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
Start with the following rule
You need about 50
litres of hot water per person per day, plus 20 litres for the
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
So a 3 person
household using average amounts of hot water will require about 170 litres of
hot water a day.
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.
Inverter sizing and
The VOV™ inverter we
use for the HotPV™ system is sized to match your element rating, not array
So a 3kW or a 3.6kW
element would be matched to a 3kW inverter. 3kW and 3.6kW elements are common
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
There's also a bonus
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
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
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!
Using 3 person
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
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
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
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
(Note that for completely off-grid
households, the controller is not required as grid-boost to heat water is not
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