AC/DC

I previously worked out that, with particular solar panels mounted at a specific angle, I could squeeze a little more electricity out of the sun than previously thought. With the amount of roof space we have available, we could cover emergency power (fridge and water pump) all year round, but in the height of summer, we'd only generate half what we generally need for other things (and that's already assuming I ditch my current desktop PC, and that we don't count heating and cooling).

That's a bit disappointing, to say the least.

But I had a closer look at our needs.  I had assumed that we would run everything at 220V AC, so as not to have to change any appliances - essentially everything would look and work the same, the only difference would be 'behind the socket'.  Assuming that, we need 10.6kWh per day, but taking battery and inverter inefficiencies into account, that's really a little over 13kWh per day.

I started to think that we could eliminate some of the inverter inefficiency loss by not running everything at the 220V AC that the inverter produces: The solar panels would charge batteries that would probably be 12V DC.  If we could run some things directly off the batteries instead of using the inverter to get 220V AC, then we wouldn't lose 10% power for those things.

For some appliances (fridge, washing machine, TV, hair dryer), this isn't possible without buying a new one.  But other things we have convert the 220V AC that comes out of the socket into DC at a lower voltage anyway (and presumably that conversion also isn't perfectly efficient) - the computers, internet thingumy, and phone chargers.  Could I run those directly on 12V DC?

It turns out that the only thing with an adaptor for 12V DC is the modem that connects us to the internet. The computers run on 15V and 20V, and the chargers are USB connectors; it turns out that USB runs on 5V.  It's possible to get step-up and step-down converters for DC power fairly cheaply.  They aren't perfectly efficient either, but seem to run at about 98% efficiency, much better than the inverter.

However, in looking at the modem, I discovered something surprising: the adaptor says the input from the wall socket is 220 Volts and 0.9 Amps - i.e. the input power is (220×0.9=) 198 Watts.  But the output into the modem is 12 Volts and 2.5 Amps, so the output power is (12×2.5=) 30 Watts. If that's really true, then we lose 168 Watts to the adapter! We never turn the internet off, so that's (168 Watts × 24 hours =) 4kWh per day!! Can that really be true?!! What a crappy adapter!!!

If so, that means that if we run the internet off the 12V battery, and everything else off 220V AC as originally planned, instead of needing the panels to generate 13kWh per day, we'd need 8.1kWh per day!!!

If we run everything off DC that we can - computers and chargers included, even if I assume 90% efficiency for the things that need conversion to other DC voltages, then our needs drop to 5kWh per day!!!!

We could also probably save more by using lover voltage LED lighting, but lighting is such low consumption anyway (0.2kWh/day) that I don't think that would make much difference, and it would make things complicated for when we need to use grid power because it's been raining for two weeks straight in the middle of winter.

That's a lot of exclamation points!!!!!  But they're justified because it turns my 6-panel solar system from something that barely makes sense to something that might actually almost work even in winter:

The bars are what we'd generate, the lines are what we need.
So using AC/DC mix, in Summer we'd generate more than we need.

 Doing things this way would add strange things inside our house though...

In addition to normal power sockets, we'd probably end up with a box with lots of USB sockets, for charging phones and tablets, and another collection of sockets - maybe a cigaretter-lighter-style one for 12V, which would give us juice for internet, and a couple more, for charging the laptops (these last ones should be some other shape, to ensure nothing else can be accidentally plugged into them, producing strange results).

Another complication is that using DC this way makes cabling a little more complicated - lower Volts often means higher Amps, and higher Amps means thicker cables are required, and some power is lost to resistance within the cable, so ideally they have to be short.  I have to go back to Boxwell's book, and work all that out...