What size split system do you need?

Split systems are sold on a capacity figure in kilowatts. That number is output: how much heat the unit can pull out of a room in cooling, or push into it in heating. It isn't the electricity the unit draws from your switchboard, which is much smaller.

That gap is the point of reverse cycle. The Australian Government's Energy Rating site says that in the normal Australian operating range these units can move three to five units of heat into a room for every unit of electricity they use, where a standard electric heater produces about one.

Most units list separate cooling and heating capacities, and in Melbourne the heating figure matters at least as much. Once the size is settled, the Zoned Energy Rating Label compares models, with a 1 to 10 star rating and estimated yearly energy use for cooling and heating in each climate zone.

Online sizing tools multiply the room's area by a figure. That suits an average room, and few rooms stay average once you stand in them on a February afternoon.

Even official guidance treats area as a range. For ducted systems, the Victorian Government suggests 1.1 to 1.4 kW of heating output for every 10 m² covered, depending on the individual characteristics of the home. That's written for ducted, not wall splits, but the lesson carries over: identical floor areas can need noticeably different capacity.

Picture a post-war brick veneer in Altona with a wide lounge window facing west, and a newer double-storey in Point Cook with the living area on the south side. Same floor area, very different afternoons. A calculator treats them alike. They shouldn't get the same unit.

When we size a split system, floor area gets written down first, then adjusted for the things below.

Most homes have two or three of them, which is why sizing by area alone misses, up or down.

The hardest job a split system does in Melbourne's west is late afternoon on a hot day. The Bureau of Meteorology's Laverton station averages 27 days a year at or above 30 °C and 9.4 days at or above 35 °C, most of them between December and February. Those are the days that expose an undersized unit.

The Bureau describes the pattern: warm northerly winds arrive first with higher temperatures and clear skies, then a cold front moves in from the ocean and conditions can change within minutes. Before the change, a west-facing room takes low sun straight through the glass just as the outside air peaks. A unit sized on floor area can run flat out and still lose the room.

The outdoor unit matters too. One on a west wall in full sun, or squeezed into a narrow side passage breathing its own hot exhaust, loses capacity when you need it most.

The reliable fix uses two levers: allow for the extra load, and cut the load where you can. Shading on the worst window can let a smaller unit do the job, which is worth hearing before you buy.

Air conditioning feels like a summer purchase, but the Victorian Government notes that Victorians typically use more energy for heating than cooling, and suggests looking for a unit with higher heating efficiency.

Cold weather also takes capacity away. In heating mode a reverse-cycle unit absorbs heat from the outside air, which gets harder as that air gets colder, and eventually the outdoor coil frosts and the unit pauses to clear it. The Commonwealth guide to the Zoned Energy Rating Label puts Melbourne in the Cold zone and says performance in frosting conditions, below 5.5 °C, is critical. Laverton's mean minimum in July is 5.2 °C.

So a unit chosen on its cooling figure for a big open living area can be marginal on a still July morning, exactly when you want the room warm. For living areas we check the heating capacity and the Cold-zone rating, not just the headline number.

An undersized unit runs hard all afternoon, gets louder, and the room creeps up anyway. The usual response is to drop the set temperature, which doesn't make the air much colder; it just keeps the unit working longer. The Victorian Government suggests 24 to 26 °C for cooling living areas and says every degree lower adds around 10% to running costs.

Oversizing is harder to notice. A unit that's too big reaches the set temperature quickly, then backs right off or cycles on and off. Temperatures swing, and the air feels cold but clammy because the unit hasn't run long enough to pull moisture out. Inverter units cope better, but every model has a minimum output that can still exceed what a small bedroom needs.

If an existing unit seems too small, rule out the simple things first. Energy Rating says dirty filters significantly reduce airflow and efficiency, and the Victorian Government suggests cleaning the dust filters about twice a year, following the manufacturer's instructions, with regular servicing. A unit that used to cope and no longer does usually needs a service or repair, not a bigger replacement.

A sizing visit is mostly looking. We measure the room and ceiling, note which way the glass faces and what shades it, check the ceiling insulation from the manhole where that's safe, and ask how the room is used. Then we place the indoor head so its air reaches the room, not the hallway, and find the outdoor unit clear space.

The electrical side is checked at the same visit. We look at whether your switchboard needs an upgrade before a new reverse-cycle system goes in, and with one local team for electrical, heating and cooling, you hear the answer before you commit.

Installation is licensed on more than one front. Installing a split system needs a refrigerant handling licence, and in Victoria refrigerated air-conditioning work is a class of plumbing work regulated by the Building and Plumbing Commission. The electrician completes a Certificate of Electrical Safety for the connection and must give you a copy within 30 days.

For a small bedroom with ordinary windows, a supplier's sizing chart will often land close. Get someone to look when the room is large, open to other spaces, faces west, sits upstairs or has high ceilings, or when an existing unit never kept up. A wrong guess there costs comfort for as long as the unit lasts.

Total Comfort is family-run and based in Altona Meadows, with 19 years in the industry. Quotes are free and no-obligation, you get the price before we start, and if a smaller unit plus better shading is the honest answer, that's what we'll tell you.