Installation of Solar System

12.5kVA - is that a maximum or a day-weighted average?

It's just that the average load here over a 91-day quarter is a little over 550W. So 12.5kVA seems a lot.

It's enough to run Twelve 1000 watt heaters and a 500.

Do you want grid tie? Off grid with batteries? Where did you get 12.5kw? This is much more power (and related expense) than most homes need. You say load capacity. Have you determined this is what your house would draw from the grid with everything in the house on? I suggest you do some reading and come back for round 2.

First find the load requirements. Load is Kilowatt hours used per day. See your electricity bill and divide the monthly electrical usage by 30 days to get the daily Kwh used.

Then WHY would you want to use Solar power, if you already have the much cheaper Mains electricity? The calculation below if you want it.

A solar panel will produce an average Annual yield, in Kilowatt hours depending on climate. In a Hot Tropical climate, Crystalline solar panels will produce 980Kwh/Kwp. Thin Film will produce 1,300Kwh/kwp. Annual yield. This becomes 3.56kwh/kwp per day for A-si, Thin Film or 2.68Kwh/kwp per day for Crystalline, in hot climates.

So if you use 5Kwh per day, and want to use 240v AC power you will need an Inverter. (NEVER use inverters with solar power systems). An Inverter of 98% efficiency will lose average 15% of the power during the day, so you will use 6.6Kwh after the inverter losses are added. You must replace this much into your battery. Battery has 30% losses so you must put 9.4Kwh into the battery. So if you get 3.56Kwh from 1Kwp Thin Film, you will need 2.7Kwp of PV panels. (Much more if you use Crystalline)

If you use the DC power directly, then an LCD TV or Lap top computer, will use 12v dc at 2amps (24w) and does not need an inverter or AC to dc converter. If you use an inverter, then you must use the AC to DC converter as well and you will need to use 12v 4 amps, plus pay for the cost of an inverter. Use DC lamps, NOT LED, DC fans, Gas cooker, gas fridge and you will need only 500wp Solar panels instead of 2,700wp needed for AC house.

More technical details at www.solarvoltaic.com but this may be considered advertising and be removed.

Re: NEVER use inverters with solar power systems.

This is the second time that I've seen this statement (I'm not sure if it was from you both times). What is the reason for this statement--is it only efficiency or is there some other reason?

I understand the efficiency reason, but there will be times when you don't have equipment that runs at 12(?) VDC and you will have to use an inverter to power some AC equipment. In fact, that is probably the most likely case when someone is in the early phases of making a transition to solar electricity. Phrasing it as "NEVER use inverters ..." without mentioning the reason seems a little unhelpful.

Sorry, Inverters result in almost 50% losses and cost a lot of money and are the main cause of breakdown in any PV system. 65% of all failures are in the Inverter. This is why we never use them. My assumption was that you were aware of these facts.

Now like you said you may have some AC equipment already, like a CRT TV. So you could power this TV with solar power. It is an AC TV, so you will need to BUY an Inverter. The cost of an Inverter is say $500. So you could spend $500 on an Inverter and then use $3,000 for the solar power system. But if you really wanted to save energy and get a proper PV system, you would do it properly and save a lot of cost by buying a new LCD TV, with the AC adapter on the outside of the case. You can then remove the AC to DC adapter and plug the 12v DC power directly into the LCD TV saving the Inverter and AC to DC losses of nearly 50%. This will cost you $300 for the NEW LCD TV and saves you $500, the cost of an inverter, then saves you another $1,500 off cost of solar panels, (50% less power). So it costs much LESS to go completely solar than to try and do half solar and half AC grid power. Also more reliable because you don't have an inverter to break down.

I guess the other comments are from people selling inverters with twice as many solar panels.

Hope it makes sense now.

Good point / good answer, and makes extreme sense for a lot of things like a TV. Just thinking out loud to myself, how common (and how expensive, and how efficient) are things like 12 VDC:

• refrigerators and freezers (I know they exist, no idea how expensive--I'm guessing they're almost certainly more expensive than the least expensive 115 VAC models)
• microwave ovens
• electric ovens--ok, you'd probably almost certainly switch to gas, even if you have to buy bottled propane
• toasters
• ???
  

• deep well pumps (for water supply)--I'm sure they could be made in 12 VDC, but, are they, and how big are the cables? (I mean, does cable size become an impediment to their use / installation?)
  

This post is chock full of factual errors. Most have been pointed out. See previous GA post about hiring a pro to access your demand, create a potential power supply solution, use some cost estimates and look at ROI. You will need to assign some cash value to attributes that alternative energy/solar will provide. It may be worth it to you, if you can afford it. It is for many people.

