Build your own ultimate 7.5kWh transportable power station

Owen Hill describes his beginner-friendly, modular energy storage system that requires no hard wiring and can allow you to make use of the three hours of free electricity each day under the new Solar Sharer Offer.

Over four million Australian homes currently have rooftop PV solar systems, and many are now considering installing home battery storage utilising the Cheaper Home Batteries Program.

The Australian Government announced the Solar Sharer Offer scheme (SSO) to commence operation on July 1, 2026, making at least three hours of free power available to all electricity users with a smart meter who have registered with their chosen electricity retailer. This is a welcome opportunity for Australian householders to save on energy costs, including apartment dwellers, renters and those who, for any other reason, are not able to directly benefit from installing rooftop solar PV panels.

Renew 164 published an article by Lance Turner, “Deleting the Genset”, which described a compact, portable 12 volt battery backup system with a built-in 1200 watt inverter based on lithium iron phosphate (LFP) battery storage. This is an example of a 2000 watt-hour battery storage power station that replaces the need for a petrol-driven generator. Such a system could be recharged by plugging the battery charger into a plug-in time switch set to operate during the designated three hours of free power available under the new Solar Sharer Offer. Simply set the time switch to turn on at, say, 11:00am and off at 2:00pm every day.

My system
My recent Ultimate 7500 project described below is intended as a basic 7.5kWh capacity solar battery/inverter modular unit that can be expanded as future needs emerge. It serves three purposes:
1) Battery storage for those registered on the Solar Sharer Offer
2) Emergency backup power unit for protection from grid power outages
3) An ideal modular DIY project for those wanting to learn more about solar power systems and who may wish to expand the system to meet their future household needs.

This article describes a modular, beginner-friendly DIY 24 volt, 3000 watt, 7.5kWh plug-in battery backup storage unit that requires no permanent installation and is ideally suited to renters, apartment owners and others who, for any reason, might not have access to existing rooftop solar PV power. With minor modification, it could be readily adapted to provide portable electrical power for sheds, caravans and motorhomes. Ultimately, you might decide to engage a qualified electrician to connect the system permanently into the home’s electrical switchboard.

As previously noted, this project is also designed to appeal to those who may want to learn more about the emerging solar renewable energy market and might be looking for a small project to start saving on home energy costs, and that can be readily expanded, as budget allows, to meet future needs or as new technology improvements arise.

System components
This system is built around the Victron MultiPlus-II 24/3000/70 inverter/charger system. While this popular MultiPlus is not the lowest-cost solar inverter/charger on the market, it is widely supported by an extensive local network of trained distributors and comes with a five-year warranty. Victron also supplies detailed manuals and extensive support documentation. In addition, there is a large community of DIY YouTube videos demonstrating the use of Victron products. The Clean Energy Council (CEC) recently updated its approval for the Victron MultiPlus-II to comply with Australian Standard AS/NZS 4777.2:2020.

Important note: this project is intended strictly for off-grid operation only and must never be connected to supply any 230-volt AC power back to the main household energy supply grid.

Some smaller plug-in solar power systems available on the local market supply only limited power, such as 1000 watts, often with limited storage capacity. This ‘plug-and-play’ power unit project requires no installation and its storage batteries can be recharged from a single plug connection to any existing 10 amp household electrical circuit. It produces enough power to operate virtually any household appliance that can be plugged into a standard household 230 volt, 10 amp general purpose outlet (GPO). Subject to the actual energy consumed, it can provide emergency 230 volt backup power for up to several days.

Project Description
The Victron MultiPlus-II inverter/charger is powered by a 24 volt, 300 amp-hour capacity lithium iron phosphate (LFP) battery. Lithium LFP batteries are both lower cost and not nearly as heavy as older lead-acid batteries with the same capacity.

Despite that, a single 24 volt, 300 amp-hour lithium battery weighs around 50kg and can be expensive to freight, and is probably too heavy for a single-person lift. A solution is to use two separate 12 volt, 300 amp-hour batteries (each weighing a more manageable 24kg) connected in series using a 150 amp circuit breaker. This circuit breaker serves multiple purposes: it replaces the required series connecting cable between the batteries; more importantly, it acts as an emergency fuse to protect against excessive overcurrent and also serves as a means to switch off and isolate the battery supply current when required. One other thing to note is that, because 12V LFP batteries have an inbuilt battery management system (BMS) to balance and protect the four cells in the battery, you must buy batteries with BMSes that allow batteries to be connected in series—not all batteries do!

