and what besides memory changes?
  • The consumer restarts. Where does it resume from, and what does the feed look like for the first 30 seconds?

Track a rolling failure rate per destination for circuit decisions

easy
sliding windowring buffercircuit breaker

The egress service delivers about 1,500 webhooks per second across roughly 40,000 destinations, each call bounded by a 10 second timeout. Maintain, per destination, the failure rate over the trailing 60 seconds so a caller can ask before dispatch whether the circuit should open. Attempts arrive as (destination_id, finished_at_ms, outcome). Requirement: amortised O(1) per attempt, with total memory bounded by the destination count rather than by traffic. Give the structure, its exact memory, and the rule that stops a destination with three attempts from opening a circuit.

Approach
  1. Name the exact-deque version and then reject it as the default. Holding timestamps and advancing a tail pointer past anything older than now minus 60 seconds is a correct two-pointer window at amortised O(1) per attempt, but its memory tracks in-window traffic, so one destination in a retry storm holds hundreds of thousands of entries while thousands of quiet destinations hold none.
  2. Use a ring of 60 one-second buckets per destination, each bucket a pair of counters for attempts and failures. On an attempt, advance the ring by the elapsed whole seconds, zeroing at most min(elapsed, 60) buckets, then increment the head. That is amortised O(1) with a fixed footprint per destination.
  3. State the footprint: 60 buckets times two 4-byte counters is 480 bytes of payload per destination, so 40,000 destinations is roughly 20 to 25 MB with per-entry overhead, bounded by the catalogue rather than by the rate. The cost is granularity, since the oldest bucket ages out in whole seconds, which is far tighter than the decision needs.
  4. Require a minimum sample before the circuit may open. A destination with three attempts and three failures reads as 100 percent and is not evidence; a floor of roughly 20 attempts in the window makes the ratio meaningful, and below that floor use a run of consecutive failures as the trigger instead.
Follow-up
  • The fleet is 30 instances and each sees roughly a thirtieth of a destination's traffic. Where does the rate actually live, and what does a per-instance answer get wrong?
  • A destination answers in 9.5 seconds and succeeds. It is not failing but it is consuming your per-destination concurrency. What signal should open the circuit here?

Built from the rounds and topics On.Energy candidates report.

Small steps. Visible outcomes.0 / 7 completed
ONE WEEK · YOUR PACE

Prepare, practise & reflect

One practical outcome each day. Spend longer where you need it.

0 / 7 done
01Map the On.Energy loop
  • Write out the reported sequence: Recruiter Alignment, Technical Screenings, Onsite Interview.
  • For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.

Deliverable: A one-page map of the 3 reported rounds, with the weakest marked.

02Work RHEL (Red Hat Enterprise Linux)
  • Spend the session on RHEL (Red Hat Enterprise Linux), which On.Energy candidates report being tested on.
  • Write one worked example in RHEL (Red Hat Enterprise Linux) and time yourself on it.

Deliverable: One timed worked example in RHEL (Red Hat Enterprise Linux).

03Work Energy Storage Systems (BESS)
  • Spend the session on Energy Storage Systems (BESS), which On.Energy candidates report being tested on.
  • Write one worked example in Energy Storage Systems (BESS) and time yourself on it.

Deliverable: One timed worked example in Energy Storage Systems (BESS).

04Work Energy Management Systems (EMS)
  • Spend the session on Energy Management Systems (EMS), which On.Energy candidates report being tested on.
  • Write one worked example in Energy Management Systems (EMS) and time yourself on it.

Deliverable: One timed worked example in Energy Management Systems (EMS).

05Answer out loud: Software & Controls Fundamentals
  • Answer aloud, timed: This category tests your core programming knowledge and your ability to interface software logic with physical control systems.
  • Answer aloud, timed: How would you design a real-time data ingestion pipeline for high-frequency telemetry from battery management systems?

Deliverable: Spoken answers to 2 reported Software & Controls Fundamentals question(s), under time.

06Answer out loud: System Architecture & Scalability
  • Answer aloud, timed: This category evaluates your ability to design robust, scalable software architectures that integrate with industrial automation and cloud platforms.
  • Answer aloud, timed: How would you architect a scalable telemetry database to store multi-year operational logs for numerous remote sites?

Deliverable: Spoken answers to 2 reported System Architecture & Scalability question(s), under time.

