diff --git a/.gitignore b/.gitignore
index 8286174..4f26831 100644
--- a/.gitignore
+++ b/.gitignore
@@ -32,3 +32,6 @@ home-assistant_v2.db*
 *.temp
 .DS_Store
 Thumbs.db
+
+# Planning documents (work-in-progress)
+planning/
diff --git a/AGENTS.md b/AGENTS.md
index 15a803d..531e8bb 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -105,6 +105,129 @@ If you detect 3+ related minor changes (e.g., multiple constant renames during r
 
 This ensures the documentation stays accurate and useful as the codebase evolves, while maintaining user control over what gets documented.
 
+## Planning Major Refactorings
+
+**Purpose**: Large-scale architectural changes require careful planning before implementation.
+
+**Planning Directory**: `/planning/` (git-ignored, safe for iteration)
+
+**When to Create a Planning Document:**
+
+Create a detailed plan when:
+
+-   🔴 **Major refactoring** (>5 files, >500 lines changed)
+-   🔴 **Architectural changes** (new packages, module restructuring)
+-   🔴 **Breaking changes** (API changes, config format migrations)
+-   🟡 **Complex features** (multiple moving parts, unclear best approach)
+
+Skip planning for:
+
+-   🟢 Bug fixes (straightforward, <100 lines)
+-   🟢 Small features (<3 files, clear approach)
+-   🟢 Documentation updates
+-   🟢 Cosmetic changes (formatting, renaming)
+
+**Planning Document Lifecycle:**
+
+1. **Planning Phase** (WIP in `/planning/`)
+
+    - Create `planning/<feature>-refactoring-plan.md`
+    - Iterate freely (git-ignored, no commit pressure)
+    - AI can help refine without polluting git history
+    - Multiple revisions until plan is solid
+
+2. **Implementation Phase** (Active work)
+
+    - Use plan as reference during coding
+    - Update plan if issues discovered
+    - Track progress through phases
+    - Test after each phase
+
+3. **Completion Phase** (After implementation)
+
+    - **Option A**: Move to `docs/development/` if lasting value
+
+        - Example: `planning/module-splitting-plan.md` → `docs/development/module-splitting-plan.md`
+        - Update status to "✅ COMPLETED"
+        - Commit as historical reference
+
+    - **Option B**: Delete if superseded
+
+        - Plan served its purpose
+        - Code and AGENTS.md are source of truth
+
+    - **Option C**: Archive in `planning/archive/`
+        - Keep locally for "why we didn't do X" reference
+        - Don't commit (git-ignored)
+
+**Required Planning Document Sections:**
+
+```markdown
+# <Feature> Refactoring Plan
+
+**Status**: 🔄 PLANNING | 🚧 IN PROGRESS | ✅ COMPLETED | ❌ CANCELLED
+**Created**: YYYY-MM-DD
+**Last Updated**: YYYY-MM-DD
+
+## Problem Statement
+
+-   What's the issue?
+-   Why does it need fixing?
+-   Current pain points
+
+## Proposed Solution
+
+-   High-level approach
+-   File structure (before/after)
+-   Module responsibilities
+
+## Migration Strategy
+
+-   Phase-by-phase breakdown
+-   File lifecycle (CREATE/MODIFY/DELETE/RENAME)
+-   Dependencies between phases
+-   Testing checkpoints
+
+## Risks & Mitigation
+
+-   What could go wrong?
+-   How to prevent it?
+-   Rollback strategy
+
+## Success Criteria
+
+-   Measurable improvements
+-   Testing requirements
+-   Verification steps
+```
+
+**Example**: See `docs/development/module-splitting-plan.md` for a completed plan (moved from `planning/` after successful implementation).