I don't see a "chock full" of factual errors. I see two:

Re: An Inverter of 98% efficiency will lose average 15% of the power during the day, so you will use 6.6Kwh after the inverter losses are added.

Without thinking about it, I don't see how a 98% efficient inverter loses 15% of the power during the day. (But I do wonder if there are 98% efficient inverters, and I know there will be power losses.)

Re: If you use an inverter, then you must use the AC to DC converter as well and you will need to use 12v 4 amps.

Somehow, he makes the assumption that the combination of an inverter and an AC to DC conversion will be 50%, which sounds extremely low.

Do you see others? Can you point them out?

I install off grid and grid tie power systems. With Outback Power Systems their inverters are 96% efficient at full output. In standby they draw about 18 watts and in sleep mode 6 watts.

What is the inverter efficiency at 25% output?

This data log shows 90% efficiency at 25% load for a 48 VDC (nominal) system. With a drop to about 82% @ 10% output.

You have a good point about not inverting DC to AC and back again for such things as TVs, BUT it is hard to come by good DC appliances in the USA without paying alot and, still, these appliance are not that good. USA manufacturers just don't put much into DC units because of the readily available (for 98% of the people) AC grid. Also, running a low voltage pump (or other high draw load) for high lift and/or high volume is not practical due to wiring limitaions at low voltage.

Re: Your average appliances in US ARE DC appliances.

Not my main point, just an aside: I guess it depends on how you create the average--by power usage, I think big appliances like refrigerators, freezers, washers, dryers, fans predominate in terms of power usage.

Re: Anything with microchips in it must be DC, low voltage already.

Again, not my main point, just an aside: Modern refrigerators, freezers, washers, dryers have microchips, but the main power user is typically a 115 (or possibly 230) VAC motor.

Re: (gas refrigerators): You can get them and other fridges easily at low cost. http://gasrefrigerator.en.alibaba.com/productlist.html

This is the main thing I want to ask some questions about.

How energy efficient is a gas refrigerator or freezer compared to an electric refrigerator or freezer? I'd be interested in an answer in terms of both energy units and then a comparison in dollars. (Because I'm not sure the energy units will be easy for me to compare--kwhrs for electric appliances, cu.ft. of gas (of a certain BTU value) per year for gas appliances?

For gas appliances, is there any rating / testing done by an agency (perhaps a Federal (US) agency to get annual power cost as is done for electric appliances?

Going to the link you mentioned, it looks like all of those appliances are quite small. If my math is right, take a 250 liter unit--that is about 8.8 cubic feet--about half the size of the refrigerator / freezer in a typical US home. (Not to say maybe we shouldn't aim towards downsizing.)

The price doesn't sound terrible (or very exact on the page I looked at)--$200 to $300, but:

• double that or so to get a US sized refrigerator
• add the tariff / duties associated with buying them from China--I don't know what they are, but I've heard some horror stories (and it won't be shipped in a small envelope to your home by the US Postal Service and be overlooked by the customs people (or be below the $200 exclusion which I think might exist on small value items.
• add shipping cost from China to the US, then trucking cost from the customs port/dock to your home.

I quite sure it will no longer be "at low cost" ;-) (But I'm willing to be educated.)

These inverters run about $2000 USD. They are also stackable up to 16 (x 3.5 kVA) for the creation of micro-grids.

Good gas refrigerators are not cheap. A good one starts at $1000 USD. I can get an AC one for free, and, theoretically, can produce my own electricity, not my own gas. So, relying on gas is reliance on a grid system for the acquisition and delivery of the gas.

I have set up quite a few DC pumps, but these are limited in volume. For any application where a large volume and/or high lift of water is required low voltage will not work. Delivering 1200 watts to a pump at 12 VDC (100 amps) would require cable pairs costing $10/ft. 120 VAC (10 amps) can run on cable with a negligible cost. And this is an example of a small pump.

Not errors. I will explain- An inverter is rated at its MAXIMUM efficiency. That is at its very, very BEST, at its optimum input with its optimum output. So it may be 95% efficient when it is a 1Kw inverter with 750 watts load and 750 watts input at the optimum voltages and loads. Now if you use this 1kw inverter to light only a 100w bulb, at night, the efficiency is no longer 95%. because it does NOT run at its maximum efficiency all the time, probably most of the time it is not at maximum, the actual efficiency it is running at, is often much less than the MAXIMUM efficiency. So instead of using the very MAXIMUM efficiency in your calculations you use an average working efficiency. A real efficiency.

The losses using an Inverter are measured losses NOT an assumption.

