October 20, 2020

5 must-cover topics before choosing a solar battery system

Two houses with solar panels on their roofs.


Solar energy has definitely become more popular in New Zealand in recent years and there are a lot of options in the market for homeowners to choose from, especially when it comes to systems with solar batteries.

In this article we will cover

Solar energy has definitely become more popular in New Zealand in recent years and there are a lot of options in the market for homeowners to choose from, especially when it comes to systems with solar batteries. Different terms such as "battery ready", "hybrid", "AC coupled", and so on, can be very confusing. To help you understand more about the different aspects of solar energy systems with batteries, we have created our 5 major things that you should ask in order to choose a suitable system.

1. Hybrid and battery ready

We often hear the reference to solar energy systems being "battery ready", while the truth is, technically speaking, all solar energy systems are "battery ready", as we can always add batteries to the existing system by including a battery inverter to control the charge and discharge of the battery, and understand the excess generation from the solar inverter. This is commonly referred to as an "AC coupled" system.

A "Hybrid" system has a solar inverter with built-in battery control functionality, providing a more optimised solution to send solar energy directly to the house loads and control the battery component of the solar energy system. In this, the hybrid handles the conversion of solar energy from DC to AC for use in the home (only converts what is needed), battery charging and discharging, control of the batteries, and supply to essential loads (in the event of a power cut) via an in-built Uninterruptible Power Supply (UPS) or Emergency Power Supply (EPS) if either function is included in the inverter (with UPS being a higher level of back-up supply).

A benefit of hybrid inverters is that the system has a 'single brain' handling the full operation of the system, while AC coupled battery systems involve two systems (solar and battery) working separately from one another but with technology that allows the battery to know when to charge and discharge.

An AC coupled solar energy system

AC Coupled Solar Diagram

A Hybrid inverter solar energy system

Hybrid Solar System Diagram

2. Battery charge and discharge rates

When choosing a hybrid inverter or other battery based solar energy system, you may look at the size of the inverter's solar generation conversion capacity and determine it's sufficient for what you want to achieve. While this is an important factor, unfortunately many people adopting battery based solar energy systems easily miss or dismiss the maximum battery charge and discharge rates of the inverter. These maximum charge and discharge rates control how much power can be charged to and drawn from the battery at a single point in time - this has a great impact on the performance of a hybrid or any other battery based system.

Below we can see an example of how the maximum charge rate affects the performance of a hybrid solar energy system. Assume in the middle of the day, your system generates 4.5kW of electricity, your internal usage is not high and is only consuming 0.5kW of the generation, leaving you with excess solar generation of 4kW.

solar with battery system with max charge rate 2kw diagram

If your hybrid system has a 2kW maximum charge rate, you can only charge 2kW of the 4kW excess solar generation to the battery, with the remaining 2kW being exported back to the grid (typically earning a small credit on your electricity bill), even if the battery is not fully charged.

solar with battery system with max charge rate 4kw diagram

If your hybrid system has a 4kW maximum charge rate, you then have sufficient capacity to charge all 4kW of electricity to the battery and avoid sending the excess generation back to the grid.

With a higher maximum charge rate, a hybrid inverter can better utilise the excess generation and lower the chance of exporting to the grid before the battery is fully charged.

This maximum charge/discharge rate also impacts on the ability to deliver energy from the batteries to higher electrical loads in the home.

Solar Inverter with 2kW Max Discharge Rate Diagram
Solar Inverter with 4kW Max Discharge Rate Diagram

If you look at the example in reverse - any electrical loads greater than the 2kW that can be delivered from the battery would need to be met by using energy from the grid. Again, this minimises the effectiveness of the solar energy system.

3. Back-up load capacity (limitation of power to essential loads)

Another popular benefit of a hybrid solar energy system is the back-up function (where available). Many people may think that a system with back-up means they can have their whole house supported by the solar battery when there is a power cut. However, this is not how it works. The back-up function is actually enabled by the dividing the output of the hybrid system into two circuits:

One is for the essential loads that you want to keep functioning during a power outage (back-up loads). When wiring the back-up loads, it actually functions as a mini 'off-grid' capable system. However, power delivery limitations mean this circuit cannot be overloaded or it will be at risk of tripping (except for any "surge capacity" available for a short time - we will talk about this later).

Another circuit is for all of the remaining loads in the house (normal loads) that are connected directly to the grid and will immediately lose power when there is a power outage.

Hybrid solar system backup circuit diagram

Due to this, a larger back-up load capacity will allow you to continue using more of your appliances during a power cut - consider this in the same fashion as a system with a higher available charge / discharge rate. While you may think 'why don't we get a hybrid system with all of the loads wired to back-up?' In theory this is possible, but the cost will increase dramatically because you are actually asking for an off-grid system to support the whole house and connecting it back to the grid.

4. Surge capability for back-up loads

As mentioned above, with a hybrid solar power system, there is a maximum load that can be put onto the back-up circuit and the system will actually trip if overloaded. However, with some appliances the immediate load will rise for a short period of time when it starts up.

For example, if you turn on a 1kW household water pump, it can draw 3 to 4kW of load for a few seconds and then return back to its normal 1kW draw. With this in mind, the majority of customers don't want the risk of this or a similar event triggering the tripping of their system. Based on this, it is important to have a good level of surge capability to handle these short-term surges and avoid system tripping.

It pays to be wise when choosing a hybrid system, including evaluating the surge capacity and the time allowed for overload, otherwise you may experience the pain of your solar energy system tripping out under low loads.

5. UPS and EPS

Another aspect of the back-up function is the way of disconnecting the back-up circuit from the grid in the event of a power cut. Imagine if you are in the middle of drafting a document and there is a power outage, the last thing you want is for your computer to switch off and for your document to be lost. This speaks to the difference between UPS vs EPS.

UPS - Uninterrupted Power Supply. As its name says, UPS automatically disconnects and switches the back-up circuit from its grid connection to only being powered by solar and batteries. This will happen immediately once power outage is detected, without stopping the power supply to the back-up circuits.

EPS - Emergency Power Supply. Operates in a similar manner as above in wanting to support back-up loads in a power outage, however, it first needs to disconnect the inverter from the grid and then create an internal mini-grid. This can take anything from a few seconds to a minute or more to achieve.

If you are looking for a system that gives an immediate cut over and a truly carefree experience in a power cut, then a UPS based system is what will provide this.

Be wise in choosing your solar battery system

Having a solar energy system with solar batteries is a valuable investment that not only provides good returns, but can also give you the LiveFree experience that so many people are after. You just need to be wise in choosing a suitable system that can meet your requirements. We highly recommend that you ask the above five questions to your solar energy company - when asked and answered correctly, you will have a well-rounded understanding of the major functions of the systems offered to you.

If you want to know more about which systems suit you the best, please feel free to fill in the form below and we will be in touch to provide you with more information.

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