andrew-l wrote:Just getting ready to start designing my fantasy PV system, and I saw this quote over at the 12voltz website:
Did you know that low voltage lighting used through a transformer uses the same amount of power at 230v. ie. Six 12v, 20 watt downlighters powered through a transformer will use 120 watts @ 12v, however, because a transformer is used the output at 230v will be the same 120 watts, so no saving is made
Now, using my schoolboy calculations, Power = Current x Voltage, Conservation of Energy etc., that statement seems a bit of a nonsense. Surely there is the same impact on the batteries - 12v lighting without a transformer will just be drawing a higher current than the same lighting powered through a 230v transfomer.
Surely the only difference is the losses from stepping up to the 230v and then down again in the Transformer - or am I way off the mark here?
I think part of me would like to keep everything at 12v if possible, as that just feels more "efficient", and there seem to be plenty of 12v lights, small appliances and even fridges - but I still have plenty to understand before we get to the panels-on-roof stage!
If 12 volt lamps are worked from a 230 volt supply by means of a transformer, then the total power used will be increased due to the losses in the transformer, often about 10%.
If the 230 volts is obtained from a 12 volt battery by means of an invertor, then additional losses occur in the inverter, again often about 10%
As a rough guide a 12 volt 50 watt lamp worked from 230 volts via a transformer will use in total about 55 watts, and if the 230 volts is from an invertor the total would be about 60 watts.
On small installations, lighting is generally best worked at 12 volts direct from the battery.
Useing standard 230 volt lighting from an invertor is certainly possible but has a number of drawbacks
1) the invertor has to be on 24/7 to ensure that lighting is available whenever needed. The standby losses in the invertor may require an extra two PV modules to supply these losses
2) A large invertor working a single lamp is not very efficient, sometimes less than 50% at very low load.
3) Failure of the invertor will lead to a total blackout, most inconvenient, and possibly dangerous, therefore invertors in such systems should be duplicated, which adds to the already substantial costs.
4) Most 12 volt lights are more efficient than mains ones
5) 12 volts is safer, especialy near water, pets or children.
Refrigeration is often also best at 12 volts for somwhat similar reasons, not vulnerable to invertor failure, no need to leave invertor on 24/7, and often 12 volt refrigerators are more efficient than 230 volt ones.
24 volts is worth considering on larger schemes, high efficiency 24 volt refrigeration is available, though the choice of other appliances is more limited than for 12 volts.
24 volts presents very little electric shock risk, but is more of a fire risk than 12 volts.
3 wire DC systems that give two voltages are worth considering, though they are more complex, and not recomended for beginers.
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