Sunsynk Not Charging on Cheap Rate — Octopus, Agile, Go and TOU Schedule Fixes
Battery should be charging overnight on Octopus Go/Agile but it's not. This is the most common Sunsynk configuration failure in the UK. BST/GMT clock offset and TOU schedule mismatch account for >80% of cases. We cover every tariff and every fix.
This diagnostic guide walks through every setting. Most cases are resolved by Step 2 — the BST/GMT clock offset.
Book Remote Diagnostic — from £75 → Octopus tariff guide →We configure Sunsynk for all major UK tariffs remotely in a single session.
⚡ Safety Warning
Do not open your inverter or interfere with DC cabling. Solar panels produce live DC voltage whenever exposed to light. Always use your DC isolator switch and contact a qualified solar engineer for hands-on fault diagnosis.
6-step diagnostic process
Work through these steps in order. Most cases are resolved by Step 2.
Confirm your cheap-rate tariff window from your energy bill
Check your energy supplier's portal or your latest bill for the exact cheap-rate hours. Common windows: Octopus Go 00:30–04:30 (GMT/BST) · Octopus Intelligent 23:00–06:00 plus peak windows · Octopus Flux off-peak 02:00–05:00 · Economy 7 approximately 00:30–07:30 (varies by region). Write these times down exactly — you'll need them for Step 2.
Check inverter clock and compare to local time
In SolarMan, go to Device → Information and check the inverter's current clock time. Compare it to your local time right now. If the inverter clock is 1 hour behind (e.g., showing 23:00 when it's actually 00:00 BST), this is the problem. During British Summer Time, many Sunsynk inverters don't auto-adjust the clock. The charge window will fire exactly 1 hour late as a result.
Check Time of Use schedule in SolarMan
Navigate to Device → Settings → Time of Use. Find your charge window and verify the start and end times match your tariff window exactly. Common mistake: if Octopus Go is 00:30–04:30 but SolarMan shows 23:30–03:30, the window is offset by 1 hour. Correct it to 00:30–04:30.
Verify grid charge is enabled for the charge window
In SolarMan, check the grid charge settings (usually Device → Settings → Grid Charge). Confirm that grid charge is set to ON for the charge window. If disabled, the battery will only charge from solar generation, not from cheap-rate electricity. Grid charge must be ON for the system to force charge from the grid.
Check charge power target and SoC targets
In Device Settings, find the charge power limit (if set). If too low (e.g., 500W), the battery will charge very slowly during the window — a 5kWh battery might only gain 500Wh in 4 hours. Set charge power to the inverter's rated capacity for maximum charging during the window. Also verify SoC target is high (e.g., 90-100%) and minimum SoC is low (e.g., 10-20%) to allow meaningful charge/discharge cycles.
Monitor the next charging cycle and verify with SolarMan data
After making any adjustments, check SolarMan's battery SoC graph the following morning. Look for: battery SoC increasing during your charge window, grid power showing as positive import during the window, and battery reaching your target SoC by the end of the window. If charging still hasn't started, book a remote diagnostic — there may be a secondary issue.
About cheap-rate tariff integration
Octopus Energy grew explosively across the UK from 2020 onwards, and many Sunsynk systems were installed without proper tariff-specific configuration. The combination of BST/GMT clock confusion, unclear SolarMan UI, and inconsistent Time of Use scheduling has created a situation where the majority of Sunsynk owners report that their cheap-rate charging doesn't work reliably. In reality, nearly all of these systems can be fixed in a single 30-minute remote session.
STS is a Sunsynk and Octopus specialist. We've configured hundreds of Sunsynk systems for Octopus Go, Agile, Intelligent, and Flux tariffs. The fix is always the same: correct the clock, verify the TOU window times, enable grid charge, and set appropriate SoC targets.
Cheap-rate charging questions
The most common cause is a BST/GMT clock offset. Check the inverter clock in SolarMan — if it's 1 hour behind your local time, that's the problem. Correct it to your current local time. Also verify your Time of Use window in SolarMan matches Octopus Go's window (00:30–04:30) exactly. If both are correct and charging still hasn't started, grid charge may be disabled — enable it in Device Settings.
Octopus Agile prices change every 30 minutes and are published by 4pm each day. The simplest approach is to set a conservative window (e.g., 23:00–05:00) that covers most cheaper periods. For advanced optimization, you can manually update your TOU schedule based on the Agile prices each day. We can configure this remotely as part of our tariff setup service, including setting up multiple windows for different patterns.
Not reliably. Many Sunsynk installations require a manual clock adjustment when clocks change in March and October. If your charge window is consistently 1 hour late (e.g., charging starts at 01:30 instead of 00:30), check the inverter clock in SolarMan and adjust it manually to match your local time. This is the #1 cause of Octopus cheap-rate charging failure and the easiest to fix.
A smart meter isn't strictly required for force-charging during cheap-rate windows with Sunsynk, but it helps your system track import and export accurately. Without a smart meter, you may not see real-time grid consumption data in SolarMan, making it harder to verify that charging is happening at the right times. Your installer or STS can check your metering setup during a diagnostic visit and recommend any upgrades needed for optimal performance.
Octopus cheap-rate not working?
Tell us your tariff and what you're seeing. We'll check your SolarMan data and configure your system to work correctly with your Octopus tariff — most cases are fixed in a single session.