Discussion in 'Home Power and Microgeneration' started by Damien, Jan 12, 2004.

1. ### DamienGuest

What advantages are there to using low voltage appliances when using a RAPS system? Obviously, you could get away with a
smaller (and therefore cheaper) inverter as the load from 240V appliances would be less, but what about other
differences such as actual energy requirements? I know they consume less power, but how much less? Obvious candidates
for low voltage are refrigerators and lighting, but what sort of savings would you achieve under typical usage
conditions? Would it mean fewer solar panels and a smaller battery bank would be required, or would you still base your
calculations on the assumption of 100% 240V appliances, and treat the savings in energy requirements as insurance
against a (literally) rainy day, when you might not otherwise generate enough power? And finally, does anyone know of a
site somewhere that already has these answers, to save having to post them here? I'm looking, but it's hard to refine
the search terms enough to get anything genuinely useful! TIA

Damien

2. ### BughunterGuest

Try a search for "ohms law" or "ohm's law".

This simple mathematical formula describes the relationship between voltage,
current, resistance and power.

It is not necessarily true that low voltage appliances consume less power.
They in fact may have some significant disadvantages.

RAPS system? Obviously, you could get away with a
would be less, but what about other
power, but how much less? Obvious candidates
would you achieve under typical usage
would be required, or would you still base your
savings in energy requirements as insurance
enough power? And finally, does anyone know of a
here? I'm looking, but it's hard to refine

3. ### George GhioGuest

Lets start with a direct comparison of two systems each using 5kWh a day.
First high volts

240 VOLT WATT HOURS PER DAY 5000 W HOURS

TOTAL Ah DEMAND PER DAY 231.48 A HOURS

NUMBER OF DAYS AUTONOMY 5

MAX. ALLOWABLE DEPTH OF DISCHARGE 0.7

CAPACITY BATT. BANK 120 Hr RATE 1878.91 A HOURS

DAILY DEPTH OF DISCHARGE 12.31 %

NUMBER OF PARALLEL STRINGS 12.96 STRINGS

Next low volts

24 VOLT WATT HOURS PER DAY 5000 W HOURS

TOTAL Ah DEMAND PER DAY 208.33 A HOURS

NUMBER OF DAYS AUTONOMY 5

MAX. ALLOWABLE DEPTH OF DISCHARGE 0.7

CAPACITY BATT. BANK 120 Hr RATE 1691 A HOURS

DAILY DEPTH OF DISCHARGE 12.32 %

NUMBER OF PARALLEL STRINGS 11.66 STRINGS

Note that the daily depth of discharge is the same while the lv system
requires two less panels and some 200 Ah less batteries.

After this things get a bit trickier. Choice and placement of lighting
can save you a lot of energy (with either system).

your ability to think outside the box as they say.

I use ELV (12 volts) for everything in the house except the computer and
printer which run off the inverter. If the inverter dies everything else
still works.

There will be a lot of posts on this. Hope you like the circus.

George

4. ### Guest

Sounds like your 200 amp resistors

Nick

5. ### Scott WillingGuest

I wholeheartedly agree. I bought a house that started out with 12VDC
power only, and then had an inverter added later (all before we moved
in). An intermediate owner tore out all the DC sockets, so now, most
of my lights are DC, and a few desk / floor lamps and most everything
else run AC.

Since power is the product of current and voltage (ignoring power
factor for the sake of this discussion), if you cut the voltage by a
factor of 10, (e.g. 24VDC vs 240VAC) the current must increase by a
factor of 10 to deliver the same power to the load. And, since power
can also be expressed as the square of the current times resistance
(again keeping it simple here) that means that the power lost in the
wire goes up by a factor of 100! As other posters have pointed out,
that suggests using larger guage wire to minimize wire losses. Then
there is the need to run both AC and DC wiring. Yuck.

DC appliances are *sometimes* more efficient, and it's true that they
can be run directly from the batteries without firing up the inverter
(incurring fixed overhead + conversion efficiency loss therein).
However, unless they draw little power and are situated close to the
batteries, your savings could be eaten up by increased wire losses.

Since I run a very modest setup and keep a tight control on
consumption, it does annoy me to fire up a big inverter for the sake
of running some little dinky gizmo. And, I'm a big fan of redundancy
and simplicity, so I really appreciate knowing that I can still have
some DC-powered light to see my damn inverter if it blows up.

But if I had to do it all from scratch, I would wire completely for
normal line-voltage AC and provide a few emergency DC lights, even if
I was just doing a cabin or something of that nature.

Another benefit of sticking to AC distribution is that this confines
the "weirdness" component to the power room. The rest of the system
remains "normal" so far as regular folks, future owners or
electricians are concerned.

-=s

6. ### Scott WillingGuest

I wholeheartedly agree with the other posters here. I bought a house
that started out with 12VDC power only, and then had an AC inverter
added later (all before we took over). An intermediate owner tore out
all the DC sockets, so now, most of my lights are DC, while a few desk
/ floor lamps and most everything else run AC. I get to experience a
mixed voltage system and develop an opinion without having my mind
warped by the original conditions and decisions that fed into it.

