Off-Grid - How Much Power Do You Need ?

1 - What Do You Want To Power ?

With any off-grid power system, careful selection of the devices to be powered can massively reduce the size and cost of the system required to power them. Efficient devices often make all the difference to the viability of a project.

To demonstate how dramatic these savings can be, listed below is a simplified comparison of two ways to acheive the same overall goal. One scenario costs almost 10x as much to power as the other.

For the purposes of this example - let us assume that each device needs to run for 5 hrs per day in winter in the UK - imagine a very small off-grid home office.


Example Scenario 1 - Normal Mains Appliances 

Desktop PC @ 100W x 5hrs = 500 Whrs

A/B Class Stereo Amplifier @ 25W x 5hrs = 125 Whrs

Incandescent 100W Light x 5hrs = 500 Whrs

Total per day = 1125 Wh + 20% Inverter Losses = 1350 Wh



Example Scenario 2 - Highly Efficient Appliances

Laptop PC @ 20W x 5 hrs = 100 Whrs

Digital Class Amplifier @ 3W x 5 hrs = 15 Whrs

CFL Light Bulb @ 18W x 5 hrs = 90 Whrs

Total per day = 205 Wh (+ no inverter losses if devices are DC).

The difference in the costs to power these two simple scenarios is huge, Scenario 2 will cost about 10% or 15% of the cost of Scenario 1. The first scenario would require approx 6x the size of solar array, battery bank, cabling, support structure, delivery & installation costs ... as well as an inverter.

Therefore you can see why it is so important to select high efficiency devices. As shown in the example above, with careful planning, this is where you can save serious money on the cost of a remote power system.

To make these high efficiency devices easier to source, we now offer some of the more ultra-efficient dc lighting, audio and ventilation equipment in our product range.

2 - How Much Power Does It Use ?

It is important to take the time to establish how much power each device uses. Most mains devices can be easily tested using a simple, low cost, plug-in energy meter.

One feature to look out for with a power meter is an internal battery, and it also helps if it's portable and simple to install. See our suggested Digital Energy Meters.

The power figure from the energy label on the back of the device or the manufacturers specification sheet is often peak power, which is normally far higher than the average. Conversely, many mains powered devices draw power even when they are supposedly 'switched off'.

Testing smaller DC powered devices may need a multi-meter or the manufacturer may be able to supply some data for average power use. Our experienced system design team can help if you get stuck here - we have figures for many appliances.



3 - When Does It Need To Run ?

To minimise running times throughout the system, use timers, PIR motion sensors and well designed controls with your appliances. These convenient devices offer huge opportunities to reduce energy requirements.

Recently a new type of energy saving multi-plug has appeared on the market. These 'smartplugs' shut down multiple devices based on one. For example, when a PC is turned off ... the printer, scanner, speakers and possibly network equipment is all automatically shut down as well.

On small off-grid power systems we recommend trying to avoid the need for an inverter to be running 24/7. All inverters use power on standby and also loose power in conversion from DC to AC. Therefore DC lighting, fridges, freezers and IT equipment all cost less to power from DC.



4 - Bringing It All Together

Buildings or larger off-grid power systems can have numerous loads, so for easy calculation, we suggest listing them in a spreadsheet.

Click here to download our free off-grid power estimator in spreadsheet (xls) format for completing off-line.

If you only have a few loads - please complete our system design enquiry form available from our system design page.