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Solar Hot Water – How It Works

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In addition to the cost and greenhouse gas emissions savings, the beauty of a solar hot water system is its relative simplicity and durability.

There are two types of collectors in solar hot water services:

  • Flat plate collectors (suitable where tank roof mounting is necessary)
  • Evacuated tubes (more efficient and great for frost-prone areas)

Flat Plate Collector Solar Hot Water Heater

Flat plate solar collector

Evacuated tubes

Evacuated tubes

Flat plate solar collectors

Flat plate collectors work on copper pipes running through a glass covered collector, often connecting to a water storage tank on the roof. The sun heats the copper pipes and the resulting hot water is thermo-siphoned out of storage tank.

Evacuated tubes

Evacuated tubes consist of two glass tubes fused at the top and bottom. The space between the two tubes is evacuated to form a vacuum. A copper pipe (called a heat pipe) running through the centre of the tube meets a common manifold. That then connects to a slow flow circulation pump that pumps water to a storage tank below, thus heating the water during the day. The hot water can be used at night or the next day due to the insulation of the tank.

Evacuation tube systems are superior as they can extract the heat out of the air on a humid day and don’t need direct sunlight. Due to the vacuum inside the glass tube, the total efficiency in all areas is higher. There’s also better performance when the sun is not at an optimum angle. This could include times such as when it’s early in the morning or in the late afternoon.

considering a solar hot water quote

Evacuated tube vs. flat panel – which is more efficient?

The short answer is evacuated tube. It’s not only more efficient but also more durable and cheaper to repair should there be accidental damage. Learn more about evacuated tube vs. flat plate collectors.

The water tank and thermal mixing

As heat rises, the hot water delivered to the tank from the solar collector comes from the top for use in the house.

The water tank and thermal mixing

The temperature of the water may exceed 60 degrees Celsius (a requirement to kill bacteria). In order to ensure a safe temperature at the taps, a tempering valve will reduce the temperature to around 45 degrees.

Electric and gas boosters

While the sun can provide sufficient energy to provide you with plenty of hot water for most of the year, there may be extensive periods of cloudy, cold days where your system will need a helping hand. This functionality comes in the form of an electricity boosted or gas booster system.

Electric and gas boosters

Boosting using a tankless gas water heater is the most efficient method as boosting only occurs when hot water usage occurs. With an electric system, the tank heats as soon as the internal temperature drops.

Hot water and household energy usage

Hot water heating accounts for around 25% of a household’s greenhouse gas emissions and accounts for 15% to 27% of the total energy use of a house. By installing a solar water system in Brisbane, you will reduce these emissions and also save up to 75% of your water heating costs

Household energy usage (average percentage). Source: 

A gas-boosted solar water heater is the most environmentally friendly hot water system available, due to its low emissions over a 10-year period.

Annual water heating costs comparison

Using the table below, determine an approximate annual cost of water heating with your current unit and what it would cost with a solar hot water system. We have used Melbourne, VIC as an example. As you’ll see, solar hot water system savings are substantial. The information is from Sustainability Victoria: Compare Water Heating Running Costs  and assumes energy tariffs in Melbourne of:

  • Natural gas (2.58 c/MJ 1-2 person household, 2.52 c/MJ 3-4 person household)
  • LPG (based on bulk supply in Melbourne area)
  • Peak electricity (22.88 c/kWh),
  • Off-peak electricity (16.54 c/kWh)

Energy tariffs do not include the annual supply charge or cylinder rental fee. Melbourne STC solar zones were factored into the solar and heat pump efficiency levels.

Please note:

  • Gas systems are energy star rated. You can read more about energy star ratings here. The higher the star rating, the more efficient the system will be. This is reflected in the difference of running costs between a lower and higher rated system.
  • Solar hot water systems do not currently have an energy rating label. The energy rating on each system is industry regulated.

Cost of a solar hot water system

Water Heater Type50L per day
1 person
90L per day
2 people
120L per day
3 People
150L per day
4 People
Gas storage - Natural gas (4 to 5 star)$290 - $250$375 - $335$440 - $395$505 - $455
Gas instant - natural gas (6 to 7 star)$135 - $125$235 - $210$310 - $275$380 - $340
Gas storage - LPG (4 to 5 star)$615 - $535$800 - $710$940 - $840$1,080 - $970
Gas instant - LPG (6 to 7 star)$280 - $250$490 - $440$645 - $580$795 - $715
Electric storage - peak tariff$340$525$660$795
Electric instant - peak tariff$265$465$620$770
Electric storage - off peak tariff$290$415$505$595
Solar/natural gas boosted (high efficiency)$50$75$100$130
Solar/natural gas boosted (standard)$60$95$125$160
Solar/electric boosted - peak tariff (high efficiency)$60$125$185$255
Solar/electric boosted - peak tariff (standard)$110$190$265$345
Solar/electric boosted - off peak tariff (high efficiency)$55$100$150$200
Solar/electric boosted - off peak tariff (standard)$90$155$210$270
Solar/gas - LPG boosted (high efficiency)$65$120$170$230
Solar/gas - LPG boosted (standard)$90$160$225$295
Heat pump - peak tariff (high efficiency)$90$135$170$205
Heat pump - peak tariff (standard)$125$175$215$255
Heat pump - off peak (high efficiency)$70$100$120$145
Heat pump - off peak tariff (standard)$90$125$150$175

These savings, in addition to solar hot water rebates, make the solar hot water system the right environmental move and a financially rewarding one in most circumstances!

Why not make the switch and get a FREE solar quote today! Complete our quick quiz and you’ll be contacted by one of our local installers. Select ‘Other Renewables/Energy Saving’ when prompted.

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Updated January 2023