It is difficult to be certain if your system is performing poorly just from looking at power alone. This is because there are so many things that can reduce its power output without there actually being anything wrong with it. These include:
• The angle of the sun: When the sun is low in the sky, whether due to the time of day or the season, less power will be produced.
• Solar panel orientation: Panels facing east or west will generate less power than those that face north.
• Clouds and haze: Less sunlight reaching the panels means lower power output.
• Heat: High temperatures reduce panel efficiency. In a heatwave, solar panels can reach 65 degrees and this can reduce power by 20% compared to panels that are at 25 degrees.
• Wind: While sunshine heats panels a nice breeze helps cool them down.
• Dirt: Bird poo, leaves, and grime in general can greatly reduce output.
• System losses: Wiring resistance results in about 2% of power being lost while modern inverters often have losses of 3-4% as they change the DC power from the solar panels into the AC power homes use.
You should therefore measure your power output in good conditions.
If the energy output of your system is still considerably lower than what it should be, then your installer is your first port of call.