Living the Dream

Solar P.V (Photo-voltaic) refers to the panels on your roof which are then ‘tied’ or ‘coupled’ to an inversion system. A ‘grid tie solar PV’ system is the technical term for your typical domestic or commercial solar system and is commonly made up of panels and a single inverter. This system converts or inverters DC (direct current) electricity produced from the solar panels into AC (alternating current) Electricity, to be used within the home at 240v A.C

This is where we run the solar P.V into a regulator, which also acts as a battery charger (D.C to D.C). This then connects directly to the batteries, which are then connected to the hybrid inverter. This is an older more traditional or typical structure for a full off-grid system in years past.

Modern Off-grid systems are now AC coupled or a maybe a mixture of AC plus a small amount of DC coupling.

So now that we’re all on the same page where to from here?

What is the right system for you? Which batteries should you use? Is it cost effective and why install hybrid Solar?

For most people, the common scenario is going to be an AC coupled system using a Hybrid inverter, such as a “Selectronic” inverter or “Redback” Inverter – Both Australian Companies. A system like this can be selected as an add-on, and allows for complete off-grid functionality at a later date. There are a number of design considerations to look at when selecting a hybrid system. These can have a dramatic effect on operation and costs involved so should be considered closely.

It is important to look at storage not just in terms of peak overnight use, but also in terms of ‘days of autonomy.’ This refers to the number of days one expects to be without sun and usually varies between 2-4 days. Storage is generally measured in amp hours, but can easily be converted to kWhrs by multiplying amp hours by the battery bank voltage. For example, a 200AH battery at 12V gives 2.4kWhrs of storage. Most modern systems are now designed in 48v, with luxury homes systems having 120v battery banks.

OK let’s look at a typical Australian home using 18kWhr’s per day with roughly 8-10kWhr’s of that at night. This home has the following features;

1 Electric storage hot water

2 Electric Oven

3 Gas or electric hot plates

4 Ceiling fans

5 Intermittent AC used only during the day

6 Fully featured AV, TV and computers

7 Lighting

The first thing is to ensure that the electric hot water and Air conditioning where possible are set to run during the day. This allows us to put up more highly cost effective solar P.V, and at the same time reduce the required battery capacity. We then use these appliances as a load for excess P.V generation meaning that we don’t ‘export,’ as the power is either used into your appliances. This allows us to factor most production against the tariff rate of $0.30c per kWhr inc GST. There is no need for night/economy tariff’s as the loads are allocated against P.V, which has an installed cost of around 7c per kWhr.

The next trick is to maximize PV capacity to ensure that even on cloudy days we are still generating ample amounts of power. For some people this won’t be possible due to a lack of roof space, however once all roof faces are considered most homes can fit the required 5 to10kW of panels. To ensure that we have enough generation, the minimum is 1kW of panels installed for every 1kWhr of storage. This is a rough guide and depends on day time usage. If Roof space is limited, it may require us to use high efficiency solar panels, such as our premium SUNPOWER panels in your design.

This refers to a standard grid tie system ‘coupled’ to a hybrid inverter, which then charges batteries and outputs to the home and/or grid. We expect this type of system to become the most popular, as it will be a great way to upgrade from existing P.V and it is also extremely cost effective as a new install.

This is the highest amount of power one expects to draw at any one point in time usually measured in kW or watts, but sometimes in amps. Since peak is a limiting factor in any hybrid or off-grid installation it makes sense to ‘load shift’ devices, so that they’re not all on at once. For example, you may tell the hot water system to heat, followed by the next highest load – all outside of peak (ie cooking hours). It may also be beneficial to change appliances such as electric stoves to gas, and it will certainly make sense to change the lighting to LED. The latest Inverters can deliver a high sustained load or short term peak load. This will handle the average home right up to Luxury homes and for larger installations again they can be chained together to make a three phase system.

There are a lot of battery technologies available on the market with massive differences in performance and price. It is important to consider firstly the application of the system and then also its requirements in terms of discharge and life cycle. If one is designing a backup system which will be rarely used, then ‘number of cycle’s’ becomes less important, as the system is only intended to take infrequent shallow discharges. This may be suitable as a gate opener, or for a holiday home, and is commonly used in telecommunications systems as secondary backup.

For the home or business however, the batteries will be discharged on a daily basis and it therefore becomes important to consider how many times this can occur before the batteries will begin losing capacity, or fail altogether. Finally one needs to consider the rate of discharge and any environment factors such as physical space, temperature and weight.

The home in the above example may run Air conditioning partially off mains power, which makes this a ‘hybrid’ rather than true ‘off grid’ solution. If they wanted to run electric hot plates and the AC that would be possible, but it would also incur significant additional costs. The rest of the home will run comfortably off P.V and batteries some 360 days of the year with the remaining 5 darkest of days drawing some power from the grid. Once again for full autonomy one could increase the battery size, however this once again increases the initial outlay.

Therefore the most cost effective method is to operate hybrid solar, which would give us total system costs starting at approximately $10K giving a payback period of 5 to 10 years and an ROI of about 20+% p.a. It’s important to remember that the life cycle of the system is 15+ years with most modern battery systems

It is also worth remembering that electricity prices will not remain the same. If electricity prices rise then the final result will be far more beneficial. Furthermore at the 15year mark, the batteries may need to be replaced, however the panels will continue producing for up to 25 years resulting in a second investment cycle with much higher returns.

Hybrid Solar Calculator

The calculation of hybrid solar production values and hybrid costs is extremely complicated and involves both modelling software and very detailed spreadsheets. If you are interested in a hybrid solar calculator specific to your requirements, please contact us and we will model something for your specific situation.

Hybrid and Off-Grid systems if configured and or installed incorrectly can have a high failure rate. If you are looking at purchasing any type of battery enabled system a ‘cheap’ solution will almost certainly result in a bad investment with equipment failure and ultimately need replacing or expensive repairs.

ACDC Energy are battery system specialists and as part of our design process we provide detailed production estimates guaranteed in writing thereby ensuring that you get what you paid for , but most importantly you get a system that will perform as required year after year.