What size HotPV™ system do I need?

Sizing a HotPV™system involves two steps: sizing how much hot water you need, and sizing to improve spring, autumn and winter output...and get rid of your reliance on coal fired power stations in winter


Dot point Start with the following rule of thumb:

You need about 50 litres of hot water per person per day, plus 20 litres for the house.

If you know you are a hot water scrooge, use a lesser number; if you have someone in the home either with long hair or/and enjoys long showers, probably add a margin.

So a 3 person household using average amounts of hot water will require about 170 litres of hot water a day.


Dot point We use the ratio of 1 kWh heating 20 litres of water*...

Using the 3 person household above, this is 8.5kWh a day on average. A 2kW array (about 8 panels) will more than do this in summer in NSW.

*4.18192 kJ/kg/deg K x 20 litres x 45 deg temp rise / 3600kJ/kWh = 1 kWh. The 45 deg temp rise is from 15 deg C cold to 60 deg C hot.


Dot point Inverter sizing and "overclocking"...

The VOV™ inverter we use for the HotPV™ system is sized to match your element rating, not array size.

So a 3kW or a 3.6kW element would be matched to a 3kW inverter. 3kW and 3.6kW elements are common sizes.

There are many 4.8kW elements used in NSW, which is no big deal when you are on-grid, but it means you would need a 5kW inverter for the HotPV™ system. We recommend swapping these 4.8kW elements down to 3kW or 3.6kW units.

The VOV™ inverters are able to take more solar panels than their rating, without up-sizing the inverters. This is known as "overclocking". For example, some of our 5kW inverters have 7kW of panels on them, and some of our 2kW inverters have 4kW of panels on them.

We use this capability to improve the performance of your array in spring, autumn and winter. For example, adding another 2 panels will improve your spring and autumn water heating energy by about 2kWh each day.

And for the 3-person household example, in winter an additional 4 or 5 panels would bring the family close to 100% winter solar water heating capability - on good winter sun days.

There's also a bonus to overclocking....


Dot point What unique environmental bonus do I get from the HotPV™ system....

Because the HotPV™ system doesn't overheat in summer like older solar hot water systems, we are not limited to how much energy we can collect. Overheating in summer is the single biggest reason why older solar hot water technologies cannot be sized to cover your winter hot water needs.

The HotPV™ system can be sized to give you almost all your water heating needs in winter, even when the winter sun is much weaker than the summer sun. Of course you will still need to boost when there is no sun, but that is the only time you need to boost. Most older technology solar hot water systems have to use external boost throughout winter.

What this means is that when you install a HotPV™ system and size it for good winter output, you will greatly reduce your need for boosting from the grid - which often comes from coal fired power stations in winter. So when we size your HotPV™ system for winter, we are sizing your system towards almost 100% self-sufficiency, and your hot water can come from almost 100% renewable energy, all year long!

Would you like to get rid of coal fired electricity back-up as much as you can? Then a HotPV™ solar hot water system will do it for you!


Dot point Size for me...

Dot point Using 3 person household example:

So using the 3 person household example, you would need about 170 litres a day of hot water. If the element is 3.6kW, you would get a 3kW VOV™ inverter.

To provide good spring and autumn performance you would install between 8 to 10 panels (260W to 300W rating each). To knock off winter as much as possible you would need ~15 panels.

The system you would install includes a HotPV™ hot water controller which you can program, for you to decide when you want to use hot water each day so that the system can run grid-boost times accordingly, to ensure you have hot water at the times you've specified.

Dot point 4 person household:

220 litres, 11kWh/day, 12 to 15 panels for spring/autumn coverage, ~20 panels to cover most of winter, with a 3kW inverter for 3.6kW element

Dot point 5 person household:

270 litres, 13.5kWh/day, 16 to 20 panels for spring/autumn coverage, ~26 panels to cover most of winter, with a 5kW inverter for 4.8kW element

(Note that for completely off-grid households, the controller is not required as grid-boost to heat water is not used.)

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