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Do I Need a Battery in My Grid Tied Solar System?

Most people are aware that solar power is only available when the sun is shining. So they ask themselves a question; how can I store the Solar power produced during the day so that I can use it at night, or during overcast periods? To do this, a reliable battery power storage solution is needed. In this article, we will take a closer look at the main considerations when adding Batteries to a Grid Tied Solar System.

Normal Grid Tied Solar System

In a normal Grid Tied Solar System, any Solar power produced is used by the house loads, and any excess is exported to the grid. When the Solar System does not produce enough power for the house loads, then power is imported from the Grid. Electricity companies typically charge about 3-4 times more for imported Grid power than they pay you for exported Solar power. This reduces the financial viability of a Solar System, and increases the payback period. The other main disadvantage of this type of system is that if there is a Grid outage, then the Solar System also shuts down and the home has no power.

Use of Solar Power Storage

Solar batteries are a great way of storing excess Solar power for your own use. This offsets the amount of Grid power that you are importing, so it reduces your electricity bill further. Because of rising grid power prices, coupled with improving technology, battery storage for Solar systems is becoming increasingly popular in New Zealand. 

How Solar Batteries Work 

When batteries are installed as part of a Solar system, they can store excess Solar power, rather than exporting it to the grid for a low return. Your home can then draw on the power stored in the battery as it needs – at night, during overcast periods, or during times of high power usage.

The Benefits of Using Solar Batteries 

If your budget allows for it, we highly recommended installing batteries with a grid-tied Solar system. While there are other options, batteries are the optimal solution. Here is why. 

Consume the Power Instead of Sending It Back to the Grid

If you do not have solar batteries installed to store energy in a grid-tied system, any excess power is sent back to the grid. Electricity retailers usually pay very little for this. For example, you may be paying 35 cents for every unit of electricity you buy from your Electricity retailer (import), when you sell them back the excess power from solar (export), they will generally only pay you around 7 to 8 cents. With solar batteries, this issue can be decreased or avoided, as the excess Solar power you produce will be stored in the battery for your own use, rather than sold back to the grid for very little money.

Choose your Solar Inverter carefully.

A Solar Inverter converts the DC power from your Solar panels into AC power that can be used by your home. Virtually every modern Grid Tied Inverter manufactured since 2015 can claim to be “Battery Ready”, however this can be a very misleading term. This does NOT mean that you can connect a Battery directly to a standard Grid Tied Inverter. All it means is that you can add an expensive AC coupled battery, ie; a specific type of battery with its own additional Battery inverter. Apart from this being a very expensive option, there is a very limited range of these available and they have a lower efficiency, so AC coupled batteries are not a viable option for many people.

A better option is to buy a Hybrid Inverter – this allows a battery to be directly connected to the Inverter. This is a much more cost effective and efficient option and there is a wider range of batteries available. Broadly speaking, there are 2 types of Hybrid Inverter.

1)     Some basic Hybrid Inverters use excess solar power to charge a battery & just allow you to use it as required. However, should there be a Grid outage, this type of system will also shut down leaving the home without power.

2)     A better type of Hybrid Inverter is the Backup type. This will have an additional Backup Output which will continue to function during a Grid outage, making use of Solar and stored Battery power. This Backup Output will normally be wired to a separate “Essential Loads Switchboard” for some basic loads that you would need to continue functioning during a power cut such as lights, fridge, water pump/septic pump, internet, gas pilot lights, etc.

Increase Your Power Supply Resilience

The grid in New Zealand can sometimes be unreliable depending on where you live. Installing solar batteries can mean that you have some protection from power outages. You no longer have to worry about the next time the grid goes offline. Instead, you can have a reliable power supply back up, no matter the time of day. However, this only works if you have a Hybrid Inverter with a Backup Output and if you have essential load circuits connected to it. Otherwise, your solar system may shut down along with the grid.

You Do Not Have to Change Your Schedule  

People with a solar system without batteries often have to optimise their energy consumption habits to use electricity when the sun is shining. While this may work for some people, not everyone has this flexibility. For instance, some people might not be able to do the laundry when the sun is shining brightly, because they are at work. Those who have less flexibility in their lives may consider installing batteries.

Why choose Lithium Batteries

Although there are different types of battery storage technologies available on the market, Lithium batteries are by far the most popular option when it comes to grid-tie solar systems. The reason for this is that they have a longer lifecycle without losing much of their capacity. Additionally, these batteries tend to have a greater Depth of Discharge (DoD) of up to 95%,  where lead acid batteries typically only allow a DoD of 50%, and Lithium Batteries are physically smaller. Lithium batteries are also more efficient, with the best brands only losing 5% of the energy stored trough the charge and discharge cycle, compared to a typical 15% for lead based batteries.

Some considerations when choosing Lithium batteries are;

1)     Safety – LiFePO (Lithium Ion Phosphate) is the safest type of Lithium battery available.

2)     Compatibility. Most Hybrid Inverters will only work with a specific range of approved battery models. You need to ensure that the Lithium Batteries that you choose are compatible.

3)     Storage capacity – This is measured in Kilowatt Hours (kWh). For example, if you have 10kWh of usable battery storage, and you are powering a 1kW heater, then you can run it for 10 hours.

