In addition to the utility meter on the outside of your house, each solar installation comes with an online monitoring system for you to view the production of your solar array. With two different sources for production information, the figures may not be equal. Why is that? And what information can we gather from them?

It is normal for the monitoring system and production meter values to be similar but slightly different. The two systems measure the same thing — the cumulative lifetime generation of the solar array — however, they have different mechanisms by which they measure power, causing the slight variation in the total energy production. 

Additionally, your utility net meter shows two readings: the 'net-import' and the 'net-export.' The utility net meter can only "see" what travels through it. The piece missing is the amount of energy consumed on-site, prior to any surplus being left over and thus exported to the grid. 

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For example, a 3.575kW system that produced 3,762 kWh of energy throughout the year may have utility readings of 2,835 kWh (energy exported back to the utility) and 2,741 kWh (energy imported from the utility). As mentioned earlier, the utility meter does not read the amount of energy consumed prior to going into the grid (how much solar power was consumed), which is the difference between the cumulative production calculation and the utility meter – in this case, 3,762kWh - 2,835kWh = 927kWh. The variation in the two utility metering calculations is the difference between energy produced and consumed by the household. This example household is not only net-zero, they are net-positive: total net usage for the year is -94 kWh!

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