Which system size actually suits your household?

By Oliver Hill

Start with what you buy, not what fits

The useful question is how much power you purchase and how much of it you use while the sun is up. Roof capacity sets a ceiling, but it should not set the target. An installer who opens with how many panels will fit is answering a different question to the one that decides your bill.

Daytime occupancy is the biggest single lever

Two households on identical systems, identical roofs and identical bills can get very different results if one is home during the day and the other is not. Power you use as it is generated is worth what you would have paid for it. Power you export is worth a small fraction of that under current Victorian feed-in rates.

That single difference moves the answer more than most things people worry about, and it is why a bigger system is not automatically a better one.

What pushes a household up a tier

In practice it is a short list, and it is about load rather than floor area:

• Ducted heating and cooling, particularly cooling, which runs when the sun is strongest
• A pool, especially if the pump can be moved to the middle of the day
• An electric vehicle, present or arriving within the year
• Electric hot water on a daytime cycle
• Someone home on weekdays
• Moving off gas

One of these might not change the tier. Two or three usually do. Large family homes through Caroline Springs, Hillside and Taylors Hill often have several at once, which is why bigger systems come up there more than in smaller households.

Roof space is the first ceiling

A 13kW system needs somewhere to go, and usable roof area is often smaller than total roof area, particularly on multi-faceted project homes through Tarneit and Truganina. It is worth establishing what your roof can actually carry before deciding what you want. Can your roof fit the solar you need covers this properly.

Export limits are the second

Your distributor sets how much you can send back to the grid. On a larger system that limit can become the binding constraint: generation above it has nowhere useful to go unless you are using it or storing it. This is a genuine reason a household that could physically fit 13kW is sometimes better served by less, and it is worth asking any installer what limit they are designing to.

Where bigger stops earning

The extra panels in a larger system pay for themselves through power you would otherwise have bought. Once your daytime consumption is covered, further capacity mostly exports, and exported power earns very little. That is the point at which a bigger system stops being a better one and starts being a more expensive one.

The exception is a household that is about to add a large new load. Sizing ahead of an EV is reasonable, because a second visit costs more than the panels would have. Sizing ahead of a vague intention is not.

Three households, three answers

A two-person home in an older inner-west house, out on weekdays, with gas heating: the smaller tier usually does the job, and the money is better spent on getting a good price than on extra capacity. Households in Werribee and Point Cook with one person home and moderate loads often sit in the middle. A family home with ducted cooling, a pool and a car on the way is where the larger tiers start to make sense.

Then get it priced properly

This page is about which tier suits you. What each costs after rebates in Victoria, and how long each takes to pay back, is on the pages that deal with exactly that: what a 6.6kW system costs in Victoria and what a 10kW system costs. If you want the sizing worked through from a bill rather than from a list of loads, What size solar system do I need in Victoria does that.

Common questions