In this article we will share our approach on designing a suitable residential solar storage system.
Solar energy storage systems combine solar and battery systems. To bring a lower LCOE and higher rate of utilization, both battery and solar system shall be taken into consideration in storage system design based on specific scenarios.
Note:
This article focuses on general system design. Specific regulation requirements or government support mechanisms are not taken into consideration.
Misleading theories in storage capacity design:
1. Focus on battery capacity only
2. The “perfect kW/ kWh ratio” on all scenarios. (There is no one fixed ratio for all scenarios)
What should be considered to exactly define the combination of solar and battery?
Conventional Solar Radiation Level
Knowing the average production ability of solar in your area is essential to help you define solar capacity. The production ability should cover the expected daily, monthly or yearly house consumption. This information is readily available online.
System Efficiency
There is always around 10% overall power loss in a solar storage system with battery cycling. So system efficiency is an important point, including:
DC/DC efficiency
Battery cycling efficiency
DC/AC conversion efficiency
AC charging efficiency
It is hard to cover all losses like transferring loss, line loss, control loss etc. but should care as much as possible to make sure the designed battery
capacity is enough for a specific request. As system efficiency is concerned, the real battery capacity should be:
Usable Battery Capacity
Refers to the real usable battery capacity working with a storage inverter. On the battery datasheet, we can see “Total Energy” and also “Usable Energy”, which is the meaningful specification for systems design.
You can also find the usable energy of battery in the recommended DOD.
Another concern is the real recommended battery DOD working with a specific inverter. Sometimes the preset DOD is different with different storage inverters.
Parameters Combination
On both inverter and battery datasheet, parameters like Max Charge/Discharge Current, are crucial. If they do not match, then the system will follow the lower of the two. The designer should follow the lower data to calculate charge/discharge speed and power supply ability especially under backup power supply mode.
Inverter Parameters
Battery Parameters
Like in the example above, if using the inverter and battery together, the system max charge/discharge current is 14.3A and max power 5kW. This is a big concern for backup supply design and battery capacity.
Operation Scenarios
Operation scenario is also important to consider in storage system design. Most residential storage systems used to shift electricity cost to solar power supply or to reserve power for backup use in the application scenarios include:
TOU (time of use)
Peak load shaving
Backup power supply
Maximize self-use of solar power
Each scenarios adopts different designing logics. But all the design logics are based on the specific power consumption of the house.
TOU (Time of Use) Scenarios
If the storage system is designed to cover loads during peak hours to avoid high electricity prices, then you should bear in mind:
A. The TOU strategy (Peak and Off-peak time)
B. House energy consumption at peak stage (kWh)
C. The general power consumption (kW)
The battery usable energy should ideally be higher than peak energy consumption. And battery charge/discharge ability higher than general consumption power.
Peak Load Shaving Scenarios
Battery is discharged to supply loads when grid demand is higher, to avoid high electricity prices in a tiered billing system. To design a proper storage system, you have to know:
A. Billing tiers
B. House energy consumption at peak demand (kWh)
C. General power consumption you want shave (kW)
Battery energy should be higher than the daily house energy consumption above the target tier to make sure a lower overall billing cost.
Backup Power Supply
If the battery is designed to store solar power or charge from grid for energy demand during grid outage, you must have an expectation of outage time and energy consumption and especially peak load power during grid outage, to make sure you have enough power supplement during a power outage.
Maximize Solar Self-Use
To increase self-use of solar power, a battery system is adopted to store excess solar energy instead of injecting solar power to grid or restricting solar production. The stored energy is discharged to support house consumption when solar production is weak.
So the general logic to design the storage system (including solar system) is based on power consumption and the constant peak consumption power during a day to at least make sure the total solar production covers load consumption during a specified time.
Usually, applications of solar storage system are designed for multiple scenarios on a same site. So the design of a solar storage system is based on a comprehensive consideration of all target uses.
Moreover, the design of a solar storage system should consider the return on investment (ROI) and local policies. Designing a solar storage system requires knowledge and expertise and there are many variables to consider. You may need consult your installer or specialist.
We would suggest to add suitable additional storage capacity in case of any unexpected load consumption or to cover solar and battery life attenuation.
