Cooling is one of the easier big loads to run off-grid, because hot days are usually sunny days and an efficient inverter split system can run largely on solar while it shines. Heating is the hard part. Demand peaks on cold nights and grey winter days, exactly when the array produces least, and reverse-cycle efficiency falls in frosty weather. That is why many off-grid homes use reverse-cycle air-conditioning for summer and the shoulder seasons, and rely on a slow-combustion wood heater through the depths of winter.

Why inverter reverse-cycle units suit off-grid

Modern split systems use an inverter-driven, variable-speed compressor, which brings three advantages for an off-grid system:

• Gentle starts. The compressor ramps up rather than starting at full speed, so start-up current is low compared with older fixed-speed units.
• Efficiency. A reverse-cycle unit typically delivers around three to five units of heating or cooling for each unit of electricity, depending on the model and conditions.
• Part-load running. Once the room reaches temperature, the compressor slows and draws much less power.

As a rough guide, a small split system with 2.5 kW of cooling capacity commonly draws well under 1 kW of electricity at full output, while a 7 kW unit may draw around 2 kW. Check each model's specifications and its zoned energy rating label, which shows performance in hot, average and cold climate zones. The cold-zone rating is especially relevant on the tablelands.

Run cooling while the sun shines

The best off-grid cooling strategy uses the house itself as a store of coolness:

1. Start the air-conditioner in the late morning, before the house heats up, rather than waiting until late afternoon.
2. Cool the main living area rather than the whole house, and close internal doors.
3. Choose a moderate setting; every degree cooler makes the compressor work harder and longer.
4. Close blinds on sun-facing windows early in the day.
5. Let the house coast into the evening once the sun goes down, using ceiling fans, which draw very little power.
6. Open up for cross-ventilation when the evening breeze arrives.

With that routine, a split system on a hot, sunny day often runs mainly on energy the battery could not have stored anyway once it was full.

The winter heating limit

Heating with a reverse-cycle system is still far more efficient than a plain electric heater, but off-grid winter heating runs into three limits at once:

• Timing. Heat is wanted most in the evening, overnight and early morning, when it has to come from the battery.
• Weather. Efficiency falls as outdoor temperatures drop, and in frosty conditions the outdoor unit periodically runs a defrost cycle.
• Generation. Short days and cloud reduce solar output just as the heating load rises.

Running a split system through a frosty highland night can drain a battery that would otherwise have lasted until morning. Practical approaches include warming living areas in the afternoon while solar is available, relying on insulation and heavy curtains to hold that warmth, using electric blankets rather than heating bedrooms, and never relying on portable resistive heaters as a main heat source.

Wood heaters: the classic off-grid partner

A slow-combustion wood heater produces heat with almost no draw on the battery, and it works hardest in the coldest, cloudiest weather. Points to plan for:

• Choose a heater certified to the current Australian emission and efficiency standards.
• Installation must meet flue and clearance requirements, and your council may require approval, so use a qualified installer.
• Plan a reliable supply of dry, seasoned firewood and a covered place to store it.
• A wetback can also heat water, if designed and installed by a licensed plumber.
• Ceiling fans on low, in winter mode where available, help move warm air down from the ceiling.

Have the flue inspected and swept regularly, and keep a working smoke alarm in the living area and bedrooms. A heater that is part of your winter energy plan needs to be reliable as well as efficient.

Choosing and placing split systems

How a split system is selected and installed affects its energy use for its whole life. A few points matter more off-grid:

• Size each unit to its room. An oversized unit cycles on and off and is less comfortable; an undersized one runs flat out for hours. Heat loads depend on room size, insulation, glazing and orientation.
• Several smaller units beat one large one. Separate units let you condition only the rooms in use, and each starts with a smaller load on the inverter.
• Shade the outdoor unit from harsh afternoon sun without blocking its airflow.
• Plan for frost. In cold areas, mount outdoor units clear of the ground and direct defrost water away from paths, where it can freeze.
• Use licensed installers. Refrigerant handling requires a licensed technician, and the electrical connection must be completed by a licensed electrician.

What this means for system size

Whatever you choose, include each unit's rated electrical input in the load list, so your designer can check both the daily energy budget and the inverter's capacity when the air-conditioner runs alongside pumps, the fridge and cooking.

Next steps

Plan the summer and winter pattern with seasonal load planning, and see how a generator fits into cold, cloudy spells in generator versus battery backup. To have heating and cooling included in a proper system design, request a free assessment from Blue Energy Solar. Efficient Air-Conditioning is listed in the energy market from $2,990 for a split system, supplied and installed; prices are indicative and confirmed after a site assessment.

Frequently asked questions

Can I run a portable or window air-conditioner off-grid?

It is possible but rarely ideal. Many portable and window units use fixed-speed compressors with high start-up currents, which can trip a small inverter, and they are usually less efficient than a split system. Portable units also leak heat back into the room through their exhaust hose. If you already own one, check its rating plate against the inverter's surge rating and run it only in daylight.

Is evaporative cooling a good off-grid option?

In dry inland areas it can be. An evaporative cooler uses a fan and a small water pump, so its power draw is low compared with refrigerated cooling. It works poorly in humid weather, which limits it closer to the coast, and it uses water, a real consideration for homes on tank supply. It cannot provide heating, so winter still needs another solution.

Should an off-grid home have ducted air-conditioning?

It can be done, but it needs care. A ducted system has a larger compressor and tends to condition more of the house than necessary, which increases both peak power and daily energy use. Zoning helps, as does running it mainly in daylight. If you choose ducted, look for good part-load efficiency, set up zones so a single area can run on its own, and have the designer check the indoor fan and controls in the load list too.