sun; and the roof must be insulated.
Thick curtains will have to be used during
winter and closed just before sunset to retain
heat within the house.
The entire household will have to maintain a
maximum peak-hour consumption of 4.2kWh
(the average usage is less than 1kWh).
If the family can implement and stick to these
adjustments for three years — at a hefty cost
— the household would save more than half of
their average monthly electricity bill.
Only once they have made these changes
could they contemplate buying a Powerwall.
The average family would need three 7kW
daily cycle batteries which cost $3,000 each
to store at least 21kW a day if they were
to use solar power exclusively to charge
their batteries. The daily battery bank must
always exceed daily kWh use for this to work
effectively as most batteries are designed not
to cycle at more than 50% daily.
For pure solar power, the family would need a
sizeable back yard — or a large surface area
such as a roof. They should purchase 50 of
the 300W solar panels at around $400 each —
these would take up at least 200m².
The installation of the panels will take about
five days, with expected daily labour crew
costs of $400 on average. If they are charged
up for five hours daily, the solar panels will
provide 15kW of electricity.
A qualified electrician with specific expertise
in this field would need to install three 5kW
DC-AC inverters, which cost about $3500
each. The inverter is a main component of any
independent power system that requires DC
power, and it runs household appliances by
converting the DC power stored energy in the
batteries into AC power.
All in all, the family would have to shell out
around $13 000 and a further $9,000 for the
complete installation of the Powerwall.
As with any investment, it’s all about the
FARMERS GAZETTE
November 2015
41