- If you not at home when your PV system is making it's maximum power, or if you sleeping at night when your wind turbine is spinning, energy will flow back to the grid
- If there is no wind or no sun and you have your washing machine, kettle and play station on then energy will flow from the grid
- If you have 3kW PV system on your roof giving 3kW of power (perfect sunny conditions ) and you have your 2kW kettle on that means 1kW of power is flowing to the grid.
understanding those 3 bullet points is a key to understanding energy balance and designing system for maximum self consumption rate.
It is important as you pay 17 cents for each unit of energy imported to your house, and they pay you only 9 cents* for each unit exported to the grid.
*9 cents apply to domestic only from Electric Ireland until end of 2015, It is nothing! for export for others (shame as in UK and many other EU countries they pay you at least the same amount for import as for export )
If you monitor your energy usage (at least you note your meeter readings) your energy profile can be assumed and based on that more precise calculations can be made. Below you can see example of energy balance graphs for one of the systems we have design (health center with annual usage of 60MWh and air conditioning) . If you can use more than 50 % of energy your system is producing I would call it a good balance.
To increase self usage of energy you can think of installing energy router. It is a thyristor device what direct energy to your water heater instead to the grid. Examples immersun, solariboost