Let’s explore an example sizing scenario, component by component, with a method Solar Energy International (SEI, see Access) uses in its classes to size stand-alone systems using an maximum power point tracking (MPPT) controller:
Who: The Ackerman-Leist family
Where: Pawlet, Vermont, approximately 3/4 mile from utility service
Solar window: 8 a.m. to 4 p.m.
Average daily solar resource: 4.6 peak sun-hours*
System backup: 4 kW backup engine generator
System voltage: 24 VDC
Projected energy use (AC and DC): 2.2 kWh per day
Expected avg. ambient temperature for batteries: 60°F
Record low temperature: -35°F
Desired days of autonomy: 3
Desired battery depth of discharge: 50%
Battery: 6 V nominal, 225 Ah, deep-cycle flooded lead-acid
PV modules: 12 V nominal, 80 W STC , array tilt equal to the latitude (43°)
Charge controller: MPPT, 60 A
Array mounting: Pole-mount
*Peak sun-hours are based on Concord, New Hampshire, values, which more accurately reflect the site’s latitude and weather patterns.
Step 1: Estimate Electric Load
Determine the amount of energy (kWh or Wh) that will be consumed on a daily basis. If it is for a home not yet built, this can be a very involved and time-consuming step. A designer will need to work closely with the homeowner/builder to realistically estimate the daily and seasonal energy requirements.
The power (W) of individual loads and their estimated energy consumption (Wh) can be tallied to calculate the household’s average daily load. This step will help identify opportunities for efficiency improvements and pave the way for sizing the system components. The table below lists the electrical loads found in the Ackerman-Leist household. The family heats their home with wood, cooks with wood and propane, uses a propane refrigerator, and heats their water with a solar thermal system and a backup propane boiler, so those are not factors in the load analysis.
According to the table, daily household loads average 1.8 AC kWh and 0.36 DC kWh (from the chest freezer), totaling almost 2.2 kWh a day.