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How to select a home solar battery or off-grid power system

Jason Svarc

This article is designed to help the general public and solar installers understand the capabilities of solar battery storage systems, as well as an introduction to system sizing and the costs involved.

The huge leap forward in home battery storage technology has seen an immense amount of interest in people looking to go off-grid, store their own solar energy (self-use) and become energy independent. Although the rapid pace of technology development has resulted in much confusion over what is achievable and which system is best suited to an individual's needs. While trained solar installers offer very good advice, much of the public have little understanding of the various solar / battery storage system capabilities and limitations.

 Common questions...

"Which battery or hybrid storage system is best?"

"What size system do I need to go off-grid?"

"What battery capacity do I need?"

"How many solar panels do I need?"

These might sound like straight forward questions but they are not easy to answer simply because there is no one-size-fits-all solution. Every households energy needs are different and there are a huge variety of hybrid and battery systems available.

For those not familiar with hybrid solar and home energy storage read this first. Also you can learn more about the many different hybrid/off-grid inverters and batteries here.

I have found one very effective way to answer these questions, especially for those not well informed is to describe several scenario's which would apply to the modern 'efficient' Australian home. (The true method to accurately size a system is through load analysis on the home, but this level of detail will be discussed in another article). Before we describe the various options it is important to determine exactly what you want to get out of a solar battery system.

Why install a solar battery system?

For many people it is quite simple; to reduce energy consumption from the power grid and store solar energy for later use, but there are quite a number of good reasons.

  • Reduce cost - save money by using less energy from the grid
  • Reduce emissions - reduce pollution from coal/nuclear/gas power generation
  • Be blackout safe - Have power available during a blackout or emergency
  • Become energy independent - operate off-grid where possible
  • Reduce peak demand - peak lopping and grid stability **

Depending on how you answer these will point you towards one of the 4 following systems. As expected they become progressively more expensive with the first option being the cheapest.

Note: prices are in Australian dollars and are estimates only - including all parts and labour for a hybrid solar installation on a efficient 3-bedroom home with a typical energy use of 12kWh per day. If your energy use is much higher, for example double this (24kWh per day) then the cost is not necessarily double as much of the installation cost is the same, although it would be around 65-85% more due to the cost of additional solar panels and batteries.

The four common solar battery system types

  1. Hybrid solar system for generation and self-use ($9-12K)

  2. Hybrid solar system for self-use and limited back-up power ($12-15K)

  3. Hybrid solar system for maximum self-use and back-up power ($15-20K)

  4. Hybrid/Off-grid solar & complete energy management system ($20-50K)

Click on the options above to see full details, capabilities and cost breakdowns.


Energy Efficiency

Naturally if you build an more efficient home which consumes less energy the costs will be lower. Unfortunately most Australian homes are very inefficient and use around 20kWh or more per day. The amount of energy used per day (measured in kWh) should be listed on the electricity bill and the more information you have about your energy use the better. If you can obtain peak, shoulder and off-peak energy use or preferably a daily energy profile (using an energy monitor) will result in accurately sizing a system to suit.

There are many ways to optimise solar use such as running electric hot water systems directly on your solar energy during the day rather than using off-peak power. Creating an more efficient home may mean you can achieve more savings by just using a standard gird-connect solar system rather than installing a battery system on an inefficient home.

Efficient heat-pump hot water unit

Efficient heat-pump hot water unit

For information about how you can reduce your energy use and create a more efficient home refer to this article on our companion site.

It is often worthwhile investing in new, more efficient appliances like fridges, freezers and solar hot water systems to reduce total energy use and get the most out of a solar / battery system.

Do I need batteries to take advantage of solar?

For many people the cost of a battery system is simply too high, but this doesn't mean you can't take advantage of solar for your home. For those who are generally at home during the day or work from home it can be far more economical to install a common (lower cost) grid-connected solar system and use the solar energy directly as it is being generated. For example it is possible to run energy intensive appliances such dishwashers, air-conditioners and washing machines (not all at the same time) for free on your solar energy during the day. Many newer solar multi-mode inverters have inbuilt ‘demand management’ controls which can automatically switch on ‘smart’ appliances when there is excess solar energy, rather than sending it to the grid for very little return. Also a number of new grid-connect solar inverters are 'battery ready' so it will be easy to add battery storage to your home in the future.

