Solar Storage System Design¶

Solar energy systems that are not connected to an electrical grid system usually require back-up or storage equipment to provide energy during unusually cloudy days. Stand-alone photovoltaic systems must size storage to bridge consecutive low-irradiance days so that the load is met during periods of below-average insolation battery autonomy for off-grid systems is conventionally specified as a number of consecutive days of reserve capacity. Unusually cloudy conditions occurring over consecutive days continually draw reserve power from batteries or other storage devices for solar systems not connected to an electrical grid. Storage devices must be designed to withstand continuous below-average conditions in various regions of the globe. Sizing for the worst-case sequence of cloudy days is a standard reliability practice in solar engineering design Duffie & Beckman, 2013; Luque & Hegedus, 2011.

Different industry organizations use different methods to size either battery or other types of backup systems.One example is the WHO Prequalification — E003: Refrigerators and Freezers, whose PQS specification E003/PV01 requires solar-powered vaccine refrigeration systems to provide a minimum of five days of autonomy under low-solar-radiation conditions, with battery capacity never sized below three days and the exact requirement determined per site from long-term local solar radiation data. Other requirements or methods of storage need different solar insolation parameters. The following sections expand the updated parameters along with their symbols and equations provided in POWER V10 data set. The underlying surface insolation is derived from NASA satellite-based flux product CERES SYN1deg Ed4.2.

Minimum and Maximum Available Insolation Over a Consecutive Day Period¶

Available on a climatological basis, minimum or maximum available insolation over a consecutive p-day period (p equal to 1, 3, 7, 14, 21 or a month) is the extreme value during multi-year (January 2001–December 2020) period for each month. The running summation is computed over the indicated days within each month. The January 2001–December 2020 climatology window aligns with the CERES-based POWER solar record used for these statistics.

Solar Insolation Deficit Over a Consecutive Day Period¶

Solar radiation deficits below expected values incident on a horizontal surface over a consecutive p-day period is calculated as follows, using previous parameter declarations:

The deficit quantifies the shortfall of available insolation relative to the long-term climatological mean over the same p-day period. It provides the climatological input to the equivalent no-sun days parameter, which expresses that shortfall in the days-of-autonomy terms conventionally used to specify battery reserve capacity for stand-alone systems, defined as the number of days a fully charged battery can satisfy the load with no contribution from the PV array. Because the limiting condition for a stand-alone system is the design month, the month of minimum array-generation-to-load ratio, which need not coincide with the month of fewest sun-hours deficit and no-sun-day parameters are provided on a per-month climatological basis rather than as annual values McNutt et al., 1999.

Surplus Insolation Over a Consecutive Day Period¶

Surplus insolation in each month is the difference between the maximum available insolation sum and the climatological averaged monthly value over the p-day period in the given month. It is the counterpart to the solar insolation deficit: where the deficit quantifies the worst-case shortfall a storage system must bridge, the surplus quantifies the largest excess above expectation — energy that a stand-alone system's battery and charge controller must either absorb or shed. It is calculated as follows, using previous parameter declarations:

Equivalent No-Sun Days Over a Consecutive Day Period¶

Equivalent number of black or no-Sun days is based upon the deficit solar radiation below expected multi-year monthly averaged value and calculated as follows:

The parameter expresses the deficit as a count of days of average insolation effectively lost during the worst observed p-day window: an equivalent no-Sun days value of 5 over a 7-day period means that window delivered only two days' worth of average insolation, as though five of the seven days had been entirely dark. It therefore characterizes the storage requirement a site imposes, in the same units as the days-of-autonomy figure conventionally used to specify what a battery provides — the number of days a fully charged battery can satisfy the load with no contribution from the PV array. The value is bounded above by p, reached only where the minimum p-day insolation sum is zero.

Minimum Insolation Percentage Over a Consecutive Day Period¶

Minimum insolation percentage is the ratio of minimum solar insolation over a consecutive p-day period to the climatological expected insolation over the same p-day period, expressed as a percentage. A value of 100% indicates the worst observed window matched its long-term expectation; lower values indicate the severity of the worst case relative to normal conditions at that location and month. Because it is normalized, it is comparable across sites and seasons in a way the absolute deficit is not.

Solar Storage Systems Parameters (p=1,3,7,14,21 and month)¶

The parameters below are computed from the POWER surface insolation record and follow standard solar-resource and PV-performance conventions NASA POWER, 2024; IEC 61724-1, 2021.