Sizing the power output of a photovoltaic system to cover the running load of your home

May 27th, 2010 by Gavin Lanoe

The running load or standing load as it is sometimes known is the amount of electricity your house uses while you are not home.

This electrical load is from the devices that are switched on all the time like refrigerators and alarm clocks etc.

For my home this list includes….

Computer Equipment

• Server PC with UPS

• Broadband Router

• Network Switch


• Cooker (clock / timer)

• Fridge Freezer

• Chest Freezer

Safety Equipment

• six mains powered smoke alarms

Everything Else

• Sky Digibox (for Sky+ scheduled recordings)

• Alarm Clock

• Alarm Radio

• Power usage meter

• three Rodent Repealers

The electrical load of all this equipment adds up to approximately 350 watts.

In order to cover this standing load with power generated from a solar photovoltaic system we must generate more than 350 watts during the day.  Covering the standing load of your home is a good idea because during the day electricity is most expensive.  Guernsey Electricity’s standard domestic tariff for customers with a single rate is 14.5 pence per kW.h.  The Super Economy 12 Tariff has two rates of 15.16 pence per kW.h at the peak rate and 6.29 pence per kW.h for the low rate.

Installing a system that can generate more power than is being used is a good idea but only to a point. Over-sizing the installation slightly will help to compensate for dull days.

If the rate at which your local energy provider will buy power from you is reasonable (currently about 6.5 pence per kW.h) then you can use the surplus power you generate to sell back to the grid and off-set the expense of the watts used overnight.

So we are looking for a solar photovoltaic (PV) system that will generate 350 watts of power reliably.

This article is also available on my website.

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