By doing this you are effectively using your hot water as a battery, with the energy stored as heat.

Electric hot water systems are extremely energy intensive and often account for around a third of annual energy costs (unless you have gas hot water). Electric element hot water systems are usually set to heat your water using cheaper off-peak electricity but a solar system together with a clever device to divert your excess solar energy to your electric hot water system can maximise your energy generation, saving you money. By doing this you are effectively using your hot water as a battery, with the energy stored as heat. There are several well known solar diverters available such as those from ImmerSun and AWS SunMate.

** How can home batteries prevent grid wide blackouts?

It is not well known that smaller household batteries have the ability to stabilize the grid by selling power back during peak times (such as during heatwaves) thus eliminating the need for expensive gas backup power stations used only several days a year. This works by having a 'remote' control system which can feed energy back into the grid from multiple home and commercial battery systems simultaneously. An Australian startup called 'Reposit Power' are already using this model and the German battery manufacturer 'Sonnen' is also setting up a similar system.

Various solar hybrid inverters - A quality, reliable inverter is essential to a setting up a solar hybrid system

Various solar hybrid inverters - A quality, reliable inverter is essential to a setting up a solar hybrid system

Going Off-grid

To answer the common question about going completely off-grid I would say to most people "if you already have the grid connected to your home then it makes no sense to go completely off-grid simply because the cost is too high; you will need a very large battery system to store energy for 3-5 days of bad weather along with a quality auto-start back-up generator for the winter months (especially in colder area's)". There is also the cost of fuel to run the generator, maintenance, etc. so for most people it just doesn't make economic sense. Why not use the grid as your back-up power source and then feed your excess solar energy into the grid for some return?

Still want to stick it the man and go off-grid anyway? If you are determined to become energy independent or you are planning on building an off-grid home we have put together a building off-grid guide on a companion site - Go hybrid/off-grid

Off-grid location

If you are living or have built a standard home in an off-grid location then I would recommend an advanced mulit-mode or interactive inverter system. There are many cheaper hybrid systems available which can operate off-grid but most are only suitable for smaller cabins and summer use. Read more information and reviews about the advanced off-grid systems here.

Back-up Generator

Living full-time off-grid means that for most of year you can rely on your solar/wind generation but there will probably come a time when the weather turns sour and you just don't have enough juice left in the battery. Also at some stage there may be a fault or repair needed in the system so you will usually need some kind of back-up generator. In the temperate, colder climates where the winter days are much shorter (depending on your energy use) you will generally need to run a back-up generator at least once a week to top up the batteries during the few winter months. If you have continuous bad weather for a longer period of time then the generator will have to run for longer. Most advanced mult-mode or interactive inverters have in-built generator control systems to automatically start and run the backup generator.

Wind turbines

This is a tricky one and a question I get asked very often is can I use a small scale wind generator instead of solar. The short answer is no, unless you live on top of a hill or by the ocean with consistant strong winds. Large scale winds turbines work extremely well and are located in area's which have been recorded to have consistent strong winds. Small scale residential turbines generally have poor performance due to the location. Many people assume the wind is well suited when in fact it is usually very turbulent and not efficient for a small scale turbine. In order to achieve full output from a turbine it needs to be installed at twice the height of the surrounding trees and buildings. Then there is the cost, a small 600W turbine can easily cost $5K installed on a 15 meter pole. If you compare this to solar, for the same cost you could purchase 18 x 250W panels (4500W) installed on a tilt system, plus solar panels have no moving parts and virtually no maintenance, so the numbers speak for themselves. Although there are situations where a combination of wind and solar will work extremely well.

Learn more about hybrid solar systems and battery technology here


The complete guide to solar storage batteries for off-grid or hybrid systems

The complete guide to solar storage batteries for off-grid or hybrid systems