Metering Solar Power Panels

Solar Array Graphic The sun's energy is captured with solar power panels or with solar thermal energy collectors.   An active solar collector captures energy from the sun by heating a fluid which is then circulated to transfer the energy to its point of use.   A passive solar collector refers to a material that can absorb the and store the sun's heat directly such as a tile or concrete floor.  These materials have a high thermal mass.   Solar power panels, on the other hand, convert the sun's energy directly into electricity.

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These panels will produce up to 120 watts of power each.

This conversion process utilizes solar panel arrays which vary in efficiency according to design chemistry and lighting conditions.  The power produced can be used to charge batteries, is consumed immediately at home or is sent back out on the grid.

We will explore how to measure the output from solar power panels across the various types of solar generation systems which are differentiated by the way in which they tie into the grid.

Grid Tie Inverters  l   Off Grid Inverters  l   Grid Tie Emergency Back-up

Before looking the methods of how to measure solar power panels lets visit the economics of installing such a system.  Your latitude, weather patterns, average hours of sunshine per day, available rebates, electrical rate and the price you can expect for selling power back to the grid are all factors to consider when sizing a home solar energy system.

Rather than walk you through an extensive set of calculations we found this Solar Estimator to be a useful tool.  Simply enter your zip code to locate weather patterns for your home.  You can adjust your monthly electric bill, the size of the system, cost per watt and interest rate if financing.

The Estimator shows total system cost, total credits available, net cost and cumulative savings over 25 years.   Annual cash flow, estimated electric bills and expected PV output on a daily and monthly basis are also presented.   Adjust the inputs to see how the costs and return on investment respond.  Study the information carefully.   Solar power panels are still not that economically viable in many locations.

For those locations where it is economically viable, solar power panels offer a clean, reliable offset to one's electric bill.  Installing an energy monitoring system that tracks solar power generation against purchased power and load is a wise move. One can monitor system output continuously and have a check-and-balance to verify accuracy of the utility bill - especially if selling power back to the grid.

There are basically three different types of solar photo voltaic systems, namely grid-tie, off grid and grid-tie with back-up emergency power which is a combination of both.  The type of inverter used will determine the configuration of the system.

Grid-Tie Inverter Systems

Inverter Sine Wave Synthesis Grid-tie systems are the least expensive because they do not include any battery chargers.  This also means they do not provide any back-up power in the event of an outage.

Inside the inverter, DC (direct current) solar power is converted to AC (alternating current) power by mimicking the AC sine wave with a series of positive and negative digital pulses.  Inverters also synchronize this synthesized sine wave with the power coming in from the grid.   Once synchronized, alternative energy can be fed directly into the main panel through an appropriately sized circuit breaker.

Any power generated beyond that which is needed in the home is sent back out on the grid.  Utility meters count backwards when this occurs by subtracting kilowatt hours from your total usage.  At the end of the month you receive a bill for the net result of grid kilowatt-hours minus excess solar kilowatt-hours generated.

However, this net result is just a summary.  It does not tell you when credit was generated or how much was generated.   A home energy monitor system can provide this information and serve as an independent means of verifying the utility is crediting you properly.  Below is an example showing how the eGauge monitor tracks solar PV generation against grid power. Other examples can be found on our Dashboards page.

Solar Photovoltiac Output on Egauge Dashboard
Click here for a larger image.

Deciphering this graph, the red line shows total load while the pink shading underneath represents energy purchased from the utility.  The green line shows the power produced by the solar power panels and the green shading shows surplus solar energy that is sold back to the grid or credited to the electric bill. Summary data for the month shows running cost and consumption data in the table below.

Where is the best location to capture this information when wiring a home solar energy system?  Obviously, we recommend following the manufacturer's recommendations for metering solar photo voltaic with any home energy monitor system you install.  As a general rule, though, current transformers (CT's) should be installed on the AC output side of the inverter before power is routed into the circuit breaker in the main panel.  A second set of CT's is also needed to monitor the net grid load, i.e. grid minus excess solar, as shown here:

Grid-Tie Inverter Metering Diagram

Off Grid Inverters

Off grid inverters synthesize AC from alternative energy DC to run appliances and serve as a battery charging system.  They are not designed to synchronize their sine waves with the grid as they operate independently of it.  Off grid inverters are typically used in remote or mobile applications where grid power is not available.

The additional charging system with its heavy duty wiring adds cost to an off grid inverter system.  Higher-end off grid inverters can tie in an emergency generator to supplement charging or meet household demand when solar power panels alone cannot satisfy the need.

A solar array must be capable of generating enough power to drive an inverter in order to use a home energy monitor designed for 120 volt systems.  Wiring an energy monitor into an off-grid inverter is similar to wiring one into a grid-tie inverter except that there is no measurement of net grid load.   CT's are located on the AC outlet side of the inverter before power enters the main circuit breaker panel as shown here:

Off Gird Inverter Metering Diagram

Grid-Tie with Back-Up Emergency Power

Combining the features of grid-tie and off-grid inverters allows one to sell excess power back to the grid and have back-up emergency power.   These types of inverters are the most expensive because they must synchronize sine waves and provide battery back-up to essential appliances utilizing an automatic transfer switch.  An essential circuits panel and appropriate wiring must also be added.

When there is an outage the transfer switch must be able to convert from grid power to back-up power immediately.  The transfer switch feeds a separate panel of essential circuits such as lighting, the refrigerator, a computer and home entertainment systems.

Heavy loads such as air conditioning, the hot water heater and the clothes dryer are not on this panel.  These high loads can drain battery back-up power too quickly.  In this event, current draw may increase to unsafe levels causing breakers to trip on the low voltage battery back-up wiring.   Some inverter models also tie in emergency generator back-up to augment charging and carry some of the load during extended outages.

The complexity of these home solar energy systems will dictate uniquely how and where to measure with your home energy monitor system.  The following diagram provides a general idea of where these measurement points should be placed.

Back-up Emergency Power Inverter Metering Diagram

Systems of this caliber often have control panels that display and monitor output from solar power panels or wind generation.  If you are considering one of these systems, be sure to confirm the energy tracking software will monitor whole house load and alternative energy generation if you want to use them in lieu of a home energy monitor system.

Remember, it is wise to have a way to verify the utility company credits you properly for any kilowatt-hours your system returns to the grid.  Plus, a home energy monitor system, or equivalent, will give you the tools to track the performance of your investment in solar power panels.  At a minimum a multi-point monitor should be used to track solar power panels but a circuit level monitor is recommended.

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