Data speaks louder, upgrade grid-tie system to storage makes more money?


In recent years with the popularization of new energy, the scale of grid-tie system has increased significantly. However, due to the primitive power grid construction in a lot of areas, decreased or  canceled Feed-in-Tariff, and low PV self-consumption rate, the phenomenon of feed-in power limitation and solar station abandonment has frequently occurred, resulting in resource waste, investment return reduction or even deficit. In view of this situation, our company put forward the energy storage AC-couple retrofit scheme and has been widely applied. Grid connected energy storage system can store the surplus power and improve the self-consumption ratio. It is applicable to the scenarios that PV power cannot be sold to utility grid, and self-use electricity price is higher than the feed-in price, or solar power generation and consumption are not in the same period.


For the owner, what can be done to improve the power generation revenue?

Now it’s going to be introduced with a real case.


It is a grid-tie PV project located in an industrial park in Zhengzhou, PV installed capacity is 520kwp with 100 kW grid connected inverter CP100X5 units in parallel, the system is to supply loads in the factory.


Original equipment list




PV panel


1764 pcs

Central inverter

Growatt CP100

5 units

AC distribution cabinet

Growatt  AC cabinet

1 unit

According to the owner's description, the original design purpose is mainly for the factory self-consumption plus earning feed-in subsidy. As the policy changes, turns out the feed-in price gets too low, which resulted in a plunge in investment profit.


The load list and service time of the factory are as follows:






From above we can see that the annual active power generation is 700.86MWh, load power consumption is 552.98MWh, and PV has supplied 371MWh to the load, 302.79MWh PV power has been sent to the grid and 181.57MWh taken from grid. Annual PV self-consumption rate is 67.17%.


If the feed-in price is 0.1USD/kWh, electricity price is 0.3USD/kWh. In a year, the owner will have to pay 182000 * 0.3-303000 * 0.1 = 42500 US dollars for electricity.


In order to improve the self-consumption rate and save electricity cost, we designed the following retrofit solution for the owner:


Added equipment list




Bidirectional battery inverter



Battery module 

ATESS 7.68kWh battery

14 packs in series X 6 strings in parallel


In this case, BYD battery B-box bro 13.819 packs in series X 6 strings in parallel is used for simulation.


System operation mode: the output of PCS500, grid inverter, load, and grid common AC BUS input and output are in dynamic balance. The grid inverter and PCS500 operate independently. PCS detects the power on the grid side, judges the power size and flow direction to decide whether to charge or discharge. When the PV power is greater than load power, PCS will detect the power transmission to the grid, so as to start charging mode and absorb the remaining PV power to charge battery; otherwise, when the PV power is insufficient for the load, PCS will detect the power flows from the grid to the load, so as to discharge to offset the power deficiency, to ensure that the load preferentially uses the PV and battery energy, minimize power draw from grid, and improve the PV self-consumption.


After the retrofit, the simulation results from PVSYST are as follows:





We can see that the annual PV active power generation is 700.86MWh, the load power consumption is 552.98MWh, PV-to-load power raises to 510.19MWh, sent to grid 133.37MWh, taken from grid 42.794MWh, and annual PV utilize rate increases to 92.26%. Assume the feed-in price is 0.1USD/kWh, the electricity is 0.3USD/kWh. In the first year, it will be 43000 * 0.4-133000 * 0.1 = 392000 US dollars on the electricity bill.


What are the bonuses this retrofit brings?


1. Before the retrofit, the annual electricity cost is US $42500. After the retrofit, only 3900 US dollars to be paid, almost realize "free use of electricity" ,  $38600 can be saved annually.

2. The annual PV utilization rate increased from 67.17% to 92.26%.

3. 181.57MWh power taken from gird reduced to 43MWh per year, the grid power supply pressure can be greatly relieved.

4. 303MWh feed-in power reduced to 133MWh per year, significantly reduces operation and maintenance pressure on the grid.

5. According to relative policy, it can be incorporated into the smart dispatch for large grid to participate in power grid regulation, and can put on the welfare of energy storage subsidy. 

6. It is a good demonstration to reduce emissions and achieve good ecological benefits.


Then, how about the earnings the owner can actually get?


From the simulation results, it can be seen that in a year, 700.86MWh of active PV power is generated , among which the load consumes 552.98MWh. Suppose the load is constant, and the PV capacity attenuation won’t be less than 80% after 25 years. Then 700.86MWh * 0.8 = 560.688MWh, which is still greater than load power consumption, thus the annual revenue can be estimated to be constant.


Inverter + battery cost = 25700+214000≈240000US dollars


$38600 electricity cost can be saved annually. (based on China's electricity price)


The energy storage subsidy is $0.078 per kWh discharged for participating in smart dispatch and peak regulation of large grid. Assuming there is 60 times of dispatching and 100 kWh discharged each year, the annual subsidy will be about US $470 (Based on China’s subsidy)


Then the investment recovery period = 240000 / (38600 + 470) = 6.14 years


To sum up, the investment recovery period for the retrofit is about 6 years and 2 months, the life of lithium battery is normally about 10 years, so the money earned in the rest nearly 4 years will be net profit. Due to the continuous progress of battery technology, the cost of battery will reduce substantially in the following years, and by contrary the electric charge rises every year. After the battery is scrapped, it can be replaced by new battery with higher performance, lower price and continue to run the storage system.


The above is a typical case of grid-tie to storage retrofit. The ATESS PCS series model does not need to communicate with the existing grid inverter, so it can be applied with the inverter of any brand in the market, saving investment for EMS, and at the same time realizing the function of energy flow regulation and management.


Now there are countless grid-tie PV power stations running in operation. Due to unsatisfactory self-consumption rate, limitation of grid power consumption and transformer capacity, many more owners tend to choose this scheme to upgrade their system. On one hand, it’s because it doesn’t require to change or replace the original equipment, on the other hand, it brings considerable economic benefits compared with continuous use of the pure grid connected system.