Home Solar Batteries: 2.1a Charging - Annual 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

Annual PV Model

Using annualised PV generation to model the performance of a battery utilises the most basic set of assumptions possible, which ultimately conform to one of subgroups as described below.

Home Solar Battery - Annual Performance Model

1. Basic

In the most basic form there's an assessment of the level of solar PV generated energy is available over the year from which an evaluation is normally made as to what proportion is available for export, which becomes the basis of any energy saving and related system cost justification.

For example, as displayed in the the above chart, the annual solar generation & household electricity demand could simply be apportioned equally, which could be construed to show a small surplus for export or storage in a UK home with average demand & a typical solar PV installation.

The chart describes a 4kWp solar PV system which has a history of generating around 3400kWh annually (or has been estimated to do this using a tool such as PVGIS-4 or PVGIS-5) which could also have a basic assessment of 50% of energy being available for export or battery charging, therefore the energy benefit & cost savings would be based on 1700kWh per annum.

50% is a particularly relevant figure in the UK as many Solar PV & (therefore) battery suppliers tend to have historically based cost justifications on the 'deemed export' percentage related to the FiT (Feed in Tariff) scheme.

2. Capped Basic

This is a marginal improvement over the above in which the energy calculated as being available to charge the battery system on an annual basis is simply divided equally to provide a daily charge provision, which is then capped by the capacity of a single battery cycle.

For example, if the same 4kWp system resulting in 1700kWh available to charge could operating with a small 3kWh capacity battery system could have the energy benefit capped to 1100kWh (365x3kWh)

Summary

Obviously, both of these methods are extremely basic and really shouldn't be considered as being anything other than a rough & ready 'rule-of-thumb' check on information being provided related to performance, however, it has been noted that there is a strong correlation between the expected results using this method and reports of information provided to help justify home battery battery costs from a number of sources.