+
+**Integration with AGENTS.md:**
+
+After successful refactoring:
+
+1. Update AGENTS.md with new patterns/conventions
+2. Move plan to `docs/development/` if valuable for future reference
+3. Planning doc is temporary scaffolding; AGENTS.md is permanent guide
+
+**Best Practices:**
+
+✅ **DO:**
+
+-   Iterate freely in `/planning/` (git-ignored)
+-   Break complex changes into clear phases
+-   Document file lifecycle explicitly
+-   Include code examples and patterns
+-   Plan testing after each phase
+
+❌ **DON'T:**
+
+-   Start coding before plan is solid
+-   Skip the "Why?" section
+-   Commit `/planning/` files (they're ignored!)
+-   Over-plan trivial changes
+
 ## Architecture Overview
 
 **Core Data Flow:**
diff --git a/docs/development/README.md b/docs/development/README.md
index 2b830d4..c087a60 100644
--- a/docs/development/README.md
+++ b/docs/development/README.md
@@ -9,6 +9,7 @@ This section contains documentation for contributors and maintainers of the Tibb
 -   **[Testing](testing.md)** - How to run tests and write new test cases
 -   **[Release Management](release-management.md)** - Release workflow and versioning process
 -   **[Coding Guidelines](coding-guidelines.md)** - Style guide, linting, and best practices
+-   **[Refactoring Guide](refactoring-guide.md)** - How to plan and execute major refactorings
 
 ## 🤖 AI Documentation
 
diff --git a/docs/development/refactoring-guide.md b/docs/development/refactoring-guide.md
new file mode 100644
index 0000000..c46ec04
--- /dev/null
+++ b/docs/development/refactoring-guide.md
@@ -0,0 +1,414 @@
+# Refactoring Guide
+
+This guide explains how to plan and execute major refactorings in this project.
+
+## When to Plan a Refactoring
+
+Not every code change needs a detailed plan. Create a refactoring plan when:
+
+🔴 **Major changes requiring planning:**
+
+-   Splitting modules into packages (>5 files affected, >500 lines moved)
+-   Architectural changes (new packages, module restructuring)
+-   Breaking changes (API changes, config format migrations)
+
+🟡 **Medium changes that might benefit from planning:**
+
+-   Complex features with multiple moving parts
+-   Changes affecting many files (>3 files, unclear best approach)
+-   Refactorings with unclear scope
+
+🟢 **Small changes - no planning needed:**
+
+-   Bug fixes (straightforward, <100 lines)
+-   Small features (<3 files, clear approach)
+-   Documentation updates
+-   Cosmetic changes (formatting, renaming)
+
+## The Planning Process
+
+### 1. Create a Planning Document
+
+Create a file in the `planning/` directory (git-ignored for free iteration):
+
+```bash
+# Example:
+touch planning/my-feature-refactoring-plan.md
+```
+
+**Note:** The `planning/` directory is git-ignored, so you can iterate freely without polluting git history.
+
+### 2. Use the Planning Template
+
+Every planning document should include:
+
+```markdown
+# <Feature> Refactoring Plan
+
+**Status**: 🔄 PLANNING | 🚧 IN PROGRESS | ✅ COMPLETED | ❌ CANCELLED
+**Created**: YYYY-MM-DD
+**Last Updated**: YYYY-MM-DD
+
+## Problem Statement
+
+-   What's the issue?
+-   Why does it need fixing?
+-   Current pain points
+
+## Proposed Solution
+
+-   High-level approach
+-   File structure (before/after)
+-   Module responsibilities
+
+## Migration Strategy
+
+-   Phase-by-phase breakdown
+-   File lifecycle (CREATE/MODIFY/DELETE/RENAME)
+-   Dependencies between phases
+-   Testing checkpoints
+
+## Risks & Mitigation
+
+-   What could go wrong?
+-   How to prevent it?
+-   Rollback strategy
+
+## Success Criteria
+
+-   Measurable improvements
+-   Testing requirements
+-   Verification steps
+```
+
+See `planning/README.md` for detailed template explanation.