07Answer out loud: Reliability & Testing
  • Answer aloud, timed: This category examines your commitment to code quality, automated testing, and mission-critical deployment standards.
  • Answer aloud, timed: How do you simulate physical hardware environments in automated integration testing pipelines?

Deliverable: Spoken answers to 2 reported Reliability & Testing question(s), under time.

Expand any day for tasks and deliverables. Your progress is saved on this device.

Behavioural rounds judge the decision you made and what it cost.

Describe a time when a software deployment failed in production and how you mitigated and resolved it.

medium
Reliability & Testing

Describe a time when a software deployment failed in production and how you mitigated and resolved it.

Approach
  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.
Follow-up
  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Explain your approach to code reviews and maintaining maintainable codebases across growing engineering teams.

medium
Reliability & Testing

Explain your approach to code reviews and maintaining maintainable codebases across growing engineering teams.

Approach
  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.
Follow-up
  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

This category focuses on your communication skills, teamwork, and alignment with our mission-driven culture.

medium
Behavioral & Cross-Functional Collaborat

This category focuses on your communication skills, teamwork, and alignment with our mission-driven culture.

Approach
  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.
Follow-up
  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Tell me about a time you had to explain complex technical constraints to a non-technical stakeholder or projec

medium
Behavioral & Cross-Functional Collaborat

Tell me about a time you had to explain complex technical constraints to a non-technical stakeholder or project manager.

Approach
  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.
Follow-up
  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

Describe a situation where you disagreed with a peer on a technical design choice. How did you resolve it?

medium
Behavioral & Cross-Functional Collaborat

Describe a situation where you disagreed with a peer on a technical design choice. How did you resolve it?

Approach
  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.
Follow-up
  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?

What draws you to the energy technology space, and why do you want to build your career at On.Energy?

medium
Behavioral & Cross-Functional Collaborat

What draws you to the energy technology space, and why do you want to build your career at On.Energy?

Approach
  1. Pick a story where you made the decision, not one where you watched it.
  2. State the situation in two sentences and spend the rest on the reasoning.
  3. Give the blast radius: what could have broken, and what you measured.
  4. Name the disagreement and how you resolved it with evidence.
Follow-up
  • What would you do differently if you ran that again?
  • How did you know your change caused the improvement?
  • 01

    Describe a time when a software deployment failed in production and how you mitigated and resolved it.

  • 02

    Explain your approach to code reviews and maintaining maintainable codebases across growing engineering teams.

  • 03

    This category focuses on your communication skills, teamwork, and alignment with our mission-driven culture.

  • 04

    Tell me about a time you had to explain complex technical constraints to a non-technical stakeholder or project manager.

PracHub preparation framework ↗
How technical is the interview process for a Software Engineer at On.Energy?

The process is deeply technical, focusing on your system design capabilities, coding proficiency, and understanding of distributed or concurrent systems. Expect rigorous discussions around real-world architectural challenges and hands-on coding evaluations.

On.Energy Software Engineer candidate reports ↗
Do I need prior experience in the energy industry to succeed?

While prior experience in energy storage, IoT, or industrial automation is a strong asset, it is not strictly required. We hire talented software engineers with strong fundamentals who demonstrate a fast learning curve and a genuine passion for the energy transition.

On.Energy Software Engineer candidate reports ↗
What is the typical interview timeline from initial screen to final offer?

The process generally moves at an efficient pace, typically spanning three to four weeks from your initial recruiter conversation through the final onsite loop, depending on scheduling alignment.

On.Energy Software Engineer candidate reports ↗
Are the engineering teams distributed or centralized?

Our engineering organization embraces collaborative working models, balancing remote flexibility with close-knit regional hubs depending on the specific team and project requirements.

On.Energy Software Engineer candidate reports ↗
What is the best way to stand out during the interview loop?

Successful candidates distinguish themselves by asking insightful questions about system constraints, proactively discussing trade-offs in their design answers, and showing a deep sense of ownership for code quality and reliability.

On.Energy Software Engineer candidate reports ↗
What topics does On.Energy test in interviews?

On.Energy interviews most often cover OT/SCADA Systems, Stakeholder Management, Industrial Automation, Change Management, and Project Management. The exact emphasis depends on the specific role you apply for.

On.Energy Software Engineer candidate reports ↗
Sources & methodology 3 sources ↗

No official company page is cited. Rounds and questions come from candidate reports and PracHub editorial material; each source shows the date it was read.