Solar PV

Solar generation lower than expected

Warning Do not attempt any electrical or gas work yourself. Always use a qualified, MCS-certified, or Gas Safe registered professional for repairs.

What you’re seeing (symptoms)

Your monitoring app or generation meter shows lower kilowatt-hour (kWh) figures than your installer’s initial estimates, or a sudden drop compared to the same time last year.

What’s normal vs not normal

Normal:

  • Lower generation on cloudy or rainy days.
  • Significantly lower generation in winter (December/January can be 1/10th of June).
  • Occasional dips due to temporary passing clouds.

Not normal:

  • Zero generation on a bright sunny day.
  • A sudden 50% drop that doesn’t recover.
  • One “string” of panels showing zero output while others are working.

Likely causes (ranked)

  1. Weather conditions: Simply fewer sunlight hours or more cloud cover than average.
  2. Shading: New tree growth, a neighbor’s extension, or even a new chimney pot.
  3. Dirty panels: Thick dust, bird droppings, or lichen buildup.
  4. Inverter clipping: On very sunny days, the system may reach its maximum capacity.
  5. Component failure: A faulty micro-inverter or a blown fuse in the DC isolator.

Quick checks you can do safely

  • Visually inspect the panels from the ground—is there visible dirt or bird droppings?
  • Check for new shadows being cast on the roof at different times of day.
  • Check your inverter for any red warning lights or error codes.

When to call an installer

  • If the system shows “0W” output during daylight hours.
  • If you see a “Ground Fault” or “Isolation Fault” error message.
  • If your monitoring app shows one specific panel is consistently underperforming.

Questions to ask your installer

  • “Can you remotely check the performance of individual panels/strings?”
  • “Is the inverter firmware up to date?”
  • “Does the generation match the MCS certificate estimates for this month?”

Related NetZeroNow tools & guides

FAQs

Does heat affect solar panels? Yes, counter-intuitively, solar panels are less efficient on extremely hot days. They prefer bright, cool, and breezy conditions.

How often should I clean my panels? In the UK, rain usually does a good job. However, if you live near the coast (salt) or a busy road (soot), an annual professional clean might be beneficial.

What is ‘Shading’? Even a small shadow covering a tiny part of one panel can significantly reduce the output of the entire “string” if you don’t have optimisers.

Will they work in the shade? They will produce a tiny amount of electricity from “diffuse” light, but for meaningful generation, they need direct or bright indirect sunlight.

Is my inverter too small? Installers often “undersize” inverters slightly (e.g., a 3.6kW inverter for 4kW of panels) to ensure the system runs more efficiently during the 95% of the time when it’s not perfectly sunny.

Written by NetZeroNow | Last updated on 2026-03-02