+
+### 3. Iterate Freely
+
+Since `planning/` is git-ignored:
+
+-   Draft multiple versions
+-   Get AI assistance without commit pressure
+-   Refine until the plan is solid
+-   No need to clean up intermediate versions
+
+### 4. Implementation Phase
+
+Once plan is approved:
+
+-   Follow the phases defined in the plan
+-   Test after each phase (don't skip!)
+-   Update plan if issues discovered
+-   Track progress through phase status
+
+### 5. After Completion
+
+**Option A: Archive in docs/development/**
+If the plan has lasting value (successful pattern, reusable approach):
+
+```bash
+mv planning/my-feature-refactoring-plan.md docs/development/
+git add docs/development/my-feature-refactoring-plan.md
+git commit -m "docs: archive successful refactoring plan"
+```
+
+**Option B: Delete**
+If the plan served its purpose and code is the source of truth:
+
+```bash
+rm planning/my-feature-refactoring-plan.md
+```
+
+**Option C: Keep locally (not committed)**
+For "why we didn't do X" reference:
+
+```bash
+mkdir -p planning/archive
+mv planning/my-feature-refactoring-plan.md planning/archive/
+# Still git-ignored, just organized
+```
+
+## Real-World Example
+
+The **sensor/ package refactoring** (Nov 2025) is a successful example:
+
+**Before:**
+
+-   `sensor.py` - 2,574 lines, hard to navigate
+
+**After:**
+
+-   `sensor/` package with 5 focused modules
+-   Each module <800 lines
+-   Clear separation of concerns
+
+**Process:**
+
+1. Created `planning/module-splitting-plan.md` (now in `docs/development/`)
+2. Defined 6 phases with clear file lifecycle
+3. Implemented phase by phase
+4. Tested after each phase
+5. Documented in AGENTS.md
+6. Moved plan to `docs/development/` as reference
+
+**Key learnings:**
+
+-   Temporary `_impl.py` files avoid Python package conflicts
+-   Test after EVERY phase (don't accumulate changes)
+-   Clear file lifecycle (CREATE/MODIFY/DELETE/RENAME)
+-   Phase-by-phase approach enables safe rollback
+
+See the complete plan: [module-splitting-plan.md](module-splitting-plan.md)
+
+## Phase-by-Phase Implementation
+
+### Why Phases Matter
+
+Breaking refactorings into phases:
+
+-   ✅ Enables testing after each change (catch bugs early)
+-   ✅ Allows rollback to last good state
+-   ✅ Makes progress visible
+-   ✅ Reduces cognitive load (focus on one thing)
+-   ❌ Takes more time (but worth it!)
+
+### Phase Structure
+
+Each phase should:
+
+1. **Have clear goal** - What's being changed?
+2. **Document file lifecycle** - CREATE/MODIFY/DELETE/RENAME
+3. **Define success criteria** - How to verify it worked?
+4. **Include testing steps** - What to test?
+5. **Estimate time** - Realistic time budget
+
+### Example Phase Documentation
+
+```markdown
+### Phase 3: Extract Helper Functions (Session 3)
+
+**Goal**: Move pure utility functions to helpers.py
+
+**File Lifecycle**:
+
+-   ✨ CREATE `sensor/helpers.py` (utility functions)
+-   ✏️ MODIFY `sensor/core.py` (import from helpers.py)
+
+**Steps**:
+
+1. Create sensor/helpers.py
+2. Move pure functions (no state, no self)
+3. Add comprehensive docstrings
+4. Update imports in core.py
+
+**Estimated time**: 45 minutes
+
+**Success criteria**:
+
+-   ✅ All pure functions moved
+-   ✅ `./scripts/lint-check` passes
+-   ✅ HA starts successfully
+-   ✅ All entities work correctly
+```
+
+## Testing Strategy
+
+### After Each Phase
+
+Minimum testing checklist:
+
+```bash
+# 1. Linting passes
+./scripts/lint-check
+
+# 2. Home Assistant starts
+./scripts/develop
+# Watch for startup errors in logs
+
+# 3. Integration loads
+# Check: Settings → Devices & Services → Tibber Prices
+# Verify: All entities appear
+
+# 4. Basic functionality
+# Test: Data updates without errors
+# Check: Entity states update correctly
+```
+
+### Comprehensive Testing (Final Phase)
+
+After completing all phases:
+
+-   Test all entities (sensors, binary sensors)
+-   Test configuration flow (add/modify/remove)
+-   Test options flow (change settings)
+-   Test services (custom service calls)
+-   Test error handling (disconnect API, invalid data)
+-   Test caching (restart HA, verify cache loads)
+-   Test time-based updates (quarter-hour refresh)
+
+## Common Pitfalls
+
+### ❌ Skip Planning for Large Changes
+
+**Problem:** "This seems straightforward, I'll just start coding..."
