How home heating affects your solar PV setup

The relationship between home heating and solar PV is significantly different to that between home cooling and solar PV. There are two key differences:

  • Time of day: we need heating the most when the sun goes down. This is the opposite to cooling, which we need most when the sun is out.
  • Time of year: winter is the time of year when our solar panels produce the least electricity, as daylight hours shorten. This is the opposite of cooling, when daylight (and solar PV production) peaks.

Home heating isn’t going to make as big an impact on your solar setup as home cooling will. However does that mean you shouldn’t account for home heating at all when installing solar panels? The answer depends on 2 main considerations.

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1. What Heating Appliances Do You Use?

There are 3 main forms of heating technology used by Australian households:

  • Air Conditioning
  • Electric Resistance Heaters (eg column heaters)
  • Gas Heating (flued and unflued)

If you use gas heating, you don’t need to account for it at all in your solar calculations as a gas heater uses a negligible amount of electricity.

Air conditioning and electric heaters could impact your solar calculations, as both use large amounts of electricity.

Some rough calculations

Let’s say you’ve got to heat a house with 4 people for 1 hour during the daytime using either multiple electric heaters (less efficient) or ducted air conditioning (more efficient). For comparison, we’ve assumed that you’re heating 6 separate rooms and therefore would be running either 6 separate heaters to do the same job as a ducted AC system – but obviously you might be able to get by with less (although you might be a tad chillier).

The table below gives a rough idea of how much electricity you’re likely to use in the hour, and what percentage of your estimated hourly solar electricity production that represents. We’ve included figures for 3kW and 5kW solar systems in Sydney for comparison.

Appliance Hourly Energy Consumption (kWh)   % of Solar Utilised (3kW system) % of Solar Utilised (5kW system)
6 Electric Heaters 10.0 581% 349%
Ducted Air Conditioning 3.32 193% 116%
Gas Heating N/A N/A N/A

Figures based on heating an area of 100 sqm, with 6 areas to be heated. Calculations assume approximately 100kW of heating per sqm. Solar output based on figures from the Clean Energy Council.

The table above shows that both ducted air conditioning and electric heating are likely to consume all of the energy produced by a solar PV system if operated during the day. A 5kW system almost perfectly covers a ducted air conditioner’s energy use; however, electric heating for an entire home is so inefficient that it uses between 3.5 and 6 times the output of a regular solar PV system. We’d recommend switching to gas (if economical) or air conditioning first, so that you don’t unnecessarily over-size your solar PV system.

Air conditioning for heating?

While you may not realise it, a reverse-cycle air conditioner (also sometimes referred to as a ‘heat pump’) is generally a much more efficient and cost-effective way to heat your home in the winter versus conventional oil heaters or radiant heaters. If you’ve got an AC unit in your home (or are considering purchasing one), make sure that it’s got a ‘heat’ setting – and make it your default, go-to choice for heating up your home. (N.b. Heat pumps are not appropriate for very hot or very cold climates, but for the bulk of Australia they work just fine.)

2. Do You Need Heating During the Day?

You can’t use solar electricity for heating after dark, as solar electricity gets sent to the grid if you don’t use it immediately or store it in a battery. Therefore you’ll need to work out the average number of daylight hours you would use an electric heater or air conditioner for during the cold months. This will help you understand how much of your winter solar output you’ll end up using for heating.

To work out the average number of daylight hours you use heating, you’ll need to consider questions such as:

  • Do you use heating at home between 9am and 5pm on weekdays? If so, for how many hours?
  • How much heating do you use during the day on weekends? Is someone always home, or are you usually out of the house?

The table below shows you how many hours of sunlight per day each capital city gets during the “winter” months (when average temperatures are below 20 degrees Celsius). Compare this to the average number of daylight hours you run your electric heater or air conditioner for each day in winter. This will give you an idea of how many hours of solar electricity you’ll either send back to the grid or be able to utilise for other devices.

Average sunlight during “winter” months Number of “winter” months per year
Adelaide 5.5 hours 5
Brisbane N/A N/A
Canberra 6.3 hours 7
Darwin N/A N/A
Hobart 5.3 hours 8
Melbourne 4.7 hours 6
Perth 5.9 hours 4
Sydney 6.2 hours 4

 Source: Bureau of Meteorology, Monthly Climate Statistics, July 2017. +Brisbane and Darwin average maximum temperatures never drop below 20 degrees Celsius, so heating requirements are negligible.

Reviewing Your Heating System

If you’re using electric heating you should definitely review your heating setup before installing solar – especially if you’re in a region with predominantly cold weather. Switching to air conditioning or gas would not only dramatically reduce your overall electricity use, but would also remove distortions from your solar PV sizing calculations.

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© 2017 Solar Choice Pty Ltd

Aravind Krishnan