Solar Design and Energy Efficiency


 

Q: How does solar PV work?

A: Sunlight hits the solar panels, where photons are converted to a direct current (DC). The direct current is routed to an inverter, which changes it to alternating current (AC). In a grid-tied system, the inverter locks onto the utility power and frequency, and, if more power is being made than is being used, the power flows onto the grid.


Q: What is net metering?

A: A qualified generation system must be allowed to push power onto the grid. Net metering is the process of compensation for the owner of the generator. Different utilities interpret net metering differently. In the PNM Small Solar Program, with inverter(s) no larger than 10kW AC, power that is pushed onto the grid becomes a credit, which may be used at a later date. The PNM Large Solar Program does not have the credit carryover provision.


Q: What is a kilowatt hour?

A: A kilowatt hour (kWh) is the basic unit of power consumption. Your utility bill provides a total of how many kWh you used in a month. A kilowatt (kW) is 1,000 watts, and a kilowatt hour is 1,000 watts of power used for one  hour. It may be compared to ten 100-watt lamps on for one hour.


Q: How are PV systems sized?
A: A PV system is generally sized in kilowatts (kW) of DC power. The size in watts of each panel times the number of panels divided by 1000 is the kW size of a system.


Q: How do I know how much PV I need?

A: Within the PNM Small PV Program, the economic sweet spot is an offset of all of your annual use, or a bit less. First total twelve months of your monthly usage in kWh to get your annual usage. A good rule of thumb in the Albuquerque area is that a PV system will produce 5 kWh per DC kW of system size per day. Divide the total annual kWh by 5, then by 365 to come up with an estimated of the size in DC kW. For example, the average PNM customer averages 600 kWh/month, so 12 X 600 = 7200 kWh/year. 7200 / 5 / 365 = 3.94 kW.


Q: Can't I oversize my system and have PNM pay me for power?

A: It does not work out economically to oversize, under either of PNM's Solar Programs. With the Small Program, you wind up with a credit that is only good if you use it, and with the Large Program, you are paid for excess power at PNM's wholesale cost.


Q: How much does a PV system cost?

A: The cost of a PV system depends on a few factors, such as the price of panels, the type of mount, and the type of inverter(s). Premium high-efficiency panels can be expensive, and are best used where space for panels is limited. Any roof mount is less expensive than a ground mount, and flush mounts on a pitched roof are less expensive than ballast mounts on a flat roof. Microinverters are cost effective for a small system, but become much more expensive than string inverters in large system. Large systems in general are less expensive per watt than small systems. A ballpark working number would be in the range of $2.25 to $3.25/watt.





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