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If you are living in an average house in Sydney and considering going totally off-grid with batteries, you are likely to find that it is neither practical nor economical. There are three reasons for this. Firstly, the technology of batteries is not yet sufficiently developed. Secondly, the weather. Thirdly, you probably don’t have a big enough roof. This article is intended to show how you can calculate the situation for your own circumstances.

Of course, each situation is different. The below detail will help you with making the best informed decision.

Power and Energy

In considering solar and batteries, most people talk about energy and this leads to a great many misconceptions.  When you use electricity, you use power, which is measured in kilowatts.  When you use power over a period of time, the product of power and time is the energy.  The unit is kilowatt-hour which is one kilowatt used for one hour or half a kilowatt used for two hours, and so on.

Sizing the battery

To size a battery, you need to consider both energy and power.


In our experience, the average house with a pool uses about 11 megawatt-hours of electricity a year (energy) .  The power drain at any one time depends on what appliances are operating.  Here are some typical appliances:

So, you need to be able to draw instantaneous power of maybe 20+ kilowatts if you are having a party.  With each battery inverter providing three to four kilowatts out, this means you need five-seven battery packs at $10,000-$14,000 each, at the time of writing.


Of the 11 megawatt-hour annual energy consumption, it is important to note that the pool is usually the largest appliance in the house and amounts to about 40% of the electricity of the house, just for filtering and chlorinating.   The remainder of the power is used mostly in the morning at breakfast and in the evening around dinner time.  Solar PV generation, if you have it, is mostly in the middle of the day, when most people are not using much power at all.

The 11 megawatt-hour p.a. averages out to 30 kilowatt-hours per day, but of course, not all days are equal.  Some days have heavy electricity demand, some have light demand and sometimes you are away on holidays with essentially no demand.  So, you really need to budget for the large usage days, such as Christmas or party days, and allow a peak demand of say, twice the average.  So you need, say 60 kilowatt-hours of storage to stay off the Grid on any one day.

Based on the list of appliances above, it would not be hard to argue that it really should be at least 100 kilowatt-hours for those special days.

You can’t fully discharge a battery and, even if you could, the storage capacity of each battery reduces as it ages.  An eight year old battery is unlikely to give you more than 70% of its rated capacity.  So, taking both the age and the fact that you cannot fully discharge a battery into account, if you want 60 kilowatt-hours out after eight years, you will need close to a 100 kilowatt-hours on the nameplate when you get the battery.  If you want a 100 kilowatt-hour out, you are looking at 150 kilowatt hours on the nameplate. That will last you for one statistically possible day.

With battery capacities of about 10 kilowatt hours per battery pack, accessible energy, you need 10-15 battery packs at $10,000-$14,000 each at the time of writing.

These require a lot of space and under current Australian Standards, you must locate these outdoors in case of fire and explosion.  Many people feel that this is unreasonably restrictive, however, batteries do store a great deal of concentrated energy and you need to think carefully before putting a lot of stored energy into a house.  It is also worth noting that you cannot put a Lithium fire out using water as water accelerates the reaction.

Charging the battery

You then need to think about how much energy you must put into the battery to get 60 kilowatt-hours out again.  If the battery is new, the answer is probably 75 kilowatt-hours as the battery will lose energy during both charging and discharging.  This evidences as heat in the battery and the inverter.  If the battery is in a battery room, the room will likely need fan-forced ventilation as this heat will take it past its allowed operating temperature range.  You will need to power these fans and adding five kilowatt-hours capacity for that, is a good idea.

You can charge the battery using solar panels or from the electricity Grid, ideally using off-peak power.  There are 109 sunny days a year in Sydney on average, 129 rainy or overcast days and 127 days with at least 25% cloud.

On the 109 sunny days, you need the PV panels to provide enough energy to fill the battery by about 3 p.m.  To supply 100 kilowatt hours, as well as your other needs during the day while you are charging the battery, you need about 22 kilowatts nominal PV capacity.  Very few people have a roof large enough for this and most domestic users are restricted to 10 kilowatts in panels in any case. On the overcast days you will get very little energy while you will get some on the intermittent days.  If you want to allow for 2 weeks of rain, as sometimes occurs in Sydney, you will need a very large amount of batteries if you are totally off-grid….and some way of charging them.  You need a massive roof and a commercial grade solar panel installation if you want to do this using solar panels.  That is why many off-Grid solar/battery installations in rural areas, where the economies are different, have back-up diesel generators.

It is interesting that the first off-grid battery installation was in 1880 (in a Spanish submarine).

Partially off-grid

Of course, most people do not go totally off-grid in urban environments but supplement their power using solar PV and batteries.  If you do the arithmetic on this alternative, you will find that PV generation can be useful in some circumstances, but that it is very difficult to find any circumstances when batteries cost-in at the present stage of the technology. It is just too early.   Batteries have a life of about eight years and a pay-back time which is usually a significant multiple of that.   Simply put, the technology and cost have to improve by large amounts before batteries make financial sense for most users for either off-grid or partial off-grid applications.  The concept is fine, but the costs and storage capacities are not yet there for most users.


Instead of storing off-peak or solar energy and using it later, on peak,  why not just reduce the amount you need?

Pooled Energy offers a service that reduces the 40% of total household electricity used by the pool, to as low as 10%, while greatly improving the quality and healthiness of the water.

Pool owners pay just $330 up-front to automate their pool under Pooled Energy’s system, as contrasting with capital expenditure of $5,000 for equivalent but less capable, conventional pool automation.  They also need about $250 for an initial supply of pool chemicals which can last for up to a year.  This is usually a neutral cost as it replaces their usual top-up of their existing chemicals.

The on-going service, which usually more than pays for itself in savings, is just $67 in pool service fee per month. An Intelligent Pool Controller, sensors and variable speed pumping are installed and Advanced Water Chemistry are used to simultaneously reduce energy and chemical use while delivering spectacular, healthier water.  Pooled Energy also ‘pools’ all of its managed pools together to help off-load and stabilise the electricity Grid, especially in times of power shortages and peak demand.  There is no cost or penalty to pool owners as the pools are managed to be biologically safe with very high quality water.  To achieve this, Pooled Energy becomes the electricity supplier to the whole house and customers purchase their electricity from Pooled Energy at standard market rates.  Pooled Energy does not offer temporary discounts but offers permanent savings, often up to 30% in electricity use, as well as substantial chemical and support cost savings.  Pooled Energy supplies pool owners and is dedicated to them!

Batteries in the future

Energy Storage is an essential part of the Grid and of enabling high levels of intermittent, renewable energy.  Batteries will be an essential part of that, and Pooled Energy will both supply and integrate batteries into its systems into the future.  Widespread use however, will require considerable improvements in the technology and cost of batteries to make them economically viable for most users and hopefully this article will give an idea as to how you can objectively calculate whether a battery is good for you or not.

We’re here to help! For more information call us on 1300 364 703 or email