Knowledge Sharing

To Solar or not to Solar – That is the Question

By Tim Herring (Collective Member #100)

When we bought our house, it had an old, small solar system (over ten years old) which didn’t work very well. After the start of the Ukrainian war, shortages caused gas and coal prices to increase significantly and flooding in Australian east coast coal mines helped keep the prices high. We decided to put in a new solar system of sufficient size and battery capacity to make a reduction of at least 60% in my annual electricity bill – at that time around $2200 per annum.

I got four quotations from local suppliers in October/November last year. The industry norm is to quote only the final figure for the system, but I had insisted on a breakdown of prices for each item. I noticed that most providers were leading with solar only (no battery) and I figured out why pretty quickly – the “basic” system of 5-6kW was being quoted as “free” after subsidies. This was undoubtedly attractive, but didn’t stand up to proper examination. There were two subsidies (Solar Vic PV and STCs) totalling around $5,000, plus a “subsidy” of $6,000 which was actually an interest-free loan – requiring repayment, albeit at zero interest. Hence the very low prices.

At my time of life, the last thing I needed was a new loan, even at those interest rates. I did all the sums and decided on a larger system with a battery which should meet my requirements. I ordered solar panels of 10.8kW maximum output for $7,500, a hybrid inverter (explained below) for $3,500 and two batteries totalling 13.7kWh for $10,500, plus installation & labour for $5,500 – less $5,000 rebates = $22,000 (all inc. GST).

I reckoned if I could get $1500 off my bill each year, that would give me 6.8% return on investment (RoI), which was about 50% more than the bank would give me at the time.

The installers came on December 19th and 20th 2023 and did a great job!

System Design

What sold me on this system was the degree of integration. It was all from the same manufacturer and designed to work together seamlessly. The hybrid inverter was only rated to supply 5kW to the house, which confused me a little until it was explained. The other 5-6kW from the roof would charge the battery at the lower-voltage DC directly. This is more efficient because other (non-hybrid) converters would have to convert the DC to 230vAC then the batteries would have to re-convert it back to low voltage DC to charge – losing efficiency and power as they do so.

The other main benefit is in the case of loss of mains power (such as after the February storm!), the system could be configured to continue working with power from the roof and from the battery, as it would automatically isolate itself from the grid (a legal, safety requirement in a black-out, otherwise power technicians would be at risk from stray currents in the mains supply wires).

A sub-set of electrical appliances in the house is created as part of the installation, which includes power in the kitchen (kettle, coffee, airfryer, etc., but NOT oven and cooktops), fridges, freezers, much of the lighting, the internet and a TV or two. All of these would be supported by the solar and battery in the event of a loss-of-power from the grid.

During the February storm we lost mains power for a few days, but all our protected devices stayed on. Each morning the sun would come up and re-charge the battery, which had powered our appliances all night as needed.

(We lost the internet anyway as the NBN fibre-to-the-node (FTTN) needs mains power in the pits, which was lost. In March we got fibre-to-the-home at zero cost (yes, NBN is doing that now!). FTTH needs no local power, except in the home!)

The new system works well, but I have noticed that in the winter months, when the sun is lower in the sky, we get much less power from the panels and the battery often fails to charge up. That means we use more mains power – but have we met our 60% target?

Economics

After the first half-year (January to June), the rather good statistics that the system provides gives the picture:

Production from the solar panels 5,430kWh

Consumption around the home & garden 5,160kWh

Bought-in from the grid 950kWh; exported back to the grid 1,140kWh

If I had no solar/battery and bought it all at the average price ($0.259 per kWh from Mycelia – using the new prices from July 2024), my consumption would have cost $1338 (I have excluded the Daily Tariff so we can compare electricity only). Doubling this would be $2,676 per year.

Calculating for what I imported, less export = $201 and doubling this would be $402 per year. (The savings will also improve if the rate per kWh later increases.)

That means my annual saving is around $2,274 per year or a return of 10.3% on my investment of $22,000. Better than my super!

To Solar Or Not To Solar

Mycelia Energy Collective

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