You can easily test the losses by using an inverter with AC to DC converter using your Lap top computer. Get the car cigarette lighter adapter for your computer model. Connect an ammeter to it and plug in the charger after you have used the computer and the battery is low. You will see an average of 12v at 1.5 amps, (18w) being used from the car battery. Now buy an Inverter, I used a 300w inverter, and connect back the mains adapter to the same computer. The mains adapter plugs into the Inverter. Measure the current into the Inverter. Now it will be 12v 2.9amps. Measure all other AC to DC conversions like LCD TV, DC Fluorescent lamps, DC Fans, this average loss remains the same. NOT an assumption. Not an error.

Ok, thanks for the explanation. Just thinking out loud to myself, yes, an inverter probably has some portion of its loss as a fixed loss (not depending on load), and maybe / probably another portion that varies with load.

Like the one poster mentioned for inverters he has used, 96% peak efficiency, but still loses 18 watts in standby and 6 watts in sleep mode. So, the efficiency at 0 or no load (or sleep) is ridiculously low. ;-)

Yes errors, unless you are talking about tiny car chargers and laptops. Moving LVDC any kind of distance invokes large losses, and requires much larger conductors, not to mention it's just not very useful for most people. Try to move 10KW around a home, safely, and in ways that allow you to watch TV, charge your Nissan Leaf, run your AC heat pump, etc. DC is fine for toys, and off grid mini systems. The increase in effective efficiency for HighVoltageAC far outweighs the inverter losses, which are quite low, in modern equipment.

But I WAS talking about tiny chargers and Lap tops and OFF GRID systems.

Yes, you are correct that moving LVDC any distance will cause huge losses. So DON'T move it long distance. Keep Solar panels on your roof, direct down, with short wires, to nearby batteries. One battery bank at one corner of the house and the other at the opposite corner. One 100wp panel (8amps max) connected direct to 1 X 100ah battery using a modular controller. Small wire, 2.4mm taking only 100wp power to its battery. You can add as many of these modular units as you need, up to mega watts if required. The controllers connect to a DC Bus but each controller also has a separate 10 amp controlled output, for a specific appliance. The controllers share power between these 100wp modules via the DC bus. So a short circuit in 1 PV panel will not affect the other 99 in an array of 100 panels. Same applies to batteries. For lighting, or fans, 1 controller goes to the fuse in the existing fuse box. Of course you have disconnected the mains power supply already, now that you are going Solar. The lighting wire was 1.8mm. Now you can change the 100w light bulb for a 12v dc 0.8 amp energy saving lamp. Calculate the losses at 0.8 amp and it is less than 3% so you can use the existing wire. AC oscillating fans were 45w so DC fans are 12v 0.7 amps. Still uses the existing wiring without losses. LCD TV, 42" with the AC/DC box removed takes 48v 2 amps. Uses 4 modules in series, still can use existing wire but the outlet must be marked 48v. AC heat pump replaced with 12v 2 amp pump, same wire. Etc.. No inverter cost. 65% less chance of breaking down. 10 X 100wp modules in series give power for your Nissan Leaf. Still 12v module and 12v battery but 120v in series. No inverter.

Re: AC oscillating fans were 45w so DC fans are 12v 0.7 amps.

You're saying that whereas a (115v) AC motor for a fan uses 45 watts, a DC motor that moves the same amount of air requires only about 8 watts? I question that. I would guess that most of the energy going into a fan motor is to actually move the air. So, are you saying that if I have a large industrial fan that uses a 1000 Hp AC motor, I could switch to a DC motor and only need a 200 Hp motor (45 / 8 ~= 5, so 1/5 of 1000 Hp)? Seems like industries would be all over that.

Re: AC heat pump replaced with 12v 2 amp pump, same wire. Etc..

See above.

Re: 10 X 100wp modules in series give power for your Nissan Leaf. Still 12v module and 12v battery but 120v in series. No inverter.

That makes sense--well at least using batteries in series to get higher voltage when needed makes sense. As I started to write that, I got to thinking that the batteries (or, at least, the individual cells) in the Nissan Leaf are certainly not 120 volt, so there it might make sense to have a different interface to the car for charging from a home solar source--multiple 12 volt circuits in one plug (for convenience).

But the OP was not. He is asking about 12500 watts. Granted the question is a little muddy. A house is not generally characterized as having an electric "capacity" That is however, not an unusual number, representing about 60 amps at 220V.

Serial DC voltage will indeed stack up to 120. Let me know how that works out when you supply the 120VAC charger that comes with the Leaf, or the power supply to your refrigerator.

LOL

Love the link, too.

GA