Boosting Solar Hot Water with Electricity


An electrically boosted solar hot water heater is substantially less expensive that a gas boosted solar HWS as there is no need to purchase a separate continuous flow water heater or the additional plumbing labour to fit it.

Most solar hot water systems come with a choice of Mid or Bottom mounted electric elements and these are usually connected to off-peak power when available. Because the mid element only heats the top half of the tank your electricity costs are going to be lower – it is more efficient and therefore attracts a larger solar hot water rebate (STCs).

In a 400 litre solar tank, a mid element will heat the top 200 litres of water at night which is plenty for a household of up to four people (allowing a generous 50 litres per day per person of hot water). If there are more people in the home then a bottom element is advised. Solar hot water systems are ideally large enough to store enough hot water to last a few days as this will allow for some days of cloud and rain without the need to boost, making for a more efficient system with lower boosting costs.

It is possible to use an existing electric HWS and retrofit some solar collectors and a solar controller and pump. We can supply you with a 4 way valve that will transform the cold water inlet to the tank into a solar flow and solar return (where the solar heated water is returned to higher up in the tank via a pipe) as well as the cold water inlet. The controller sensor is then placed against the internal tank (usually near the element for ease of access). This is only a worthwhile financial proposition if you have at least 6 years life left in your existing tank as you will sacrifice the possibility of a solar hot water rebate (STCs).

Some solar hot water systems offer a choice of element size. It will take the same amount of electricity to heat a given amount of water so the element size only determines how long that process takes, and in most cases where the boosting occurs at night, this may not be urgent. If however you have a Mid element and are prepared to be interactive with your solar hot water system to maximise the solar gain by switching the element off as much as possible, you will want to recover the heat quickly with a larger element. Other considerations may be existing cabling in the house, and if you are off grid you will want a small element which you would connect directly to your generator rather than to your batteries.

If you have off peak power available then you probably would not require a timer to control when the boosting occurs. If you don’t have off peak power available then you will need to consider your household’s use of hot water for showers to adjust the time of boosting If you have solar electricity panels you may want to use the electricity that you generate and you would set the timer for the last few hours of sunshine in winter (summer will not require much, if any, boosting if you purchase an efficient and correctly size solar HWS).

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