An inverter with a continuous rating of 2,500 W and a minimum surge rating of 6,436 W will meet the household’s instantaneous power and surge requirements. The inverter model chosen must have an input voltage of 24 VDC to match the nominal voltage of the battery bank, and have an AC output voltage of 120 VAC to meet the needs of household loads. There are no 240 VAC loads in the Ackerman-Leist home, but if there were, the following options would be available: specify an inverter with 120/240 VAC output; stack two 120 V inverters in series; or use a step-up transformer for the loads that require 240 VAC. Inverter features are also important to consider, such as an inverter-integrated AC-DC battery charger. This feature is convenient for use with a backup generator when the batteries need supplemental charging. A digital interface can also be a helpful feature.
This system was sized appropriately given the design parameters and, along with the backup generator, should provide the family with a reliable and long-lasting PV system. The daily and annual energy production of any PV system is largely dependent on how much available sunlight there is and weather patterns, which vary from year to year.
It is interesting to examine how the system design would change if a backup generator was not incorporated. Using the month with the lowest peak sun-hours (December, 2.8 daily sun-hours) and increasing the days of autonomy from three to five would require 20 batteries and 20 modules—a 66% increase! Of course, higher-capacity batteries and larger modules could be used, but the increase in cost would still be substantial.
Since it was installed in May 2004, the Ackerman-Leist system has performed well and has provided the family with almost all of their electrical needs—minus about 30 hours per year of generator run time to equalize the batteries and make up for occasional shortages during the winter months. Although the system was sized for 12 modules, they started out with 10 for budgetary reasons. But with the addition of two children to the family (making them a family of five) and a few new loads, they will be adding the other two PV modules soon. In addition to the use of efficient appliances, the family is also in-tune with the weather and their energy usage patterns; they only do laundry on sunny days and only use a clothesline to dry their clothes. The system powered the entire construction of their three-level home and has since served as an educational model for them, their community, and students at Green Mountain College, where Philip Ackerman-Leist teaches.
It’s inspiring to see a family of five use so little energy and yet live so comfortably—a system of this scale would be vastly undersized for almost any other full-time residence, at least here in the United States. A testament to energy conservation, efficiency, and awareness, the Ackerman-Leist family lives with their system, paying close attention to the ebb and flow of energy.
Khanti Munro is a Green Mountain College alum, an ISPQ-certified PV instructor, and SEI’s PV online coordinator and instructor trainer. Tied to the grid since childhood, Khanti lives vicariously through his off-grid friends and clients, with ambitions to someday unplug.
The sizing method presented is the sole intellectual property of Solar Energy International (www.solarenergy.org), which acknowledges that there are many sizing methodologies available today, and assumes no liability for systems sized using this method. Omitted from this sizing exercise were some technically complex aspects including nonoptimal tilt and orientation derate factors, conductor and conduit sizing, overcurrent protection sizing, grounding, and PV mount selection.