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How to configure the batteries in the household energy storage system?

2023-04-18

With the development of battery technology and the rapid decrease in costs, lithium batteries have become the mainstream choice in household energy storage projects due to their advantages of high efficiency, long cycle life, accurate battery data, and high consistency.

 

Four common misconceptions in battery capacity selection

Myth 1: Only selecting battery capacity based on load power and electricity consumption

In battery capacity design, the load situation is indeed the most important reference factor. However, the battery charging and discharging capacity, the maximum power of the energy storage machine, and the power consumption period of the load cannot be ignored. Therefore, the battery capacity cannot be selected solely based on the load power and electricity consumption, and needs to be comprehensively considered.

Myth 2: Treating the theoretical capacity of a battery as its actual capacity

Usually, the battery manual indicates the theoretical design capacity of the battery, which is the maximum amount of electricity that the battery can release from SOC 100% to SOC 0% under ideal conditions.

However, in practical applications, the actual capacity of batteries may differ from the design capacity due to factors such as temperature and service life. And in practical applications, in order to extend the battery life, it is generally not allowed to discharge to SOC0%, and protective power is set, which reduces the actual available power. Therefore, when selecting battery capacity, these factors need to be excluded to ensure that there is sufficient battery capacity available for use.

Myth 3: Choosing a larger battery capacity is better

Many users may think that a larger battery capacity is better, but we also need to consider battery utilization when designing. If the capacity of the photovoltaic system is small or the load consumes less electricity, the demand for battery capacity is not as large, which can also cause waste of battery costs.

Myth 4: The battery capacity is exactly equal to the load's electricity consumption

Sometimes, for cost saving reasons, the selected battery capacity is almost equal to the load's electricity consumption.

However, due to process losses, the battery discharge capacity will be lower than the battery storage capacity, and the load consumption will also be lower than the battery discharge capacity. Ignoring efficiency losses may lead to insufficient power supply.

 

Battery capacity design in different application scenarios

This article mainly introduces the design ideas for battery capacity in three common application scenarios: spontaneous self use (with high electricity bills or no subsidies), peak and valley electricity prices, and backup power sources (with unstable power grids or important loads).

① Spontaneous self use

Due to high electricity prices or low subsidies for photovoltaic grid connection (without subsidies), installing photovoltaic energy storage systems to reduce electricity expenses.

Assuming the power grid is stable and does not consider off grid operation, photovoltaics are only used to reduce the power consumption of the grid, and generally there is sufficient sunlight during the day.

The ideal state is that the photovoltaic+energy storage system can fully cover household electricity consumption. But this situation is difficult to achieve. So, considering the investment cost and electricity consumption, we can choose to choose the battery capacity based on the average daily electricity consumption (kWh) of the household (default photovoltaic system has sufficient energy). The design logic is as follows: Image

If the electricity usage patterns can be accurately collected and combined with the energy storage machine management settings, the system utilization rate can be maximized.

② Peak valley electricity price

The structure of peak and valley electricity prices is roughly from 17:00 to 22:00, which is the peak period of electricity consumption:

During the day, the electricity consumption is low (the photovoltaic system can basically cover it), and during peak electricity usage, it is necessary to ensure that at least half of the electricity is supplied by batteries to reduce electricity bills.

Assuming average daily electricity consumption during peak hours: 20kWh

Calculate the maximum demand value for battery capacity based on the total electricity consumption during peak periods. Then, based on the capacity of the photovoltaic system and the benefits of investment, find the optimal battery capacity within this range.

③ Areas with unstable power grids - backup power sources

Mainly used in unstable areas of the power grid or scenarios with significant loads.

For example: Application site: Approximately 5-8KW components can be installed

Important load: 4 * ventilation fan, with a single fan power of 550W

Power grid situation: The power grid is unstable, with irregular power outages, with a maximum duration of 3-4 hours

Application requirements: Under normal power grid conditions, batteries should be charged first; When the power grid is cut off, the battery and photovoltaic system ensure the normal operation of important loads (fans).

When selecting battery capacity, it is necessary to consider the amount of electricity required for the battery to be supplied separately when off grid (assuming a night power outage without PV).

The total power consumption and estimated time of leaving the grid are the most critical parameters. Based on the estimated maximum duration of a power outage of 4 hours.

 

Two important factors in battery capacity design

① Photovoltaic system capacity

Assuming that all batteries are charged by photovoltaics, the maximum power that the energy storage machine can charge the batteries is 5000W, and the daily sunshine hours are 4 hours.

So:In the mode where the battery is used as a backup power source, the average requirement for a battery with an effective capacity of 800Ah to fully charge in an ideal state is:

800Ah/100A/4h=2 days

② Battery redundancy design

Due to the instability, line loss, ineffective discharge, and battery aging in photovoltaic power generation, efficiency losses are caused. Therefore, a certain margin needs to be retained in battery capacity design.

The design of battery margin is relatively free and can be comprehensively judged based on the actual situation of the system design.