Adding Solar PV to Existing Inverters

How to get real savings in Electricity bills and keep the batteries safe!

Electricity supply in India has improved enormously in the recent past. The Southern region is expected to have an high Peak shortage (Power cuts !). The Eastern region is expected to have an energy surplus in 2015-161. With increasing power availability, consumers are looking to add Solar PV to help in reducing electricity bills. In the absence of grid tied options, adding Solar to existing Inverter based back up makes economic sense. Adding Solar to existing Inverters allows the reuse of the investment already made in Inverters & Batteries.

With this approach, the Solar PV panels & a Charge Controller that controls the charging from both the existing Inverter and Solar PV is retrofitted to the existing Inverter. The combined system is expected to work as a single unit, maximizing the use of Solar PV and providing sufficient power back up in case of power cuts.

Most retrofit charge controllers control the Mains input to the Inverter, switching off the Mains supply, when Solar PV is available so that the loads connected to the Inverter are powered from Solar PV (and the batteries).

A good ‘retrofit’ charge controller MUST control the Charge Rate to the batteries if they aim to really save electricity bills with the addition of Solar PV. To get real savings in electricity bills, the Solar PV addition must generate enough energy to completely service the day time loads connected to the Inverter. For example, in a typical home, the day time energy consumption might be 2-3 Units (kWHr). A kWp of Solar PV panel would generate about 4-4.5 Units per day. A Solar array suitably sized would be capable of generating the required energy. The conflict arises becase of the size of battery back up available. Most existing Inverters would have Lead acid batteries of ~ 150Ah capacity. Lead acid batteries must not be charged at a rate higher than 10 -15% of their battery Ah. When charged at an higher rate, the batteries reach gassing Voltage before they are fully charged.However, a Solar PV array to take care to day time loads might deliver Current that is higher than the optimal Charge rate. Installing larger batteries is expensive & not very useful. Installing smaller PV capacity is counter productive – in the end, the energy deficit would be bridged by the use of Grid supply and will result in increased bills. Not what the user wants either. The retrofit charge controller Must monitor the net current delvered to the batteries and Must control this current to the optimal charge rate – so that Loads get the required power and the batteries are used safely as an effective buffer for the variations in load and generation.

Therefore a good solution to add Solar to existing Inverters to be of real benefit to users must allow sufficient Solar capacity to take care of day time loads and keep the batteries safe at the same time.

1Page i.. Anticipated All India Power Supply Position for the Year 2015-16. Load Generation Balance Report 2015-16. Central Electricity Authority.