The Ultimate 7500 plug-in solar power system has been designed to be modular and readily transportable. It requires little or no installation. Simply find a suitable indoor location, such as a garage, laundry or even a spare kitchen pantry cupboard, assemble the two batteries, mount and connect the MultiPlus-II inverter/charger, and you are ready to go.

To take advantage of three hours of free power under the SSO, plug the lead for the 230 volt AC inlet to the Ultimate 7500 power unit into any existing household 10-amp GPO via a simple time switch to connect AC power, allowing you to charge the lithium batteries when free power is available. Run the outlet from the Ultimate 7500 unit to a 230 volt distribution board ready to connect various extension leads to power appliances such as a washing machine, refrigerator, microwave, toaster, kettle or even a portable induction cooktop. It is preferable to use a power distribution block with, say, four switched 230 volt outlets and possibly equipped with USB outlets to charge mobile phones and other devices.

Please note the Ultimate 7500 is not intended for high-power, permanently connected loads such as electric ovens, water heaters or air-conditioning systems.

Actually connecting the cables is quite simple. Cut the extension lead at about the midpoint and remove at least 50mm of the cable sheath from each of the cut ends, being careful to not damage the insulation on the inner wires. Now strip the ends of the exposed wires on both the input lead and output (see photo).

Do not connect the battery leads at this stage.

Now take the Active, Neutral and Earth wires on the 3-pin plug cable and insert them into the marked terminals on the AC IN connector on the inverter. Tighten the screws and check that each wire is securely fastened in the connector. Repeat this by connecting the stripped wires from the socket end of the extension lead to AC 1 OUT.

Note the small black switch at the base of the MultiPlus-II near the battery connectors. This has three positions: centre (OFF), top (INVERTER ON) and bottom (BATTERY CHARGER ONLY). Make sure this switch is in the OFF (centre) position.

Connect a 1 metre red cable (4 SWG (standard wire gauge) or 25mm2 with 8mm cable lugs) between the positive (+) terminal on battery one and the red input terminal on the MultiPlus-II.

Repeat this for the negative connection, connecting the 1 metre black cable to the negative (–) terminal on battery two and to the black input terminal on the MultiPlus-II.

Caution: Avoid any short circuit across battery terminals.

With the circuit breaker switched to the OFF position (check this first), connect the circuit breaker linking the – terminal on battery one to the + terminal on battery two and tighten all the battery and inverter terminals.

Turn on the circuit breaker at the batteries and then switch the MultiPlus-II to position 1 (ON).

Future expansion possibilities
This Ultimate 7500 DIY project is a modular building block intended for beginners and has been initially configured as a battery/inverter that can be charged through a normal household 10 amp GPO activated during the selected hours when free power is available under the SSO, or possibly when rooftop solar power is available in a household without whole-of-house battery storage.

However, as previously mentioned, the Ultimate 7500 can be expanded to suit many other configurations. It can be readily adapted to operate from shore power at powered sites in a caravan or motorhome, or perhaps to serve a remote off-grid shed.

In a future issue of Renew we will describe how to add almost any old solar panels and a solar controller to provide additional charging for the batteries using solar energy produced during daylight hours. There is no need to mount the solar panels on a rooftop; you can place these portable panels on a sunny balcony or even lay them on the ground.

With the help of a licensed electrical contractor, the Ultimate 7500 could be permanently installed on the household grid supply at the meter box using an AC changeover switch, so that the house runs either from the grid or the inverter. You can add up to five more Victron MultiPlus-II inverter/chargers in parallel. Why not discuss this with your Victron supplier? Of course, you can always install additional 24-volt LFP batteries to increase the storage capacity.

By carefully shopping around online, the cost of parts should be about $2500. Why not start constructing this minimal, energy-saving solar power project with the goal of learning the basic principles of solar energy before moving on to future projects?

[Editor’s note: The author is a member of the Renew Policy and Advisory Working Group. Neither Renew nor the author is affiliated with any product manufacturers, and the personal views expressed by the author are not necessarily those of Renew. Readers are reminded that contact with electricity is potentially dangerous and subject to various regulations. You should consult a licensed electrician or authorised technician and discuss your specific application if required.]