Power 3: Off-grid power system, 12V or 230V?

A guest post by Skvez.

Part 1 of this series is ‘Choosing your back-up/off-grid power source’

Mains power in the UK is 230V A.C. (standard across all of Europe, the tolerance on this is +10% -6% and includes the 240V that used to be the standard for most of Britain). The A.C. is Alternating Current, the Voltage changing direction forwards and backwards 50 times a second in a sine wave. The main benefit of A.C. is that it’s easier to transmit long distances as it can be transformed to a higher Voltage with a transformer. Power lost in a transmission line is proportional to the current squared times the resistance (I² * R), so by doubling the Voltage you half the current and quarter the power lost in the transmission line. This is why power is transmitted at high Voltages. However it’s not useful to consume power at high Voltages so it’s stepped back down to lower and lower Voltages the closer to the point of consumption.

DC (Direct Current) is easier for small appliances to use. It’s also easier to convert to chemical power to store in a battery. (Low Voltage) DC is not suitable for transmitting long distances, around your house is OK but between distant buildings you probably want a higher Voltage (24V D.C. / 48V D.C or 230V A.C).

230V can be transformed to lower or higher voltages and rectified to DC or a switched mode power supply can provide DC from an AC supply. This process is not 100% efficient and some power is lost as heat.

12V DC can be converted to 230V A.C. this requires an inverter and the process is usually much less than 100% efficient.

When you’re running off the mains you probably don’t care too much about the losses. What losses there are will be to heat and in the UK we want to heat our homes year round anyway. Losses become noticeable when you’re running off-grid.

Most standard domestic appliances are wired to accept 230V A.C. Even things like computers that actually use a variety of low Voltage DC supplies require 230V A.C. to power the power supply.

The sort of thing you are likely to want to power in a grid-down scenario are;

• Central heating (pump burner for oil central heating, while you still have oil) – will be 230V
• Fridge / Freezer – will be 230V unless you bought one specially capable of running on 12V
• Lights – Probably 230V, even if you have 12V lighting (such as some MR16 lamps) they are probably wired to a transformer and will require 230V. (In a long grid down situation I’ll be doing a lot of re-wiring of lights for 12V)
• Battery Charger – for small batteries (AA etc) can be 230V or 12V, some are 12V with a 230V ‘wall wart’ (a transformer built into the plug that transforms the Voltage down to a low Voltage and usually (but not always) rectifies it to D.C.
• Radio receiver (commercial radio) – probably actually low Voltage DC but may have a power supply expecting 230V
• Ratio transceiver (CB / Ham radio) – probably actually low Voltage DC but may have a power supply expecting 230V

I’m not going to cover big items such as cookers, power showers and washing machines as the power demands are too much for the average prepper (unless you are going to go totally off-grid).

You will need to decide if you want your back-up power system to be a DC system (probably 12V), an AC system (230V) or capable of both.

If you have a petrol/diesel/gas generator as your back-up power source then 230V equipment is fine.

If you have solar / wind then you want a DC system, probably 12V equipment although you can use 230V equipment with an inverter (but at an efficiency loss).

If you want to make use of a battery bank to store power when it’s not available (for instance only running your generator briefly each day to top up the batteries) you want 12V/24V equipment although you can use 230V equipment with an inverter (but at an efficiency loss).

When looking at DC systems remember the problems of transmission losses, cable is rated for carrying current, the Voltage has no effect on the conductor (at the Voltages we are discussing), the insulation may be rated for a Voltage but the same copper that carries 10A at 230V can still only carry 10A at 12V. This becomes a problem on a big DC system, for example a 1mm cable can carry 10A. That is 2300W at 230V but only 120W at 12V. For this reason big DC systems tend to run at 24V or 48V but for now I’ll assume we’re starting small with a 12V system.

In my next article I want to look at a few load types and the practicality of powering each of them from ‘home made’ electricity, but I’ll cover the fridge now since it proved to be an eye opener to me on the 12V/230V issue.

Fridge
Initially I thought that it would make sense to source a 12V DC fridge as powering this at 12V would avoid the inefficiency and complication of an inverter to step up to 230V however I discovered that 12V fridges tend to be optimised to be small and portable. As a result they have limited space for insulation and tend to leak heat much worse than their full-size 230V counterparts.

As an example one Portable (12V) Fridge I found was rated at 85W continuous, drawing current 24hours a day. This works out to 2kWh/day (I also fear the manufacturer knows it’s not ever going to cool adequately if it’s designed without a thermostat, they know it never needs to switch off because it never gets too cold).

By contrast my relatively modern fridge/freezer is rated at 300kWh/year which is less than 1kWh/day.

The efficiency of your inverter isn’t great but is better than 50% making the full size 230V fridge/freezer more efficient than the much smaller 12V fridge (which is smaller and doesn’t have a freezer!)

Taking a few measurements from my own fridge (rated for 300kWh / year). It draws about 400mA (0.4A) when the compressor is running (current when not running is negligible) suggesting it runs on average about 8 or 9 hours a day consuming an average of 0.82kWh per day.

Unfortunately when the fridge turns on there is a momentary current spike up to about 5A as the motor in the compressor is starting. This is typical of all motor loads but it means I would need to size my inverter to handle this momentary current spike rather than for the running current. So while the fridge runs at 0.4A * 230V = 92W, I need an inverter rated at 5A * 230V = 1150W to get the fridge started. I could probably risk briefly overloading the inverter and go with a 1000W inverter but I can’t afford to risk pushing it too hard in a world without replacements or spares.