UBM Tech
UBM Tech

Add battery back-up power option to existing grid-tied PV and solar systems

-July 24, 2013

Editor’s note: The following is a very practical white paper that Outback Power has written regarding battery backup for storing energy from your PV systems while tied to the grid and using it during utility outages and emergencies. My experience with Outback articles has been that they are technical, straightforward and to the point and simple to read and implement as a designer. I think you will find this white paper useful and educational.

Overview:  Homeowners and businesses have bought into the concept of distributed power by installing solar energy systems. There are a lot of factors that have caused a significant rise in solar energy production. These include federal tax incentives, renewable energy incentives, cheaper photovoltaic (PV) solar modules, immediate and projected rises in energy costs, and increasingly stronger desires for energy independence.


Almost all residential, community and light commercial PV/solar systems fall into three types, the first being the most common:

  • grid-tied capable of reducing dependency on utilities and saving money,
  • off-grid capable of supplying electricity where a grid is non-existent, and
  • grid-interactive where the presence of a connected energy storage system (usually in the form of a battery bank) enables the user to achieve both grid-tied benefits with off-grid independence.

The last type is especially useful in scenarios where the grid is down for a variety of reasons, when grid power is inadequate or problematic, or when grid power is costly making it advantageous to “offset” the grid using renewably-generated stored energy. Concerns about grid stability and even availability are more valid than ever, even in the developed world, as life-changing events such as historic storms, tsunamis, and other disasters are combining with increasingly routine brownouts, blackouts and other interruptions to raise anxiety levels about electricity supply to meet growing demand worldwide.


In areas where a lot of renewable energy is added to the grid, the effect of all this extra “peak demand” electricity can actually destabilize the “load demand” grid which relies on more conventional and less flexible or “dynamic” sources—once the sun stops shining or wind stops blowing PV arrays and turbines are effectively turned off, and that loss of peak electricity places even greater demands back on the relatively in-elastic grid which can’t always meet the need.

Figure 1 – Typical Grid Tie Inverter Utility Connection


For these and other reasons, the benefits of storing up renewably-generated electricity for use when it is more advantageous are obvious. Energy storage can offset electricity use during peak times, provide off-grid independence during outages and emergencies, and contribute to greater grid stability to ensure that renewably-generated electricity can remain a vital and proactive partner in the energy mix. This is why energy storage-based systems represent the fastest growth area in solar installation today, and why industry surveys show battery-charging capable inverters to eclipse grid-tied “string” inverters in the next two years (Photon International 2012).

Next: The basics

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