Solar Hot Water

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Household Electrical Wiring Diagram & Notes

In April 1999 I had a Solahart BC (Black Chrome) XII solar hot water system installed on the roof of the house. The main components of the system are:

- 2 collector panels
- A 300 litre storage tank mounted immediately above the panels
- Heat transfer fluid contained in a closed system flowing between the panels and the storage tank
- A thermostatically controlled electric booster.

When sufficient heat is generated by the sun shining on the panels, the heat transfer fluid within the panels rises and heats the water in the storage tank. This is known as the thermosyphon principle. At preset times of day if there has been insufficient warmth from the sun to maintain the water temperature above a pre-determined level (50C in my case), the electric booster will switch on and heat the water to the desired temperature.

Electric HW Booster usage

A lot of people have a manual switch fitted to control the operation of the booster. While this system can work well, the users have to remember to turn the booster on a little before required so the water has time to be heated, and off again afterwards. If the booster is left switched on on a sunny day, electricity may be used to do most of the heating that the sun would normally do. If hot water is not required until later in the day, this is a waste of electricity and defeats the purpose of the solar hot water system.

To overcome this problem a timer switch has been installed. It consists of a 24 hour clock with on/off switches for each 15 minute period, and controls the times within the Off-Peak 2 period when, if the water temperature is too low, the booster is allowed to switch on. The timer can be manually over-ridden should the need arise. I currently have the timer 'on' periods set for an hour or so mid to late afternoon (just before off-peak 2 power switches off for the evening) and again around 3am. This ensures hot water both in the evenings and mornings.

I've had the thermostat for the electric booster turned down from the default 60 to about 50C (see discussion on Water Temperature). The booster will not kick in until the water temperature falls below this level. The water can and does get a lot hotter when heated by the sun. I haven't measured it, but I would guess that on some hot summer days the water temperature can exceed 90C.

With 1 or 2 people in the house, VERY generally, the electric booster comes on when:

When the booster does come on, it can use anything from 0.2 to around 7kWh of electricity per day, depending on how far short of the 50C mark the water temperature is. On one occasion I've had the system operate for 10 weeks without the booster coming on, and around 4 weeks several other times. It's all dependent on the weather and how much hot water is used.

See the Household Electrical Wiring Diagram & Notes page for details of how the booster and timer switch fits in with the other circuits in the house.

The Economics of the system

The system cost $3050 fully installed (including the timer switch) after the $500 rebate (See Hot Water Links) and $200 trade-in on the previous electric hot water system, which was only a few years old.

Although the Solahart was more expensive than the others, it appeared at the time to be the most efficient at producing hot water, came with a 12 year warranty which was longer than other manufacturers were offering, and has the option of having a gas booster installed instead of the current electric one if gas is connected to the house in the future. (See Hot Water Links for info & comparisons of efficient hot water systems). Note: After 12 years, it is recommended to have a service done on the system. The current price for this service is around $200.

For the following calculations, I'm assuming that the electric booster consumes a 'baseline' of 6 kWh per day for our household's hot water usage pattern, assuming no solar input or residual excess heat from previous days. This figure has been derived from observations on rainy Winter's days where the preceeding days have also been rainy or overcast. This is the amount of electricity an equivalent electric hot water system set for 50C and used under the same conditions might be expected to use. 

Electric Hot Water Booster Stats for 2001
Month Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec For 2001
Av daily kWh consumed 0.2 0.1 0.0 0.0 0.3 0.7 2.0 0.7 0.2 0.4 0.5 0.1 0.43 kWh
Total kWh consumed 6 4 0 0 9 21 61 23 6 11

14

4

159 kWh
Booster ON days* 2 2 0 0 2 6 17 7 2 4

5

1

48 days

* The number of days that were recorded on the meter. The resolution of this meter is to the whole kWh. I know from the previous meter I had installed that on some days when very little extra heating was required the consumption could be down to 0.2 kWh, in which case it may not register on this meter. From past records these occasions are relatively few, and don't alter the overall kWh consumed, so are ignored here.

For the year 2001 we used 159 kWh of electricity for hot water heating with the Solahart system. This works out to be an average of under 0.5 kWh per day, which is a saving of some 5.5 kWh per day of electricity over an equivalent electric hot water system and using similar amounts of hot water.
Electricity for the hot water system is charged at the Off-Peak 2 tariff which is around 7.5 c/kWh. This means that I'm saving around 41 cents per day on my electricity bill. This equates to a simply calculated payback period of around 20 years. If there were more people in the house then the relative savings should be higher, and the payback period would then be decreased.

This isn't taking into considerations other factors like solar hot water systems are generally manufactured to a higher standard than standard electric systems, so will generally last a lot longer. eg One $3000 solar system might last as long as two $800 electric systems.

Note: For some new systems installed in Australia on or after the 1st of April 2001 it may be possible to reduce the effective price of the system through selling 'Renewable Energy Certificates'. See Hot Water Links for more info.