Since power is the product of current and voltage (ignoring power
factor for the sake of this discussion), if you cut the supply voltage
by a factor of 10, (e.g. 24VDC vs 240VAC) the current must increase by
a factor of 10 to deliver the same power to the load. If you're
looking at running 12V, then make that a factor of 20.

And, since power can also be expressed as the square of the current
times resistance (again keeping it simple here) that means that wire
losses soar! As other posters have pointed out, that suggests using
larger guage wire to minimize losses. Then there is the need to run
both AC and DC wiring everywhere. Yuck.

DC appliances are *sometimes* more efficient, and it's true that they
can be run directly from the batteries without firing up the inverter
(incurring fixed overhead + conversion efficiency loss therein).
However, unless they draw little power and/or are situated close to
the batteries, your energy savings could quickly be eaten up by
increased wire losses. And from an economic standpoint, DC appliances
are less common and often more expensive. Couple that with more and
bigger wire and it starts to look like a Bad Idea from an economic
standpoint as well.

Since I run a very modest setup and keep a tight control on
consumption, it does annoy me to fire up a big inverter for the sake
of running some little dinky gizmo, especially if the little dinky
gizmo has a wallwart power supply to convert the AC back down to low
voltage DC!

I'm a big fan of redundancy and simplicity, so I must admit really
appreciate knowing that I can still have some DC-powered light to see
my damn inverter if it blows up.

But if I had to do it all from scratch, I would wire completely for
normal line-voltage AC and provide a few emergency DC lights, even if
I was just doing a cabin or something of that nature. For a very
small, simple, compact system (short wire runs, few appliances) I
might still go DC.

BTW, another benefit of sticking to AC distribution is that this
confines the "weirdness" component to the power room. The rest of the
system remains "normal" so far as regular folks, future owners, or
electricians are concerned. Also means that you can upgrade to a
different primary voltage (12V to 24V or 48V) without orphaning a
bunch of 12V gear or having to install a 24V/12V converter to keep it
- which is exactly the decision I'm facing now.

I may well end up completely rewiring this place yet. <sigh>

-=s

7. ### DamienGuest

That is indeed the tricky part! Time is on my side though, as it will be some time before I will be able to implement my
plans - and therefore plenty of time to make sure I get it right!
I was thinking I'd probably still have the house fully wired for 240V AC, but have a secondary 12V circuit as well,
which would run lights etc, and maybe a fridge (depending on the merits of various models available). It would mean
doubling up on some things, but as you say, if the inverter dies it wouldn't then be such a big deal. So far, the only
real negative that seems to have come up is the issue of running higher gauge wires (which would also be limited in
length to avoid the problem of dropping too much voltage - iirc, about 20m max length for a 12V system?), but I don't
see that as being such a big deal.
Indeed I will! I know enough to understand the various points that people make, but not enough to really have any
sort of an in-depth technical discussion on the pros and cons of the various options - so the more info people post, the
more I can learn! I'm trying to find out what I can, but there is so much information to wade through it's a bit hard
working what is relevant to my needs and what isn't. All part of the fun!

Damien

8. ### DamienGuest

"Remote Area Power Supply" - basically, it's a term used to cover the various power generation systems used in off-grid
setups (eg. solar, wind, micro-hydro, etc). Quite a common term here in Australia - maybe not so much in the US?

Damien

9. ### George GhioGuest

Poor Nick needs everything explained in triplicate.

240 VOLT WATT HOURS PER DAY

This is of course the Watt hours used per day at 240 Volts or your local
voltage (US -110 or 220) supplied from an inverter.

So it follows that

24 VOLT WATT HOURS PER DAY

is the watt hours used per day at system voltage (12 - 24 - 48 Volts
ELV)

George

10. ### George GhioGuest

I often recommend an ELV (12 24 48 V) lighting circuit in the ceiling
and a LV (240V) base board circuit for power.

I run all my lights, TV and stereo on 12V while the computer, printer,
and the sometime cake mixer off the inverter.

My lighting is almost all QH lamps in 5 & 10 Watts. All lighting is
placed at work stations and kept down low close to the job.

As an example the breakfast bar has three 5W lamps. The lamps are under
cupboards two foot above the counter top. This was planned at the
drawing stage for the house plans.

The same setup is used for the bread/pasta bench, the work bench next to
the stove, the stove itself and the kitchen sink. In all there are 15
seperate lamps in the kitchen/dinning area.

The pantry has a 20W flouro. This may be on for as much as 3 minuets a
day.

The house uses only 1.2 kWh a day for the electricity. My fridge and
cooking is LPG as is the new HWS. The old wood fired unit has died.

I have reading lights placed for the job near chairs. My office has two
5W lamps for the desk. One on each side to minimise shadowes.

Hope this helps you with your design work.

George L Ghio
Solar Power Consultant

11. ### Ken FinneyGuest

RAPS system? Obviously, you could get away with a
would be less, but what about other
power, but how much less? Obvious candidates
would you achieve under typical usage
would be required, or would you still base your
savings in energy requirements as insurance
enough power? And finally, does anyone know of a
here? I'm looking, but it's hard to refine