+
+**Result:** Halfway through, realize the approach doesn't work. Wasted time.
+
+**Solution:** If unsure, spend 30 minutes on a rough plan. Better to plan and discard than get stuck.
+
+### ❌ Implement All Phases at Once
+
+**Problem:** "I'll do all phases, then test everything..."
+
+**Result:** 10+ files changed, 2000+ lines modified, hard to debug if something breaks.
+
+**Solution:** Test after EVERY phase. Commit after each successful phase.
+
+### ❌ Forget to Update Documentation
+
+**Problem:** Code is refactored, but AGENTS.md and docs/ still reference old structure.
+
+**Result:** AI/humans get confused by outdated documentation.
+
+**Solution:** Include "Documentation Phase" at the end of every refactoring plan.
+
+### ❌ Ignore the Planning Directory
+
+**Problem:** "I'll just create the plan in docs/ directly..."
+
+**Result:** Git history polluted with draft iterations, or pressure to "commit something" too early.
+
+**Solution:** Always use `planning/` for work-in-progress. Move to `docs/` only when done.
+
+## Integration with AI Development
+
+This project uses AI heavily (GitHub Copilot, Claude). The planning process supports AI development:
+
+**AI reads from:**
+
+-   `AGENTS.md` - Long-term memory, patterns, conventions (AI-focused)
+-   `docs/development/` - Human-readable guides (human-focused)
+-   `planning/` - Active refactoring plans (shared context)
+
+**AI updates:**
+
+-   `AGENTS.md` - When patterns change
+-   `planning/*.md` - During refactoring implementation
+-   `docs/development/` - After successful completion
+
+**Why separate AGENTS.md and docs/development/?**
+
+-   `AGENTS.md`: Technical, comprehensive, AI-optimized
+-   `docs/development/`: Practical, focused, human-optimized
+-   Both stay in sync but serve different audiences
+
+See [AGENTS.md](../../AGENTS.md) section "Planning Major Refactorings" for AI-specific guidance.
+
+## Tools and Resources
+
+### Planning Directory
+
+-   `planning/` - Git-ignored workspace for drafts
+-   `planning/README.md` - Detailed planning documentation
+-   `planning/*.md` - Active refactoring plans
+
+### Example Plans
+
+-   `docs/development/module-splitting-plan.md` - ✅ Completed, archived
+-   `planning/config-flow-refactoring-plan.md` - 🔄 Planned (1013 lines → 4 modules)
+-   `planning/binary-sensor-refactoring-plan.md` - 🔄 Planned (644 lines → 4 modules)
+-   `planning/coordinator-refactoring-plan.md` - 🔄 Planned (1446 lines, high complexity)
+
+### Helper Scripts
+
+```bash
+./scripts/lint-check   # Verify code quality
+./scripts/develop      # Start HA for testing
+./scripts/lint         # Auto-fix issues
+```
+
+## FAQ
+
+### Q: When should I create a plan vs. just start coding?
