When getting an off-grid solar system proposal, it’s important to understand several key things
In order to properly evaluate a solar system design or a solar system proposal, it’s important that the end user understand the important design metrics and knows the right questions to ask
Anyone can give you a solar system proposal and tell you that it will work reliably, but how do you as a discerning consumer know whether their claims are valid? There are several key technical metrics that an off-grid solar system should adhere to, and anyone that is unable or unwilling to discuss these should be treated with healthy skepticism.
Array to Load Ratio (ALR)
In off-grid solar system design, one of the most important design metrics is the Array to Load Ratio (“ALR”). This is the ratio of average expected power production to the average load consumption. In terms of reliability, off-grid solar systems are limited by the month(s) with the lowest solar resources (usually December). According to IEEE 1562, in order to operate reliably year-round, an off-grid solar system should have an ALR of at least 1.1 for the worst solar resource month (array produces 10% more power than the load).
For off-grid systems that are powering critical loads or are in poor solar resource areas, the ALR should be closer to 1.3 or 1.4 (30%-40% excess power produced by array compared to load).
A knowledgeable solar system designer should be able to explain what this expected ratio is for your particular solar proposal, and how both solar resource production numbers and average load calculations were determined.
The next most important aspect of designing a reliable solar system is to verify that it has the appropriate amount of battery backup autonomy given the specifications of the project. Below are just a few of the important considerations when determining appropriate battery autonomy for your system. Remember, most solar batteries should not be discharged below 80% Depth of Discharge (80% energy discharged), as batteries that drop below this will sulfate and be damaged. Make sure to account for this limitation when calculating battery autonomy!
Critical Nature of Load – The nature of the load you are powering will dictate the level of design safety that you should incorporate into your system. According to IEEE 1562, Section 6 states the minimum battery autonomy should be 5-7 days for non-critical loads and areas for high solar insolation, and 7-14 days for critical loads or areas with low solar insolation.
Temperature Effects on Battery – Both cold weather and warm weather will affect batteries in different ways, and both are important to take into consideration when designing your system. Cold weather will decrease the amount of available energy in the battery with temperature. At temperatures of -15°C, a typical AGM or Gel battery only has about 65% of its energy capacity compared to ambient temperatures (25°C). Therefore, systems that will be installed in cold weather climates should have their capacity de-rated according to the winter temperatures at the site. The 5-7 days of battery autonomy should be AFTER the cold weather de-rating is taken into account, not BEFORE!
Off-grid systems in North America should have a tilt-angle of approximately 45° to 65°, depending on the project location’s exact latitude, in order to maximize solar energy production in the winter. Make sure that the racking system proposed will support your solar array at the optimal winter tilt angle! Tilt angle is very important, as flat (horizontal) solar panels will not perform well in the winter, because the sun is lower on the horizon, which does not allow for optimal sun angle for the solar array.
Low Voltage Disconnect (LVD)
Most off-grid solar systems should be equipped with a Low Voltage Disconnect in order to prevent the batteries from dropping below 80% Depth of Discharge (“DoD”). The LVD will disconnect the load when the batteries reach 80% DoD in order to preserve the health of the batteries. When the battery state of charge rises, the load will automatically re-connect. In order to protect your battery investment (which can account for up to 40% of total system costs), we recommend that users almost always use an LVD.
Another major factor when evaluating various solar system proposals is the quality of the components used. Below are just a few factors to consider with regards to the major system components.
Enclosures – Enclosures that are intended to be placed outdoors should contain a NEMA3R or better rating (or equivalent). Enclosures that are powder coated are more durable and will withstand the elements better than enclosures that are bare metal.
Mounts – If your project is located in a region with high wind speeds or large storm events (usually coastal areas), check to make sure that your mounts are rated for the appropriate wind speeds (usually 90-110 MPH). Check if these ratings are verified by a professional engineer, and whether they adhere to either IBC or ASCE standards (the most widely accepted building standards related to wind speed). Make sure your mount is able to tilt your solar array at the optimal winter tilt angle.
Batteries – Batteries are one of the most important aspects of your system and will usually be the limiting factor with regards to replacing parts (batteries will be first thing that need to be replaced). Therefore, choosing high quality batteries may result in higher upfront capitol costs, but will save you considerable money in the long-run as they will last longer, require fewer field trips for replacements, and be more reliable overall. A well designed off-grid solar system should have batteries that last in the 5-8 year range, systems that are getting less than this are likely designed incorrectly or using sub-par quality batteries.
The primary characteristic that defines solar batteries is their “cycle life”. This is the number of cycles that a battery can be discharged to a certain depth. Typical solar batteries are rated for between 500-1000 cycles (or more) at a 50% Depth of Discharge rating. Higher cycle life will result in longer battery life. Other factors to consider are warranty information, quality of overall design, plate thickness, brand reputation, and chemistry (AGM or Gel).
Solar Modules – When choosing solar modules, pay attention to power performance warranties, junction box design, quality of construction, and any certifications (UL, Class 1 Division 2, etc.).
Charge Controllers – The solar charge controller is the heart of your system, and also the most complex piece of equipment. Therefore, choosing a good charge controller is critical for having a reliable and well-performing system. SunWize strongly recommends going with a trusted brand-name like Morningstar Corporation, Outback Power, MidNite Solar, Phocos, Specialty Concepts, or similar.
Power Electronics – Make sure that the efficiency of all power electronics are being taken into account in the overall design. Many DC-DC converters have efficiency values lower than 80%, while many inverters have efficiency values around 90%. It’s important that each of these are taken into account, in addition to general system losses (on the order of 10%).
Another way to evaluate solar system proposals is to compare multiple quotes for the same system or project. They should all have roughly the same size solar array (Watts) and same size battery bank (Amp-hours). If some or all of your proposals are inconsistent, we encourage you to scrutinize them closely per the above criteria and ask questions!