Tech Tip: Basic System Sizing Design Concepts

Basic System Sizing Design Concepts

By Steve Bell, Technical Support Specialist

A couple of basic principles that form the foundation of system sizing are:

  1. Array to Load Ratio (ALR): a ratio of the net daily amp-hours from the solar array, stored in the battery bank compared to the net daily amp-hours consumed by the load(s).
  2. Days of Battery Autonomy: the number of days the fully charged batteries can power the loads without being recharged and without exceeding their recommended depth of discharge (DOD)

For a reliable system, experience has shown that the average daily amp-hours delivered to the battery bank, after factoring in all the system losses, needs to be at least 115% to 120% of the daily amp-hours consumed by the load(s). This is an ALR of 1.15 to 1.20. This minimum ALR needs to be met for the worst solar/weather month that the system is expected to operate, typically December or January in the northern hemisphere.

Why a minimum ALR of 1.15 to 1.20 you might ask?  Well, if the array only delivered the same amount of amp-hours per day as the load consumed (an ALR of 1.00), and the battery bank discharged due to several days of bad weather, there would not be any excess charging current available to refill the battery bank. This excess 15%-20% charging capability allows the battery to recover (recharge fully) after such occurrences.

Factors affecting the net solar energy harvested by a system include solar availability and weather, shading, array orientation, proper component selection, and various system efficiencies.

Net solar resources are the most significant factor to energy harvest, and they vary from location-to-location and month-to-month. There are multiple databases and solar calculators available online; many of them are based on the NREL database (see https://pvwatts.nrel.gov/). They can be used to determine the solar resource, on a month-by-month basis and at various tilt angles, for many locations. For most stand-alone off-grid systems the tilt angle is based on the optimal tilt angle that provides the greatest solar energy harvest during the worst month of operations. Designing to the month with the lowest solar energy potential will result in a larger solar array and excess energy production for the remainder of the year. For larger systems, and those with critical loads, a generator is often used to supplement the winter power generation. This allows for a smaller solar array with the generator making up the mid-winter shortfall of energy production.

Shading on the solar array can profoundly reduce the power and energy produced. Ideally there should be no shading on the solar array from about 9AM to at least 3PM. If there is unavoidable shading, then the size of the solar array may need to be increased appropriately. Remember, snow accumulation on the solar array is a form of shading and must be considered.

Ambient temperature affects the solar module’s performance and therefore energy production. Typically, higher temperatures decrease module performance, and colder temperatures increase module performance.

Matching the system components can also affect system performance. For PWM-type solar charge controllers, the output voltage of the solar array must be properly matched with the charge controller and battery bank’s nominal voltage. Mis-matched voltages can significantly reduce the net power harvested and may damage the PWM controller. MPPT-type controllers can both harvest more power from the solar array and compensate for most mismatches between the solar output voltage and the battery voltage.

System inefficiencies can reduce the net solar array performance by 8%-10%.  These inefficiencies include:

  • Module performance mismatch due to production tolerances, typically about 2-3% losses.
  • Wiring losses due to wire resistance and physical connections, typically about 2-3% total.
  • Soiling and dirt on the module face, typically about 2-4%.
  • Ageing of the solar modules. Solar modules lose about 0.5% to 1% of output power per year of service. Factoring in some module aging is prudent for long system reliability.

Moreover, charging a battery bank is not 100% efficient. The net charge efficiency is only about 85% to 90%.  This charge efficiency decreases further as the battery ages. So, typically the net amp-hours stored in the battery bank are about 20% less than the theoretical production of the solar array. Thus, the solar array needs to be sized about 20% larger than it might first appear. This 20% increase is on top of the 115%-120% ALR.

The battery bank is the “gas tank” of the system. It not only powers the load(s) through the night, but also bridges periods of bad weather, where there is minimal solar resource available. For many locations, 4 to 5 days of Battery Autonomy is considered adequate. For locations with significant weather issues, 7 to 10 days, or more, of battery autonomy may be required. Factors that affect battery autonomy include: the size, age, and health of the battery bank, ambient temperature, discharge rate, average and maximum DOD.  With the addition of a generator or other supplemental charging source, the size of the battery bank can be greatly reduced, often down to 2 or 3 days of Battery Autonomy.

Applying the concepts in this post and designing for adequate ALR and battery autonomy will help ensure your system runs reliably for many years.