+
+**A:** If you're asking this question, you probably need a plan. 😊
+
+Simple rule: If you can't describe the entire change in 3 sentences, create a plan.
+
+### Q: How detailed should the plan be?
+
+**A:** Detailed enough to execute without major surprises, but not a line-by-line script.
+
+Good plan level:
+
+-   Lists all files affected (CREATE/MODIFY/DELETE)
+-   Defines phases with clear boundaries
+-   Includes testing strategy
+-   Estimates time per phase
+
+Too detailed:
+
+-   Exact code snippets for every change
+-   Line-by-line instructions
+
+Too vague:
+
+-   "Refactor sensor.py to be better"
+-   No phase breakdown
+-   No testing strategy
+
+### Q: What if the plan changes during implementation?
+
+**A:** Update the plan! Planning documents are living documents.
+
+If you discover:
+
+-   Better approach → Update "Proposed Solution"
+-   More phases needed → Add to "Migration Strategy"
+-   New risks → Update "Risks & Mitigation"
+
+Document WHY the plan changed (helps future refactorings).
+
+### Q: Should every refactoring follow this process?
+
+**A:** No! Use judgment:
+
+-   **Small changes (<100 lines, clear approach)**: Just do it, no plan needed
+-   **Medium changes (unclear scope)**: Write rough outline, refine if needed
+-   **Large changes (>500 lines, >5 files)**: Full planning process
+
+### Q: How do I know when a refactoring is successful?
+
+**A:** Check the "Success Criteria" from your plan:
+
+Typical criteria:
+
+-   ✅ All linting checks pass
+-   ✅ HA starts without errors
+-   ✅ All entities functional
+-   ✅ No regressions (existing features work)
+-   ✅ Code easier to understand/modify
+-   ✅ Documentation updated
+
+If you can't tick all boxes, the refactoring isn't done.
+
+## Summary
+
+**Key takeaways:**
+
+1. **Plan when scope is unclear** (>500 lines, >5 files, breaking changes)
+2. **Use planning/ directory** for free iteration (git-ignored)
+3. **Work in phases** and test after each phase
+4. **Document file lifecycle** (CREATE/MODIFY/DELETE/RENAME)
+5. **Update documentation** after completion (AGENTS.md, docs/)
+6. **Archive or delete** plan after implementation
+
+**Remember:** Good planning prevents half-finished refactorings and makes rollback easier when things go wrong.
+
+---
+
+**Next steps:**
+
+-   Read `planning/README.md` for detailed template
+-   Check `docs/development/module-splitting-plan.md` for real example
+-   Browse `planning/` for active refactoring plans
diff --git a/planning/README.md b/planning/README.md
new file mode 100644
index 0000000..acd5fb9
--- /dev/null
+++ b/planning/README.md
@@ -0,0 +1,163 @@
+# Planning Directory
+
+**Purpose**: Work-in-progress planning documents for major refactorings and architectural changes.
+
+**Status**: 🚫 **GIT-IGNORED** (files in this directory are NOT committed to the repository)
+
+---
+
+## Why This Directory Exists
+
+Large-scale refactorings require careful planning before implementation. This directory provides a space for:
+
+1. **Detailed planning documents** - Breaking down complex changes into phases
+2. **Architecture proposals** - Evaluating different approaches
+3. **Migration strategies** - Step-by-step transformation plans
+4. **Risk analysis** - Identifying potential issues before coding
+
+## Directory Structure
+
+```
+planning/
+├── README.md                           # This file
+├── <feature>-refactoring-plan.md       # Active planning documents
+└── archive/                            # Completed plans (optional)
+```
+
+## Document Lifecycle
+
+### 1. **Planning Phase** (WIP, in planning/)
+
+-   Document created in `planning/`
+-   Iterative refinement with AI assistance
+-   Multiple revisions until plan is solid
+-   **NOT committed to git** (allows messy iteration)
+
+### 2. **Review Phase** (Ready for implementation)
+
+-   Plan is reviewed and finalized
+-   Decision: Implement or Archive
+
+### 3. **Implementation Phase** (Active work)
+
+-   Plan used as reference during implementation
+-   Can be updated as issues are discovered
+-   Still in `planning/` until complete
+
+### 4. **Completion Phase** (Done)
+
+-   **Option A**: Move to `docs/development/` as historical reference
+
+    -   Rename: `planning/config-flow-refactoring-plan.md` → `docs/development/config-flow-refactoring.md`
+    -   Update status to "✅ COMPLETED"
+    -   Commit to repository as documentation
+
+-   **Option B**: Delete if superseded by code/docs
+
+    -   Plan served its purpose
+    -   Code and AGENTS.md are the source of truth
+
+-   **Option C**: Archive in `planning/archive/`
+    -   Keep for reference but don't commit
+    -   Useful for "why we didn't do X" decisions
+
+## Planning Document Template
+
+See `docs/development/module-splitting-plan.md` for a successful example of a completed plan that was moved to documentation.
