Building new Off Grid Home in Panama

Re: Bulding new Off Grid Home in Panama

In general, if you want to power DC appliances directly, 12 or 24 volt appliances are more available.

However, given that inverters are not that expensive and (on average) will only cost you ~15% extra power usage (85% efficient), I suggest that it is, overall, easier to use 120/240 VAC inverters and very efficient AC appliances instead.

You have several limiting issues with electrial power distribution. First is voltage drop... With a 12 volt system you have about 0.5 to 1.0 of voltage drop that can be allowed in a system (~11.5 volt minimum battery voltage under load, 10.5 volt typical DC cutoff for many loads). Add that it takes a huge amount of current to sustain larger 12 volt loads (i.e., 10-20x as much current vs 120-240 VAC system).

My suggestion is to limit maximum DC current flow in a system to ~100 amps nominal maximum. If the current gets higher, you are talking about very heavy copper cables, switches, breakers, etc. This means ~1,200 watts maximum load for 12 volt bank, 2,400 watts for 24 volt system, and 48 volt for larger system.

For example, a simple 12 volt 1,200 watt AC inverter system should be on a branch circuit of:

• 1,200 watts * 1/10.5 battery cutoff voltage * 1/0.85 inverter eff * 1.25 NEC wiring+breaker derating = 168 amp rated branch circuit (round up to 175-200 amp or so wring+components)

And say you want to send it 10 feet (one way trip, many voltage drop calculators use round trip length, so be careful):

• 1,200 watts * 1/10.5 battery cutoff voltage * 1/0.85 inverter eff = 135.5 amp max current

Using a generic voltage drop calculator for 10' one way, 0.5 volt drop maximum > 2 AWG wire minimum

Using NEC table (very conservative), you would need around 1/O to 4/O cable (depending on insulation, conduit fill, etc.).

Note that most good AC inverters can support ~2x rated output as surge current (motor starting loads, etc.), so surge current can easily be 2x rated current (AC inverters do not have anywhere near enough on-board storage capacitors to supply this sort of surge energy).

Say you did the same thing at 48 volts (1,200 watts), the current will be 1/4 as much (~85 amp minimum branch circuit) and 10' of wire+2 volt drop would only need the wire to be ~8 AWG cable. And NEC table would use ~4 to 3 AWG cable (note that every 3 AWG size change is ~2x as much copper and ~2x the price per foot of cable). The difference between 4 AWG and 4/O is about 10x the cost per foot.

Add that many DC appliances have a fairly narrow operating voltage range (for "car type" appliances, around 12-14.2 volts) vs the range required for an off grid deep cycle battery system (around 10.5 to 15-16 volt maximum), it is usually easier to use a single AC inverter than to fight all of the various DC appliance voltage ranges/limits. (15/30/60 volts is not really high enough battery high voltage limit--many people, especially in cold climates where cold batteries require higher charging voltages have to turn off their AC inverter/loads when equalizing at 15-16 volt ranges or their inverters fault).

The above issues are complex and a simple "rule" may not address all of the local issues for a system. That is why I suggest a paper design (or a few designs) to see what works best for you.

But for most people, they would be better off with as high of battery bank voltage (24 or 48 volts) as they can justify--12 volt system are really only for very small systems where there is not much peak power required.

And the costs to go to higher voltage systems can be an issue... Most AC inverters at 24 and 48 volts are much larger and more expensive (not many high function 48 volt inverters smaller than a few kWatts). Good quality MPPT charge controllers can support 12-48 volt (or even higher in some cases) battery banks. A 60 amp charge controller can handle a 4x larger array on a 48 volt battery bank vs a 12 volt battery bank (note, array sizing is my "cost effective" maximum array suggestion--always read manuals for details on supported arrays/battery banks):

• 60 amps * 14.5 volts charging * 1/0.77 panel+controller deratings = 1,130 watt array on a 12 volt bank
• 60 amps * 58.0 volts charging * 1/0.77 panel+controller deratings = 4,519 watt array on a 48 volt bank

So, while you may pay for for a large inverter, you may pay less for a charge controller.

Lots of trade offs to make designing your system (and, as always, knowing your loads and lots of conservation is a very good start). In other countries, it may be difficult to find very efficient AC appliances, but in the US (North America, Europe), the Energy Star rated (and equivalent) appliances do not consume much more power than their DC equivalents (for example refrigerator/freezers), and the AC versions can cost 1/4 the price and last longer (so adding a couple solar panels and upping the battery bank AH capacity a bit are easily cheaper than the DC version of the appliances).

-Bill

Re: Bulding new Off Grid Home in Panama

Efficiency of each energy conversion device is not that bad (between 80 and 95%)... However, when you have multiple conversions, the over all efficiency is terrible. For example:

0.81 Solar Panel Marketing rating * 0.95 Controller efficiency * 0.80 Flooded Cell Battery eff * 0.85 AC Inverter Eff = 0.52 = 52% end to end system efficiency

Terrible--Right?

These are "typical numbers". A battery bank that is cycling deeply can be > 90% efficient. A large inverter running 24x7 with small loads 20 hours a day, can be 60% efficiency (when lightly loaded).

If you do DC power, you save the 85% inverter losses (but have 3% losses, or more, for wiring losses).

If you run loads during the day when the sun is up, you can avoid the 80% efficiency battery losses.

Keep your panels clean, you might get 5% better efficiency (near equator, you can mount panels flat, but then they don't self clean in rain. So you may have dust/leaves to clean off to keep panels at peak power. Or tilt to 5 degrees and have some self cleaning action).

Tilting panels to follow the sun (at equator, tracking the sun is not usually as helpful compared with being closer to the poles.

-Bill