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How to Measure Solar Energy

There are two types of solar energy measurement, based on the type of energy: photovoltaic energy produces electricity, and solar thermal energy heats water. The energy output is expressed by the amount of solar radiation that reaches the absorbent surface – be it a solar panel or solar collector.

Measuring Solar Thermal Energy

Solar thermal energy is measured in British thermal units (Btu) per square foot of collector space. A Btu is about the amount of energy it takes to heat a pound of water from 39°F to 40°F.

Measuring Solar Photovoltaic Energy

Electricity is measured in watts, with one thousand watts in a kilowatt. Using one thousand watts of electricity in one hour is a kilowatt-hour (kWh), the measurement on your utility bill.

For solar panels, the measurement of kWh refers to the amount of energy produced by the panel. This measurement is represented as kWh per square meter of panel surface.

An appliance rated at 1kW uses one kWh of energy when operating for an hour. The average electricity consumption for a household in the United States is 8900 kWh per year.

A 1kW system generates almost 1,000 kWh per year in cloudy regions, and almost double in sunny climates. The term peak refers to energy production in optimal conditions. Whether the system generates the stated amount depends primarily on the amount of solar radiation reaching the solar array.

For example, it takes 38,000 Btus to heat 80 gallons of cold water to 122 degrees. A solar hot water heating system with a rating of 30,500 Btu/day rating will be able to provide 80 percent of the daily hot water needs.

These ways of measuring solar energy is important when sizing a PV system or sizing a solar hot water heater.

How do solar panel meters work?


Photo Credit: General Electric

Power meters monitor your systems battery voltage, the level at which your batteries are charged, and the amount of power you consume. In the U.S., power providers are legally required to buy excess power from grid-tied small renewable energy system at the ‘avoided cost.’

The avoided cost of electricity is the cost to the provider to produce the power itself, and is lower than the retail cost. The monitoring process can be done in several ways. Let’s take a look at a few:

How many meters do I need?

Your provider may install two meters, with one monitoring power drawn from the grid, and another recording extra electricity generated by your system that is fed back into the grid. In this case, you pay the retail price for electricity drawn from the grid, while your excess power is bought at avoided cost. However, most often, excess power is forfeited to the utility, as in the case of the net metering system detailed below.

Solar Net Metering

Net metering involves one bi-directional meter. The meter spins forward when drawing electricity and backward when your system provides power to the grid. When winding back, it is equivalent to the provider paying you the retail price for that energy. If you use more electricity from the grid than feed back in a month, you pay the  retail price for that extra amount. If you have provided more than drawn, the power provider usually pays you the ‘avoided’ cost. Financially, this method is better for the consumer.

Can extra electricity be carried over to the next month?

Some providers let you carry over extra electricity your system generates over to the next month. This provision can be beneficial if the amount of power your system generates monthly varies (say, due to weather). At the end of the year, if you’ve produced more power than you’ve used, the excess power is forfeited to the provider.

Where us net metering available?

As of November 2010, net metering was available in 43 states, Washington, D.C., and Puerto Rico.

Is solar energy sold at peak rates?

Favorable arrangements for PV owners call for power providers to pay the customer based on the extra power being produced at the “peak rate” cost of production in the mid-afternoon time when grid demand is highest. This corresponds to when PV arrays produce most of their power. The utility pays the PV owner “peak rate” for the extra power. At night, when the PV owner draws from the grid, the utility only charges the lower “off-peak” rate since grid demand is lower.

Credits are not forfeited at the end of the year and extra kWh’s are paid for by the utility to the PV owner. A myriad of other options exist and vary greatly from location to location.


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