+
+### Required Sections
+
+1. **Status & Metadata**
+
+    ```markdown
+    **Status**: 🔄 PLANNING | 🚧 IN PROGRESS | ✅ COMPLETED | ❌ CANCELLED
+    **Created**: YYYY-MM-DD
+    **Last Updated**: YYYY-MM-DD
+    ```
+
+2. **Problem Statement**
+
+    - What's the issue?
+    - Why does it need fixing?
+    - Current pain points
+
+3. **Proposed Solution**
+
+    - High-level approach
+    - File structure (before/after)
+    - Module responsibilities
+
+4. **Migration Strategy**
+
+    - Phase-by-phase breakdown
+    - Dependencies between phases
+    - Testing checkpoints
+
+5. **Risks & Mitigation**
+
+    - What could go wrong?
+    - How to prevent it?
+    - Rollback strategy
+
+6. **Success Criteria**
+    - How do we know it worked?
+    - Metrics to measure
+    - Testing requirements
+
+## Active Plans
+
+<!-- Update this list as plans are created/completed -->
+
+-   None (all previous plans completed and moved to docs/)
+
+## Best Practices
+
+### ✅ DO:
+
+-   Iterate freely - planning/ is git-ignored for a reason
+-   Break complex changes into clear phases
+-   Document file lifecycle (CREATE/MODIFY/DELETE/RENAME)
+-   Include code examples and patterns
+-   Estimate time per phase
+-   Plan testing after each phase
+
+### ❌ DON'T:
+
+-   Start coding before plan is solid
+-   Skip the "Why?" section
+-   Forget to update status as you progress
+-   Commit planning/ files to git (they're ignored!)
+-   Over-plan simple changes (some things don't need a plan)
+
+## When to Create a Planning Document
+
+**Create a plan when:**
+
+-   🔴 Major refactoring (>5 files, >500 lines changed)
+-   🔴 Architectural changes (new packages, restructuring)
+-   🔴 Breaking changes (API changes, config format changes)
+-   🟡 Complex features (multiple moving parts, unclear approach)
+
+**Skip planning for:**
+
+-   🟢 Bug fixes (straightforward)
+-   🟢 Small features (<3 files, clear approach)
+-   🟢 Documentation updates
+-   🟢 Cosmetic changes (formatting, renaming)
+
+## Integration with AGENTS.md
+
+When planning is complete and implementation successful:
+
+1. Update `AGENTS.md` with new patterns/conventions
+2. Move plan to `docs/development/` if it has lasting value
+3. Delete or archive if superseded by docs
+
+The planning document and `AGENTS.md` serve different purposes:
+
+-   **Planning doc**: Temporary guide DURING transformation
+-   **AGENTS.md**: Permanent guide AFTER transformation
+
+---
+
+**Remember**: Planning documents are throwaway scaffolding. The real documentation lives in `AGENTS.md` and `docs/`.