Maintenance and fault-finding
How to Read Your Solar Monitoring Data and Spot Underperformance
Your inverter or battery management system logs every kilowatt-hour your panels generate. Understanding what those numbers should look like, and how to compare them against typical UK benchmarks, is the fastest way to catch a problem before it becomes expensive.
Monitoring data is not the same as your export meter reading
Your monitoring app shows generation at the inverter. Your SEG statement and electricity bill show what was measured at the grid meter. The difference is self-consumption (and battery cycling if you have one). Both figures are correct: they measure different things at different points in the system.
1. Find your monitoring portal or app
Most solar inverters sold in the UK include free cloud monitoring as standard. The portal or app varies by brand:
- GivEnergy: GivEnergy portal (portal.givenergy.cloud) or the Giv-AI app
- Solis: SolisCloud (soliscloud.com) or the SolisCloud app
- Sunsynk: Sunsynk Connect app
- Fox ESS: FoxCloud (foxesscloud.com)
- SolarEdge: SolarEdge monitoring platform (monitoring.solaredge.com)
- Sigenergy: Sigenergy Cloud or the SigenStor app
- Tesla Powerwall: Tesla app
- Older systems (pre-2018): Solar Log, SMA Sunny Portal, or a similar platform set up by your installer
If you are not sure where your data goes, look for a sticker on the inverter with a portal URL, or contact your installer. They are required to give you access to your system's monitoring as part of the installation.
2. Understand the four key metrics
Whatever app or portal your system uses, the core figures are the same:
Generation is the total AC electricity output from your solar panels in kilowatt-hours. This is the starting number: everything else describes where that energy went.
Self-consumption is the portion of solar generation used directly by appliances in your home. High self-consumption means more of your free solar electricity is offsetting bill costs rather than being exported at a lower export rate.
Export is surplus generation sent to the grid. This is what your SEG licensee measures and pays for. If you have a battery, most of the surplus will charge the battery first before anything is exported.
Import is grid electricity drawn to cover what solar did not supply. On a sunny summer day this may be near zero. In winter, import is typically high because generation is low.
On battery systems, you will also see battery charge and battery discharge figures. These sit between generation and export in the energy flow: surplus solar charges the battery first, and the battery discharges to the home in the evening, reducing import.
3. Benchmark against typical UK figures
PVGIS (the free irradiance tool from the European Commission, available at re.jrc.ec.europa.eu/pvg_tools) calculates site-specific monthly generation estimates from 20-plus years of satellite weather data. Enter your postcode, array size in kWp, panel tilt in degrees, and roof orientation to get a monthly breakdown.
As a rough guide, for a south-facing 30-degree roof in Kent or East Sussex, a 4 kWp system should produce approximately:
- January: 140 to 200 kWh
- April: 460 to 540 kWh
- July: 540 to 660 kWh
- October: 240 to 320 kWh
These are long-run averages. Any individual month can be 20 to 30 percent above or below average depending on cloud cover. Comparing against your own historical data for the same month is more useful than comparing against a reference average alone.
4. Compare year-on-year, not month-on-month
Monthly generation varies so much with the weather that comparing June to May tells you almost nothing useful. The most revealing comparison is month versus the same month last year.
Most monitoring apps provide a year-on-year view. Open it and look for any month where current-year performance is consistently 15 to 20 percent or more below the previous year. Panel degradation is real but gradual: around 0.5 percent per year on good-quality panels, so around 5 percent over ten years. A sudden 20 percent drop over one year is not normal degradation; it points to a fault, shading change, or settings problem.
If your system is new and you have less than a year of data, use the PVGIS estimate for your site as the benchmark. A first-year result more than 15 percent below the PVGIS estimate, after accounting for weather, is worth querying with your installer.
5. Spot the early warning signs
Four patterns in your data that usually point to a specific problem:
A flat plateau on a clear-day power curve often means one or more panels are shaded during that part of the day. A typical solar array produces a smooth bell-shaped curve from sunrise to sunset on a clear day. A flat section or step down mid-morning or mid-afternoon is the signature of shading from a new obstacle such as a satellite dish, TV aerial, chimney pot, or tree growth.
One string significantly lower than another on a multi-string inverter suggests a problem in that string: a failed bypass diode in a panel, a loose MC4 connector, or a damaged cable. Multi-string inverters display each string's output separately in the monitoring portal.
Persistent fault or warning codes on the inverter display or monitoring app should be looked up in the manufacturer's manual. Codes beginning with F or E typically indicate fault states; codes with W are warnings that may self-clear. A code that returns after a restart usually needs a qualified engineer.
Zero or very low generation despite good weather may mean the inverter has tripped off due to a grid protection event. Most inverters disconnect from the grid if the grid voltage goes outside the allowed range. This is a safety feature, not a fault: the inverter should reconnect automatically once the grid stabilises. If it does not reconnect, check the inverter display for a code before calling an engineer.
Frequently Asked Questions
What is a good output for a 4 kWp solar system in the UK?
For a south-facing 30-degree roof in southern England, a 4 kWp system should generate around 3,400 to 3,800 kWh per year in total. Monthly figures range from around 100 to 200 kWh in winter months up to 500 to 660 kWh in peak summer months. The PVGIS tool on the European Commission website gives a free site-specific estimate: enter your postcode, system size and roof angle.
Why does my monitoring app show different figures to my electricity bill?
Monitoring apps measure generation at the inverter. Your electricity bill and SEG statements measure export at the grid meter. The difference is self-consumption: electricity you generated and used in your home. If your app shows 400 kWh generated but your SEG statement shows 120 kWh exported, you self-consumed 280 kWh. Battery charging and discharging also sits between generation and export, so the figures will diverge further when a battery is involved.
When should I call an engineer about low solar output?
Call your installer or a qualified electrician if your inverter shows a persistent fault code that does not clear after a restart; if generation is consistently more than 20 percent below last year's equivalent month and you cannot identify a shading or weather cause; or if the daily generation curve shows a flat section mid-morning or mid-afternoon on a clear day, which often indicates a failed or shaded string. Faults in DC wiring between panels and inverter involve live voltages and are not DIY repairs.