The data below is presented to start a discussion about this topic.

The data builds on from other discussions in this ATA Forum and from discussions in the Whirlpool Forum.

Please provide corrections and comments and test the logic.

The data suggests that when compared with a solar hot water system, a grid-connected solar electricity system can provide up to eight times the return per dollar of purchase price.

The insulated tank of a hot water system stores water at 60 degreesC.

If the tank is located inside the house and if the room temperature is 20 degreesC then the amount of heat that is lost by the tank totals 1300kWhr/yr.

A person typically uses 20 litres of hot water every day of the year.

Because one kWhr (= 3.6MegaJoule) of heat is needed to heat 20 litres of water from 20 to 60 degreesC, 350kWhr/yr of heat are needed to provide hot water for each person.

For hot water, a 6 person house needs 3400kWhr/yr (= 1300 + 6 x 350) of heat and a 2 person house needs 2000kWhr/yr of heat.

A solar hot water heater with a 315 litre insulated tank and 30 evacuated heat-collector tubes that cover 4 square metres can be purchased and installed for $6000 or $3,800 after rebates.

A grid-connected 1.5kW solar electricity system with 10 square metres of mono-crystalline photo-voltaic modules can be installed for $7,700 or $1,700 after rebates.

To use electricity to produce hot water, a 327 litre heat pump hot water service can be installed for $5,000 ($3200 after rebates). The maximum electricity input to the compressor in a $5000 heat pump is one kilowatt, say 20kWhrs per day, 7500kWhr/yr. A heat pump uses one kWhr of electricity to deliver about 3kWhr of heat so 7,500kWhr of electricity will deliver 22,500kWhr/yr of heat. A second heat pump (or one larger heat pump) can be used if more than this amount of heat is needed (e.g for hydronic heating).

If an additional $2,000 per room is invested for hydronic heating, hot water can be pumped through wall-mounted radiators and used to heat the house.

Onto one square metre that is in full sun, facing due North and tilted at the optimum angle of 32 degrees to the horizontal, the NASA website showsthat the solar insolation in Adelaide is 1920kWhr/yr.

Compared with 1920kWhr/yr, the annual solar insolation decreases by less than 5% for all tilt angles between 10 and 50 degrees and for all azimuth angles within plus and minus ? 30 degrees of due North.

An evacuated tube solar water heater collects about 75% of the solar insolation or 1440kWhr/yr.

At 20cent/kWhr, 1440kWhr/yr is worth $288/yr which is a 5% return on $6000 (or 10% on $3,200 after rebates). Note that a 2 person house needs 2000kWhr/yr of heat and the booster element will be used to provide the extra heat. Note also that any heat that boils the water reduces the % return because it is not "useful".

In Adelaide, a 1.5kW solar electricity system collects about 2,500kWhr/yr.

For a 1.5kW system the collector area is 10 square metres if mono-crystalline photo-voltaic modules are used and up to 25 square metres if amorphous modules are used.

If the solar electricity is used on site to replace electricity that would have been purchased at 20 cent/kWhr, then 2,500kWhr/yr are worth $500/yr. This return is 6.5% on $7700 (or 29% on $1700 after rebates)

If the solar electricity can be fed into the grid at the SA Feed In Tarrif of 60cent/kWhr, the return is $1500/year. This return is 20% on $7,700 (or 88% on $1700 after rebates).

This return of 88% is more than 8 times the 10% return on the solar hot water system.

If electricity costs 20cent/kWhr, the return of $1500/year could be used to buy 7,500kWhr of electricity.

At the low tarrif of 10cent/kWhr, $1500 could buy 15,000kWhr of electricity.

A heat pump produces 3kWhr of heat from 1kWhr of electricity so the heat output can be 22,500kWhr of heat from 7,500kWhr of electricity or 45,000kWhr of heat from 15,000 kWhr of electricity.

The 1kW compressor in a single $5000 heat pump can use a maximum 7,500kWhr/yr of electricity to deliver 22,500kWhr/yr of heat.

In addition to $750 (half) of the electricity that is produced by the solar electricity system and not used in the heat pump, the maximum of 22,500kWhr/yr of heat ($4,500/yr)that can be delivered by using a solar electricity system (bought for $7700) plus a single heat pump water heater (bought for $5000) is 15 times the 1440kWhr/yr of heat ($288/yr) collected by the evacuated tube solar heater (bought for $6000). The combined (solar electricity plus heat pump) costs $12,700 which is 2.1 times the $6000 for the solar water heater.