# Starting calculations

Registered Users Posts: 15
Hello all. I posted my first post earlier today in the "Off Grid" forum, describing the larger goal which is upgrading solar power in a cottage. However, I'm definitely a beginner and want to make sure I'm breaking stuff down properly.

I figured to start with how much battery I needed for the goal to run my wife's and my laptops, a satellite modem and small wireless router for 8 hours a day. Is this the right approach:

Satellite modem: 2 amps
Airport: 0.3 amps
My laptop: 0.8 amps on average
Wife's laptop: 0.3 amps on average

Total = 3.6 amps x 120v = 432 watts

432 watts x 8 hours = 3456 watt-hours

3456 watt-hours x 1.25 (80% inverter efficiency) = 4320 watt-hours

4320 watt-hours x 2 (50% depth of discharge on batteries) = 8640 watt-hours

8640 / 12v = 720 amp-hours of battery

• Banned Posts: 17,615 ✭✭
Re: Starting calculations

Minnow;

There is one problem with calculating loads: the results tend to be wrong.
The biggest favour you can do yourself in planning an off-grid system is to buy a Kill-A-Watt http://www.solar-electric.com/kiacpomome.html meter and plug in everything you want to run. Try it for at least a day on each item, and see what sort of numbers for Watt hours you get. You will find that the power ratings supplied by manufacturers or in the realm of science fiction.

Your estimated 4.3 kW hours is very high, in fact. We have an off-grid cabin here, run the same equipment you desire (satellite modem, router, VOIP phone, two laptops, printer) plus refrigerator, water pumps, lights, and even microwave. We don't use more than 3kW hours a day.

Another thing: if you're in the realm of 2kW hours + per day you will not want to use a 12 Volt system. Go to 24 Volt now. The reason being loads will increase over time and it becomes more difficult to store up power at lower Voltages. Here is an explanation of why: http://forum.solar-electric.com/showthread.php?15989-Battery-System-Voltages-and-equivalent-power
• Registered Users Posts: 15
Re: Starting calculations

Hi Cariboocoot,

Thanks for the reply. Okay, I have a Kill-A-Watt here at home and used it to get the numbers on the laptops (0.8 and 0.3 amps). However, the satellite modem and Airport router are 600 miles away and won't be accessible until next year, so I was using the manufacturers' numbers from data sheets of 2.0 and 0.3 amps. However, since you're running so much more than we are, it does seem that those manufacturer specs might be rather off. I can probably find someone with the same router around here to measure it but the satellite modem is another story.

On the 12v vs. 24v front...yes, that is something I've been wrestling with. The problem is that we have a small 12v system in place. Some time ago I wanted to stop heating my cabin with propane lights, so I bought two panels, a 12v controller and a couple of 6v batteries--I didn't spec anything, just bought a minimal system and put it in place. It's worked fine for a decade but, now that we're actually working up there, it's not large enough. Replacing the batteries will happen anyway. If I were to replace the controller and go with a 24v or higher system, is there any way to keep using all the 12v lights that are throughout the cabin?
• Banned Posts: 17,615 ✭✭
Re: Starting calculations

I can tell you that our satellite modem & related equipment (router, VOIP adapter, wireless phone) uses up to 45 Watts depending on what it is doing. It averages about 40. Leaving it on is like running a second refrigerator! (Watt hour equivalent.) Fortunately it is on mostly during the day, which means it can make use of the 'surplus' power from panels after the batteries are fully charged. You just have to remember to shut it off at night. I have everything plugged into a power bar; one for the com system, another for the other office stuff. We stretch out our time by shutting the AC to the laptops off and using their built-in batteries evening and morning. It's picky, but it works.

Your 12 Volt lighting system could be salvaged with a DC to DC converter, if you know the power draw. One example: http://www.solar-electric.com/12to24or24to.html

Or you might keep the 12 Volt lighting system in place as is and plan a new one for AC around 24 Volts.

We still have propane lights, but don't use them. Stopping that and the replacing the propane 'frige saves a lot of propane. We're down to 20 lbs. per month for cooking and hot water. If I ever get that solar water heater built that will reduce further. Much better than 20 lbs. per week!

The more time you spend there, the more the electric pays for itself.
Re: Starting calculations

You can purchase 24 to 12 volt DC to DC converters--They would work but are not cheap.