4)     Depth of Discharge (DoD). This is the percentage of the batteries capacity that you can use. For example, if you have a 10kWh battery with a DoD of 95%, then you have a usable capacity of 9.5kWh.

5)     Maximum Power Draw. This is the maximum rate at which you can draw power from a battery measured in Kilowatts (kW). Ideally, this should be at least the same as the Hybrid Inverters power rating.

6)     Lifespan. Many Lithium batteries will have a life expectancy of over 6000 full cycles, or 15-20 years. As the battery ages, it’s capacity will reduce slowly, so that at the end of its useful life it may have a capacity of 60-70% compared with its new specification.

7)     Warranty. A good quality Lithium battery will typically come with a 10 year warranty.

8)     Reliability. Lithium batteries are still a relatively new technology and for that reason the Battery Test Centre in Canberra have tested most of the well know Solar Lithium batteries on the market.  We only stock brands that have passed, we highly recommend reading their latest testing reports here.

Other benefits of Lithium batteries are the excellent cycling, low internal resistance, and high efficiency. All of these advances make Lithium the perfect option for high-demand applications. However, it is important to know that Lithium batteries do not like low temperatures, especially below 5 degrees Celsius, so it is important to correctly position them.

Always Size Batteries Correctly 

One important consideration is sizing. It is important to ensure that the solar array can produce enough power to charge the batteries. Doing so ensures that the entire system proves to be of great value for money. 

While closely related, the battery and solar panel size should be calculated independently. The size of the solar panel array depends on the energy consumption of the home, while the battery size is determined by the amount of autonomy needed in a home, inverter size, and a few other technical parameters.   

For example, when sizing battteries for a home, the rule of thumb for the minimum capacity is double the size of the inverter. So, if the inverter is 5kW you will likely need about 10kWH of battery storage at a minimum.

If your objective for installing the battery is to be partially independent from the grid, then sizing comes down to how long a homeowner wants the batteries to supply power when the grid goes down. When sizing the batteries, it is important to understand the average energy consumption in a home. For instance, if a home consumes 25kWh a day for 24 hours, the battery bank size of 25kWh would cover one day consumption. This size of the battery would be very large and expensive though. So, we recommend separating the wiring in the house to have an Essential Load Switchboard with loads such as refrigeration, lights, water pumps and other essential appliances connected to it. If the essential loads consume less than 5kWH per day, then 10kWh of battery storage will give you back up power for 2 days. The Solar panels will also top up this energy depending on the sunshine available. Please remember that the Hybrid inverter needs to be capable of working without the grid.

Optimal size will depend on the budget, the autonomy time needed, and the size of the essential load circuits. The capacity of the inverter and solar panels will also determine the optimal size. It can be a complex part of the design, so we recommend engaging with an experienced solar company. 

Alternatives to batteries

If the budget doesn’t allow for immediate installation of the batteries, we recommend purchasing battery ready inverter like this Solis Hybrid unit.

There are other ways to benefit from a solar system without batteries. 

Consider Using a Hot Water Cylinder Timer or a Hot Water Diverter

In New Zealand, electric hot water heating typically accounts for up to 35% of the power bill. Consequently, any measures take to optimize hot water heating could have a significant impact on the energy consumption of a home, which could ensure that stored solar energy last longer. One of the first measures to optimize hot water heating is to use a hot water cylinder timer.

A water heater timer allows the homeowner to determine when the electric water heater will run. When the timer is set to use electricity to heat water during sunshine hours, there is a greater opportunity to use Solar Power. The timer does not know how much Solar power is being produced during those hours, so it is not perfect; it is a simple way of giving the home a greater opportunity to use Solar power.

A water heater needs around 30 minutes to a couple of hours to heat water in the storage tank (depending on the size of the tank, age and capacity of the heating element). Once heated, the water can store the higher temperature for many hours, if the tank is properly insulated. When properly timed, the water heater can provide enough water for a home’s showering and cooking needs for daily usage. Without a timer, the same water heater can run when the sun is not available, which can significantly increase a home’s power bill.

To improve efficiency, the water heater can be connected to a Solar diverter. A Solar Diverter measures how much excess Solar power is being produced at any given time, and diverts exactly that excess amount of power to the hot water cylinder instead of exporting it to the grid. One of the best options in New Zealand are Hot Water Diverters from SolarEdge. These are used in a Grid tied solar system along with a compatible Solaredge Inverter.

The main benefits of using a solar diverter is that you redirect the power you produce to store it in your water cylinder, instead of sending it to the grid. As mentioned above, utility companies pay very little for the solar power you send them. As a result, using as much of the solar power you produce as possible gives you the best possible savings and return on your Solar investment.


If you have already installed a solar system without battery storage, an expert can sometimes retrofit battery storage into the system, although it may be costly. If you are planning to install a new solar system, you might want to consider installing batteries as part of the system or installing a Hybrid inverter that will allow the addition of a battery. It will ensure you get to maximise the potential benefits from your solar system installation. If you can’t afford batteries now, an easy way to use the excess power from PV panels is by installing a hot water diverter or to put hot water on a timer.  

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