Bridging Time and Self-Consumption Quota
This section describes a method by which you can estimate bridging time and self-consumption quota for an SMA Home Energy Solution with battery-backup function. For the battery capacity, an empirical value of a typical battery-backup system is assumed and verified by means of the estimate. In the example, the assumed values for the energy demand of the loads in a private household, the peak power of the PV system and the battery capacity are characteristic of a battery-backup system in a four-person single-family household in Germany.
Step 1: Estimation of self-consumption quota for an SMA Home Energy Solution
Input data (example):
Peak power of the PV system: 5000 Wp
Annual energy demand: 5000 kWh
Total battery capacity: 10000 Wh, of which the inverter uses 50% for intermediate storage of PV energy.
The usable battery capacity therefore amounts to 5000 Wh.
Transfer the calculated values to the diagram to estimate the self-consumption quota.
Estimation of the self-consumption quota with intermediate storage
The estimate reveals that the self-consumption quota, with energy management with intermediate storage, is approximately 60%.
Step 2: Estimation of energy demand in the event of grid failure
Input value:
Annual energy demand: 5000 kWh
It can be assumed that during a grid failure, electrical energy will be used sparingly, e.g. by switching off energy-intensive loads. As a result, the daily energy demand of 13.6 kWh can be reduced. In this case, at a reduction by 40%, the energy demand of this household will be around 8 kWh in the event of a 24-hour grid failure.
Step 3: Estimation of PV generation during a grid failure
The peak power of the PV system is at 5000 Wp. In Germany in the winter, it can be assumed that 0.7 kWh/kWp will be generated. Hence, an energy yield from PV production of 3.5 kWh between sunrise and sunset is derived.
Step 4: Calculation of battery capacity required for the battery-backup function
Input value:
Daily energy demand of the household: 8 kWh
Daily energy yield from PV production: 3.5 kWh
The required battery capacity amounts to 4.5 kWh. In Step 1, a battery capacity of 10 kWh was assumed. Thus, in this example 45% of the battery capacity will be needed as energy reserve for grid failure on a winter day.
Result
With lead-acid batteries, the default value of 45% of a Sunny Island parameter battery capacity is reserved for the battery-backup function in winter operation. This value is within the value range of 15% to 60% in which the battery capacity must range ( > Battery Use by the Sunny Island). Thus, the battery used for the SMA Home Energy Solution is also adequate for the battery-backup function.
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