Solar Academy Lesson 1: How solar energy storage systems work

A solar system operating in conjunction with batteries may offer the system owner far more functionality than a solar system operating without such additional power. A system with batteries can fully power an average home for several hours as long as that home is connected to the grid. In blackouts, batteries can power selected circuits in the home around the clock because they get recharged in the daytime by the solar panels. The magic for all this extra functionality is performed by the system’s inverter. This article will examine the functionality offered by inverters that work with the LG Chem RESU 400 Volt home batteries.

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Overview of how solar energy storage systems work.

A grid-tied solar system is usually designed to produce as much or more power as a home needs. Without batteries, any power not used in the home when the sun is shining is sent into the grid. With home batteries, some of that power that would have been sent to the grid gets captured and stored for later use. That stored power can then be used to power the home when grid power is unavailable or expensive.

Four main parts of a solar energy storage system

A solar energy storage system consists of four main parts: 

  1. Solar panels – Provide electricity to the system with sufficient sunlight.
  2. Solar charge controllers – Manages the power going into the batteries, and prevents reverse current which would drain the batteries when the sun isn’t shining.
  3. Inverter – Converts DC power from the solar panels or the batteries into AC power for the home or grid.
  4. Batteries – Stores DC power from the solar panels for later use in the home.

A breakdown of how each part of the system works during the day

The charge controller will charge the batteries when there is sufficient power to charge them. Any extra power produced by the solar panels will be sent to the inverter. When the batteries are fully charged, the charge controller will cut off the charge going to the batteries except for a small float charge meant to keep the batteries fully charged. 

The charge controller and batteries 

With sufficient sunlight, the solar panels provide electricity to the system. Power provided to the solar charge controllers from the solar panels will be used to recharge the batteries. The amount of voltage, known as the nominal voltage, needed to charge the batteries rises as the batteries charge. As a result, the charge controller will increase its output voltage as the level of charge in the batteries rises. When the batteries are fully charged, the charge controller maintains a trickle charge to hold the batteries at their full charge.

When the sun isn’t shining, the charge controller will block current from flowing from the batteries to the solar panels. This prevents the batteries from discharging into the solar panels.

The battery charging process

To keep things simple, imagine we are charging a 12-volt battery. A 12-volt solar panel is used to charge a 12-volt battery. But that solar panel will actually have an output that is closer to 18 Vmp (Volts at maximum [power) when there is a load presented by the batteries. That is because batteries need a higher voltage source to accept a charge. If the charge controller supplied only the battery’s rated charge, the battery would not charge.

The inverter, the home’s meter, and the grid

The power that isn’t used to charge the batteries is sent to the inverter. An inverter converts the DC power from the solar panels or the batteries into AC power. The power drawn from the solar panels will either be consumed in the home or sent to the grid. 

If the amount of power being produced by the system is less than the amount of power the home needs, then some or all of the power produced by the system will be used in the home. The home’s meter will spin forward and the homeowner will be charged by their utility company for the power used. 

If the amount of power produced by the system exceeds the amount of power used in the home and for charging the batteries, then the excess power is sent to the grid.

A breakdown of how the system works at night

Once the sun goes down, the system may draw power from the batteries if the system is configured for it. This is often the case if the home is billed for power using Time-of-Use (TOU) billing. In California, all three major utilities charge peak TOU rates between 4 pm and 9 pm. Home batteries are sized (charge capacity) to be able to fully power the home for 6 hours. The batteries are then charged from the solar panels the next day.

The charge controller and the batteries

In the afternoon, when the power output from the solar panels declines, the charge controller will isolate the battery from the solar panels to prevent the batteries from discharging into the panels. If a float charge is desired, it will come from grid power.

Powering the home with the batteries

When desired, the charge controller will provide power to the home from the batteries. The batteries will provide continuous power up to their rated discharge level or until when they are cut off by the charge controller. To prevent over-discharging, the charge controller will cut off the batteries if they reach their maximum depth-of-discharge level. Most lithium-ion batteries will reach that level when the voltage produced by the battery’s cells drops to 3.0 V per cell. If the batteries charge is used up, the home will automatically be switched to grid power.

