Equipment Notes
by Matthew Lieber Buchman
A FEW BRIEF NOTES THIS INSTALLATION:
Photovoltaics (PV):
This home uses 12 Siemens SP-150 (150W/24V) solar-electric panels. They are wired in series in 2-panel
subarrays of 150W/48VDC. These 6 sub-arrays are wired in parallel using Outback Power PSPV string combiners
at the pole. This totals 1,800W at 48VDC.
These panels are coated in tempered glass and have a 25 year warranty. Their life expectancy is easily
double that. The Siemens SP series uses mono-crystalline silicone cells to convert the sunlight into
electricity. The surfaces of the cells are etched to "roughen" the crystals and allow them to achieve
better off-axis capture of solar radiation.
There is a planned expansion to 20 panels for a total generating capacity of 3kW.
Inverter:
The AEI MM-5000 inverter is the heart of the system. It manages the PV modules for best performance.
Converts the DC voltage of the PV modules to the AC voltage used by the lighting and appliances, and
by the utility. It also manages the batteries to keep them as fully charged as possible so that they
are ready in the event of an utility outage.
If an utility outage occurs, the inverter stops any supply of energy to the grid (thus guaranteeing
the safety of the utility's linemen) and also switches the house systems over to the batteries (though
it is still converted to AC by the inverter). The house will be run from PV and batteries until the return
of the grid.
Batteries:
A small string of batteries (220Ah at 48V) was installed to allow 2-3 day autonomy of lighting, heat,
refrigeration, and laptops in the case of a combined utility outage and such foul weather that the PV
modules can provide no substantive assistance. Concored AGM (Absorbed Glass Mat) batteries were selected due to no required maintenance beyond checking the connections twice yearly.
DC Disconnect:
An Outback Power PSDC (DC disconnect) was installed. The PV feeds and battery come to breakers in this
box and then the electricity continues on into the inverter from there. It also includes a shunt which
provides a very small but measureable drop in current which can be monitored using a meter.
AC Panels:
Unlike a typical house installation, there are two AC panels. One acts as a service disconnect between
the meter socket and the inverter. The only other loads here are the high draw AC septic pump and the
chest freezer. The freezer can survive for days without power and they can't draw on the small battery bank.
The second panel contains all of the other house AC loads and can all be fed by the inverter. Often,
more loads are placed on the first ("utility") panel but we've chosen to start with all loads run
from the inverter. This means we are in danger of running down our batteries more quickly if we don't
notice the grid is out, but we like the sense of autoonomy, even if it exists only in our sun-baked minds.
