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Solar Lighting Comparison Guide

Sizing and reporting solar powered lighting performance relies on historical databases and methods developed by various organizations over the past couple of decades and include a fair amount of judgment and experience supplied by the manufacturer.  For this reason, it may be hard to compare offers.

 

A good way to help standardize is to start with the same assumptions for calculating performance.  Your suppliers should be able provide you with their assumption values and rationale.  If assumptions differ, select a reliable source and ask manufacturers to base their calculations on those assumptions.  NREL and NASA both provide credible data, with more locations reported by NASA. 

 

Alternatively, select the assumptions of one supplier as the benchmark for your specification.  We suggest including the following values.

Assumptions

 

Sizing and Assumptions

Sizing and reporting solar powered lighting performance relies on historical databases and methods developed by various organizations over the past couple of decades and include a fair amount of judgment and experience supplied by the manufacturer.  For this reason, it may be hard to compare offers.

 

A good way to help standardize is to start with the same assumptions for calculating performance.  Your suppliers should be able provide you with their assumption values and rationale.  If assumptions differ, select a reliable source and ask manufacturers to base their calculations on those assumptions.  

Depth of Discharge

Using all or most of an AGM Battery's capacity each night will lead to early battery failure. Lithium Ion (LFP) and Nickel Metal Hydride can both be deeply discharged with little affect on battery life.  This means that AGM systems being compared with LFP or NiMH batteries should have significantly more capacity.

Design Sun Hours

Design Sun hours times Solar Capacity (W) = Watt Hours of Energy Collected per Day (should be based on the worst case time of year).  Be sure that any photometric analysis you receive is based on this condition.

Night Length

Based on your location on December 21st.

Recommended Days Of Storage

Lights remain on all night - all year round.

Temperature Sensitivity

Solar Panels and batteries behave differently in different climates.  Analysis should account for high PV voltage in cold temperatures and losses in PV production in high heat.

Battery Chemistry reacts differently due to temperature too.

Lithium Ion batteries (LFP) dramatically outperform AGM (Lead Acid) batteries in high heat.  Just an extra 10 degrees reduces a lead acid batteries cycle life by half.  Lithium Ion batteries, however, do not take a charge in below freezing temperatures.

Nickel metal halide used in our SmartLight are the most temperature tolerant.  They hold their capacity well regardless of temperature and can be charged in both extreme cold and heat.  It delivers an expected life of 10 or more years.

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Lighting

More light doesn’t always mean better light. If you don’t have a photometric comparison tool, like Photometrics Pro and time and resources allow, ask suppliers to provide a layout based on your drawings and lighting requirements. 

 

Another means of comparison is a small “typical layout” showing the spacing required for your application for example a span of a 20’ wide roadway or maximum spacing between four parking lot lights.  At the very least, observe differences in templates showing the same lamp height and measuring fc/lux at grade.  Look for a wide/even distribution of light versus a high peak light level at the center of a narrow field.  

Charging

Maximum Power Tracking Versus PWM

Maximum Power Tracking (MPT or MPPT) allows you to collect 20%-30% more energy than a PWM controller will for a given panel size.  Energy collected with an MPT plus a 100W panel is greater than the energy collected with a PWM controller and the same panel.

 

Multi-Stage Charging

Multi-Stage charging is required to use the full capacity of your batteries.  Single-Stage Charge controllers will only charge 90% of your battery capacity.

 

Features

Energy Management

Energy management functions track the state of charge of the battery and make adjustments to protect it when there is an anticipated energy shortage.  SolarOne’s RTE (Run Time Extension) is adjustable to be aggressive or conservative.  The default settings activates adjustments in less than average conditions.

 

Remote Control

A remote control can save time and money by allowing you to check and adjust your solar lighting system without opening the enclosure.  This is saves money, especially when the enclosure is located near the top of the pole. 

 

Flexible Programming

In most cases your lighting profile will remain unchanged, but you may need to make adjustments. Neighborhoods may request an earlier dimming profile or a “lights out” period; you may encounter unexpected shading or decide to use your lights in a different application.  With flexible remote-control-programming, you can make the changes without electrical services, shipping or down-time.

 

Error Reporting And Diagnostics

Controllers that can report status and report error types make maintenance faster and easier.