How the solar energy storage system works in blackouts

In a blackout, the system is isolated from the grid to prevent electrical discharge into the grid during times when the power lines may be undergoing maintenance or are damaged. During the day, the solar panels will charge the batteries. While the batteries are charging, the system’s inverter has a special circuit that allows power to be drawn from the solar panels to power selected circuits. The amount of power available is typically not enough to entirely power the home.

The amount of power available depends on the inverter

The amount of power available from a solar energy storage system depends on the type of inverter used. High-end inverters connect directly to circuits in the home that are designated to receive power during blackouts. Some lower-powered inverters simply offer outlets that you can connect an extension cord to.

How to size the batteries in a solar energy storage system

As a general rule of thumb, it’s better to have more battery capacity than you need to meet the home’s power needs. The faster you discharge batteries, the faster they wear out. If you have enough battery capacity such that regular usage only discharges the battery bank by 30 – 50% of the battery bank’s capacity, this helps ensure long battery life and gives the homeowner an emergency power reserve should they need it.

Batteries carried by Freedom Forever

Freedom Forever carries the LG Chem Cell battery. The version of the LG Chem Cell available in the US is the 9.8-kilowatt-hour RESU 10H (9.3 kilowatt-hours usable). It has a capacity of 108 amp-hours and is rated at 400 volts. Two RESU-10H batteries can be combined for a total of 19.6 kilowatt-hours. The battery is capable of outputting 5 kilowatts maximum or 3.5 kilowatts continuously.

How much battery storage capacity is needed?

To find out how long the battery will last, you’ll need to know how much power the customer needs the battery to deliver. For a grid-tied system, you can use the customer’s electric bill to estimate their power needs. If the customer needs to power their home from the battery during high peak time-of-use rates, you should estimate their needs based on their hourly power usage (if available) shown in their utility bill.

Plan for a discharge depth of 50%

Discharge depth is the amount of battery capacity that is consumed relative to the total battery capacity before that battery is recharged.  A system sized to use the batteries to 50% of their discharge depth helps ensure long battery life and provides the customer a power cushion should they need to draw extra power. For example:

Electrical usage from 4 pm to 9 pm

  • 4 PM – 5 PM = 1 kWh
  • 5 PM – 6 PM = 0.5 kWh
  • 6 PM – 7 PM = 3 kWh
  • 7 PM – 8 PM = 2 kWh
  • 8 PM – 9 PM = 1 kWh

Total demand = 7.5 kWh

Thus the ideal battery size for this example would be 15 kWh. That would leave the customer with a 7.5 kWh backup reserve. Most customers won’t use as much power as shown in the above example. Thus a single LG Chem Cell RESU 10H will provide adequate capacity in most cases.

Power available during blackouts depends on the system’s inverter

The amount of power available from the system during blackouts depends on the inverter. For the LG Chem Cell RESU 10H, there are several different inverters available. You will need to find out which circuits the homeowner wants to power during a blackout and match the inverter you’ll use in the system to their needs. 

Surge power

Appliances such as refrigerators and A/C units require a surge of power when starting up. The amount of surge power that the system can deliver in a blackout depends on the inverter. Thus you should evaluate the appliances that the customer wants to power in a blackout and choose an inverter that can deliver.

Hybrid Inverters that work with the LG Chem Cell RESU 10H

A hybrid inverter combines the inverter and charge controller into one unit. For grid-tied solar energy storage systems, hybrid inverters can be an ideal solution. 

InverterContinuous kWSurge kWBackup kW
Solax Hybrid 48 V3 – 55
Sungrow SH5K553.2
Goodwe SE4.2 – 5.46.83.6 – 4.6

Solar energy storage systems can meet the need for energy reliability

Planned and unplanned blackouts, plus high peak time-of-use electric rates make for an uncertain energy future. But a properly-sized solar energy storage system can take that uncertainty out of a customer’s energy future. Understanding how these systems work and how to size them is essential if you are to serve your customer’s energy needs effectively now and in the future.

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Disclaimers

Not all homes and solar systems are the same. Contact your Independent Authorized Dealer to who will help find the best solar solution for your particular circumstances.

Figures referenced above are purely for illustrative purposes only and are not meant to be definitive.