Calculating Energy Requirements
The first step in planning a PV installation is to determine the exact amount of renewable energy you require. You should start by reviewing your energy use to reduce electricity consumption wherever possible, which in turn decreases your system generation requirements and costs. The general rule to remember is that every dollar invested in energy efficiency will reduce the capital cost of electrical generation by about 3-5 dollars.
For example, a compact fluorescent light bulb or CFL consumes about 4½ times less energy than a traditional incandescent bulb. If your home needs 600W of electricity to light the main area and you use incandescent bulbs to do so, you would need PV panels to generate the necessary power. If the cost of PV panels is $4 per Watt, this would cost you $2,400, whereas the CFL would only require 130W and $520 for the PV module. It pays to be efficient.
Begin by checking last month's electrical bill. Average usage is 20-40 kWh per day, which can be slightly higher if your home has electric heat and hot water, or even air-conditioning. Your PV system size can also be affected by grants or low-interest loans in your area. Some states have buy-down renewable programs that offer a grant of up to 50% (subject to a ceiling limit).
Last but not least, your dealer can advise you on preferred system size depending on factors like roof area, prices of packages and other variables in the market that cannot be covered here.
- A weekend cottage with few lights, boom-box stereo, small TV and no refrigerator requires a 12V DC system of 2-4 PV modules with a small battery pack. It will produce a maximum of 200W with 1kWh/day energy.
- A fully-occupied seasonal cottage (4 residents) with refrigeration and some water pumping would require a 12V system with some direct current (DC) and alternating current (AC) outlets, depending on the load. But if expansion is in the pipeline, consider wiring for 120V AC with a small inverter to give maximum 1,500W power with as much as 2kWh/day energy. The PV generation system needs an array that can give a maximum of 500W output.
- Full-time occupancy with 4 residents who practice a great energy conservation philosophy - no clothes dryer or dishwasher, only a small TV and stereo, a high-performance clothes washer and a 120V fridge - will require a 24V battery wire system with a 2500W maximum power inverter for a maximum 3 kWh/day energy. This calls for a PV array system of 800-1,000W maximum power with a generator for backup purposes in "dark months". Different locations with more PV may reduce the generator need if the winter sun is 100% sufficient for daily load.
- Full-time occupancy that has all high-efficiency electrical comforts with a home theater system, gas clothes dryer, dishwasher, washing machine, central vacuum and refrigerator will require either a 24V or 48V battery wired system. If the total energy for each day is less than 7,000 watt-hours, use 24V, otherwise use 48V. Include a 2,500-4,000W inverter for peak output