Residential Load Considerations

Introduction

When designing a domestic off-grid system, the main loads to consider are as follows:

• cooking and heating;
• refrigeration;
• lighting;
• pumping;
• small appliances;
• large appliances.

Each of these is discussed below, within the context of ‘off-grid’.

Cooking and heating

Both electric cooking and heating draw large amounts of power. For this reason, it is often impractical build off-grid systems which contain electric hobs and appliances such as microwaves. Consequently, it is recommended that off-grid systems utilise either gas or solid fuel burners for both cooking and space heating. Good insulation and passive solar design will also help to reduce the need for heat.

If a wind turbine is included in the system, it can be used to power immersion, space and storage heaters. Peak wind power production usually coincides with colder weather. However this should not be relied upon as a primary source of power.

Refrigeration

Electric fridges and freezers should be avoided in off-grid systems unless they are high efficiency models. If possible, gas powered fridges should be used as an alternative.

Daily energy consumption estimates in kWh should be available from manufacturers to help with system sizing.

Lighting

For small off-grid systems (vehicles, caravans, boats etc.), it’s best to choose low voltage, DC lighting which can be powered straight from the battery. This doesn’t require an inverter and short wiring runs mean low gauge wire can be used; therefore the system costs are reduced.

For larger off-grid installations, it is often more cost effective to use an inverter to supply AC fittings via any pre-existing wiring. Any lights should be replaced with high efficiency compact fluorescent bulbs.

Pumping

Pumping requires a large amount of power, therefore careful considerations must be made when designing off-grid systems which include pumps. In particularly sunny climates, it is often advantageous to utilise direct solar pumping technology.

Small appliances

Most small appliances fall into two categories:

• High power, short duration appliances such as toasters and hair dryers. Since these appliance are only used for small periods of time (typically a few minutes), off-grid systems can easily cope with them, provided the battery, inverter and charging system are sufficiently sized.
• Low power electronics such as TVs, computers and stereos. Most of these appliances have DC and AC models. DC model typically use less power, however off-grid systems are able to accommodate both.

Large appliances

Most large appliances (vacuum cleaners, power tools and pumps etc.) run off of AC only. In order to use these, the system inverter has to be a suitable size.

In most domestic situations, washing machines make up the largest electrical load. Since they only run for fixed periods, it’s usually advantageous to run them off of a standby generator with any spare generator capacity being used to charge the battery.