What happens when i export more power than i import?


We are just about to get a large array of solar panels installed on our house - as such during certain times of the day we will be net exporters to the grid (we are 3 phase)

Do i still orient the CT clamps with the arrows facing into my supply and is the IotaWATT smart enough to manage the calculations etc when the values essentially reverse ?


It’s smart enough to do the arithmetic to calculate net usage. Although it’s arbitrary and different folks use different models, the model that seems to be most popular and that I advocate is to setup the CTs so that the PV is positive when providing power, and the mains are positive when importing power.

The particular math depends on how your PV is connected. Is the inverter three-phase or single-phase? Will the inverter supply power upstream (toward the meter) or downstream of the mains CTs?

Finally, will you want to know only the net power used, or will it be important to know the total exported and the total imported? If that’s the case, you should plan on uploading to PVoutput to get total imported and exported. I recommend PVoutput to anyone with solar anyway as it’s free, simple to configure and use and provides a useful mobile app.

Hey Bob,

Thanks for the quick response.

I have access to the mains cables before they hit the meter - so they will show the true power usage i.e after the solar has hit the meter and then started backfeeding into the electricity mains. So i will be upstream of the Solar.

The Solar inverter will be a 3 phase unit - it feeds into the main meter through a solar breaker - i do not intend to monitor the solar output as this will be handled by the rest of my automation system communicating directly with the inverters API

So during the day my CT clamps will be showing me with a negative usage (according to the clamps) and then a positive usage in the evenings (until i get a battery solution !)

Yep i already use PVoutput for my current solar solution and will continue to do so

I could i suppose use extra channels and clamp at my main switchboard onto the 3 phases - but if as you say IOTAwatt is smart enough to do the calcs i will not need to waste the inputs.

Thanks for your timely reply - pleasure dealing with you again !


Hi Craig,
The only issue with that plan is that you will not be able to calculate total usage without the IoTaWatt knowing what the inverter is putting out.

For example if the mains show a net -2500 Watts you are exporting 2,500 excess Watts of power, but excess of what? If the inverter is putting out 7,000 Watts, then you are using 4,500 Watts. If the inverter is putting out 9,000 Watts, then you are using 6,500 Watts. So IoTaWatt can do the math, but it needs to know the value of the variables. I would recommend that you put CTs on the inverter if you have enough inputs, and plan to upload both generation and consumption from the IoTaWatt.

If you plan to stay the course, I’s suggest you put the mains CTs downstream of the solar feed so that they measure consumption and let PVoutput determine net with the inverter data.


OK good point

Looks like i might follow your advice then !