The Solar Bucket - Using Solar Electricity

There are three intertwined factors to take into account when considering the use of solar energy.  They are Size, Use and Buffering. The reason for messing with the Solar Bucket was to acquire enough data to understand these issues.  Solar panels are all about location, location and location, most of the comments on this page relate to southern England.

Use

Solar panels generate DC current, typically at 12Volts.  A lot of familiar things, such as mobile phones and remote weather stations operate on a low voltage DC energy economy, whilst the wire coming into a typical home carries 240 volts AC.  Whilst it is possible to convert low voltage DC to high voltage AC and vice-versa by using devices such as inverters, transformers, rectifiers etc., the cost and efficiency of these devices needs to be accounted for in the design and costing of a system.

These two types of supply have different risks.  High voltage AC can deliver a fatal electrical shock, however, a ring main with a 30A fuse can supply most of the appliances found in a typical home.  A low voltage DC supply has a lower risk of electric shock, but the current which flows whilst meeting high power requirements can be high requiring heavy cable and appropriate fusing and isolation requirements.  In short, a house designed for 240V AC can't readily consume 12V DC.

Size

Whilst the rating of a solar panel is a measure of its output under standard test conditions, it is only a rough guide to the installed output which will vary over the year and according to location.  Take two extreme examples.   A solar panel which is required to power an air-conditioning unit during the summer months will be approximately one-third of the size of one needed to power a heat pump with similar energy requirements during the winter months.  If a system is required to produce a constant load over the year, then it must be sized on the basis of the expected output in December and January.  A lower capacity will require either an alternative back-up supply or abstinence (e.g. "could not pay my gas bill because the sun was not shining and my computer would not work").

Buffering

Solar panels do not produce a continuous output.  Passing over the fact that the sun does not shine at night, the output of a solar panel reflects the passing of clouds.  Also most loads fluctuate (a good example is a laser printer where the power consumption increases dramatically during fusing phase).  Where fluctuations are a problem, it is necessary to have some form of buffer in the system.  In on-grid system, the buffer is the grid itself, the grid supplies electricity at night and during cloudy days and any surplus is exported to the grid.  Off-grid systems use some form of battery (e.g. NiCd, NiMH, lead-acid etc.).  The cost of buffering can be a major element in the cost of a system and design will vary according to the load.  A system which is required to supply a small, but continuous mission critical load (e.g. an internet modem/router)  365/24/7 might require a buffer capacity of 10 days or more in order to get through December, however, the discharge rate on the battery will be low, say a few per cent of its rated capacity.  If the objective is to power a washing machine, then a different set of rules apply, the battery/buffer must be capable of supplying a short burst of current, in which case the discharge rate could be more than 50% of the buffer capacity.

Page Updated: 12-Mar-09

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