©1996 This is an original document written by ECO Centre staff.  You will find much more uptodate research and facts and figures elsewhere.

The majority of households in the UK receive 'mains' electricity via the National Grid; this provides an inexpensive and reliable supply of as much or as little electricity as is required. However, mains electricity tends to be produced at the expense of the environment - causing global warming, acid-rain, and nuclear waste.

This fact-sheet is aimed at people who are thinking about establishing their own renewable electricity supply and producing pollution free electricity from the power of the sun, wind, and water. A renewable supply can complement an existing mains electricity connection, or, where there is no mains connection, serve as the sole power source. Whichever, the first stage in planning a renewable electricity system is to assess your needs.

Renewable electricity practicalities

Most small renewable energy systems store the electricity they have generated in batteries. The electricity can then be used directly for powering low voltage d.c. appliances and lighting. Where 230v a.c. 'mains style' electricity is required and there is no grid connection, an inverter may be used to convert the d.c. into a.c. electricity (where an inverter is used there will be conversion losses of approximately 20% - therefore d.c. appliances should be employed wherever possible). The total cost of the three major components of a renewable electricity supply will depend on the required electrical consumption, the number of days reserve capacity, and the maximum instantaneous a.c. demand:

• Generating hardware: £1000 to £8000 per kW capacity.
• Battery Store: around £140 per kWh capacity.
• Inverter: £500 to £1500 per rated kW - hence, around £1000 for an inverter that can provide up to 1kW.

Although these costs are usually recouped within the first few years of operation, it is sensible to keep them to a minimum; care must be taken to ensure that only as much electricity as is required is produced. It does not make sense to install a scheme that is capable of providing 10kW of electricity to meet occasional peaks in demand, when on average only 0.5kW is ever used. Instead, for a fraction of the price, and a minimum of inconvenience, the demand is managed such that the most appropriately sized system can be installed. Therefore, before starting it usually pays to take a good look at your electricity use:

• Appropriate fuels: The first step is to ensure that any renewably generated electricity is only used for appliances that cannot be powered by other means; therefore, all heating and cooking should be with gas or solid fuel, etc. An exception to this rule is where larger hydro-electric or wind schemes provide heating as a by-product of their 'dump-load'.
• Energy Efficiency: All electrical appliances should be as efficient as possible - consider the 'Energy Rating', use low-energy light-bulbs, etc. Where there is no mains electricity, an efficient washing machine, costing slightly more than an inefficient unit, could easily save £500 in capacity costs.
• Load Management: Good-housekeeping should ensure that the limited supply of electricity is utilized as effectively as possible. Hence, lights are switched off when not required, etc. Where there is no mains connection, high-power appliances can be used one at a time, thus reducing the required inverter capacity.

Assessing your needs

To assess your needs it will be necessary to know: the total daily consumption and the average demand to establish the capacity required; and the likely maximum a.c. demand in order to size the inverter. A 'load table' can be constructed to facilitate the analysis:

Explanations:

• Wmax - The maximum demand column can often be completed on the basis of manufacturers data; the plate on the back of some appliances can provide a very rough (over)estimate - usually in amps(A): Power (W) = A x Voltage (V), i.e. A x 230. However, the most accurate method of assessment is to take readings from an ammeter connected in series with the appliance.
• Wav -Average demand can be tricky to determine; while a washing machine may use 1000W in some stages of its cycle, on average it will consume far less. Similarly with the hi-fi, cranked up to full volume it may well consume 100W; at normal levels the figure is lower. Some appliances provide data on their average 'in-use' consumption; most do not, and the figures will need to be estimated (e.g. with the refrigerator, if the motor runs 40% of the time at 50W, then Wav will equal 20W (50 x 40%).
• Net Wh/day - Net consumption per day equals the sum of the average power demand for each appliance multiplied by their average hourly daily use.
• Actual Wh/day - Actual consumption per day takes into account inverter losses of 20% when powering the ac appliances.

In this example (the table), the electrical load characteristics are representative of a renewably powered home with no grid connection:

• Overall electricity consumption is low; energy efficient appliances are used and the load is well managed.
• Where possible, dc appliances have been used.
• Gas is used for cooking, and a wood-stove provides heating.

A system powered by wind and sun would require a battery store of around 11,000Wh (i.e. 900Ah @ 12V) - sufficient to provide four days of reserve power (bearing in mind that batteries should not be discharged to below 20% of their capacity - hence 11,000 Wh = 2142 x 4 x 1.25). With a hydro powered system - where the resource is constant - the battery store could be far smaller; likewise if a back-up diesel generator was incorporated. Taking into account the conversion losses involved in charging the batteries (~25%), they would need to be supplied with, on average, about 120W (i.e. 10A @ 12V). The likely maximum a.c. demand can be ascertained from the Wmax column. A 1kW inverter should be sufficient to power the a.c. appliances - provided that the higher energy consumers are not switched on simultaneously. Hence, there would be no problem with having the TV on at the same time as the microwave; however, there could be problems with having the washing machine on at the same time as the iron. Most inverters can cope with short periods of operation above their rated output, although they work less efficiently. If greater flexibility was required then a larger inverter or a back-up diesel generator would need to be used.