Home Solar Batteries: 2.1b Charging - Monthly PV Model

Home Solar Batteries - Modelling Performance

Introduction

This post specifically addresses the charging of domestic battery systems using solar PV and forms part of a series of entries looking to establish a reasonable view as to how the combination of solar PV and battery storage system should be modelled to ensure that information available to consumers should be considered accurate enough to establish a reasonable justification for investment in storage technologies.

If this is the first time you've accessed this information it is recommended that all of the relevant posts are considered in order through accessing the 'relevant posts' links

Monthly PV Model

To take the next a step up the accuracy ladder concerning PV generation related to modelling battery performance leads us to consider data based on a monthly basis, which would most likely be derived using one of the methods below.

1. Derived from Annual figure

The most basic form of estimating the amount of energy which is potentially available to charge the battery system would be by estimating annual generation & apportioning relative to monthly generation

Apportionment is necessary due to the variability of monthly generation throughout the year. To provide an idea as to the asymmetry of the monthly generation pattern, here we have a representation of the generation by month for a typical SW facing UK system (with minor Winter shading) over a 6 year basis, along with the period average

Solar PV - % of Annual Generation by Month (6Yr Av)

Basing the apportionment on percentages derived from the above (amongst many other sources of a fixed percentage) to a 3400kWh annual generation would result in gross generation of around 442kWh (3400x13%) in June and 68kWh (3400x2%) in December.

2. PV Generation Estimation Tool

Whereas the derived method effectively employs a standard percentage and applied this across all energy apportionment assessments, next step up in accuracy would be to employ site specific monthly estimates for the specific site utilising a tool such as PVGIS-4.

In the example here we see that a specific location 4kWp system would be expected to generate a total of 3560kWh per year, of which 460kWh would be in June and 101kWh in December, which resolves to 13% & 3%, remarkably close to the derived percentages discussed above.

3. Historical Site Data

In the case where a PV system already exists it may be possible to use actual site data either from records kept by the homeowner, such as FiT (feed in Tariff) submission records etc, or through downloading monthly generation figure directly from the inverter or accumulated data systems such as PVOutput

4. The Monthly Model

Home Solar Battery - Monthly Generation Model

The chart above represents a UK household with average electricity demand & a typical 4kWp solar PV system. In comparing this with the basic annual model discussed previously, we can immediately see where applying monthly generation variation affects the level of generation which is available to either export or divert to storage.

Summary

Obviously, whichever method for predicting site generation described above is employed, the calculation of solar PV energy which could be available for export or storage is still provides a considerable degree of error which likely leads to overestimation of battery benefit, resulting in improved payback justifications.