You can even use a MPPT charge controller (even some DC to DC converters will work too) connected to a 24/48 volt battery bank and use it to directly charge a 12 volt battery bank. Not usually recommended as this adds a second battery bank and charging system--But does work fairly efficiently if you have a large 12 VDC infrastructure you need to support.

There are also 12/24 volt balancing chargers (used for RV's that have 24 volt for vehicle batteries and 12 volt loads for the living area) that can support mixed 24/12 volt loads (not sure how well these work for the long term--but seem to be popular with some RVers).

However, if you have an AC inverter anyway--You might want to think about changing to AC lights (CFL, LED, etc.). Much easier to send 120 watts (1 amp at 120 VAC vs 10 amps at 12 VDC) for any distances. Just "invest" your money in a good quality TSW (true sine wave) inverter (or inverter/charger assuming you have a backup AC genset).

Power usage is a highly personal choice. I suggest 3.3 kWH per day (100 kWH per month) as a good aim point for small/efficient off grid home with most of the usual appliances (lights, efficient TV/Laptop, well pump, washing machine, etc.). But 4.3 kWH per day is not that far off either.

Solar is difficult to expand (mixing old and new batteries, upping battery bank voltage to reduce DC current/wire gauge, adding new/different solar panels to existing array+charge controller, etc.). So--we try to have folks measure their loads to the best of their ability--Off Grid Solar Power is not cheap and having too large of system is just about as bad as having too small of system (different problems, both can suffer from increased operating costs).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 15
Re: Starting calculations

Cariboocoot,

uses up to 45 Watts depending on what it is doing

Wow, 45 watts is a far cry from 275 watts (2.3 x 120)! Assuming that there's some similarity between our systems, I can see why you say that manufacturers are in the realm of science fiction!

This was the first year my wife and I tried to work up there. We did much the same thing...worked on laptop batteries during the early morning and evening hours, then recharged when there was sun and/or when we were running the generator to pump water. We were very stingy with turning the satellite modem on but, unfortunately, our jobs require a lot of online time.

DC to DC converter, if you know the power draw.

If every single electric light is turned on right now we draw just over 9 amps. Even allowing for some more electric lights to find their way into the cabin, that converter you referenced looks like it would do the trick. I suppose we could take our existing system and move it out to the bunkhouse, which is flashlight-powered right now.

replacing the propane

We still use one propane lamp right over the dining room table when it's not too hot but the rest of them don't even have good mantles anymore...though I keep spares just in case they become necessary. The refrigerators (we have 2 since we're over an hour from the nearest store of any type) and a small chest freezer do suck up propane. However, they're only a decade old, which is quite young for something with no compressor, and the cost of scaling solar out to include them...well, it's more than we can tackle short term.

By the way, am I approaching this from the right "end"? I figured I needed to determine battery capacity, then charging but I see some people saying, "I started with such-and-such panels..."
• Banned Posts: 17,615 ✭✭
Re: Starting calculations
Minnow wrote: »
By the way, am I approaching this from the right "end"? I figured I needed to determine battery capacity, then charging but I see some people saying, "I started with such-and-such panels..."

You're doing it right: determine the loads first. The loads run off the battery bank, so they determine how big that needs to be. The panels are there to recharge the batteries.
People who start out by buying panels or other equipment willy-nilly end up with a lot of headaches. Then they come to the forum looking for aspirin.
• Registered Users Posts: 15
Re: Starting calculations

Hi Bill,

You posted while I was responding to Cariboocoot. Thanks.

I have a Samlex PS-1000-12 inverter we're using that's true sine wave. It handles the computers, modem, router, etc. but I know it's not big enough to handle a lot more stuff. On switching lights to AC: the inverter draws power while it's running...I'm wondering if that loss is a significant factor if we just want to run the cabin lights for 3 or four hours in the evening?

We do have a generator that we use for pumping up the water tanks. It was what allowed us to run the computers up there this year. Though I haven't gotten to the charging side of things--since the generator produces far more power than is needed by the water pump, I've thought about getting something like an IOTA charger to take the excess power and put it into whatever battery bank I end up with.
Re: Starting calculations

The inverters are about as efficient as a DC to DC down converter--Where AC inverters can cost you is powering small loads. A good sized inverter can consume 10-40 watts just being "on". If it is already running (computers, pumps, etc.) then the additional load of the lights will be fine (and efficient).

For the out building--I would think about burying an AC line to the building instead of moving your present system there (if distances work out). The extra maintenance/costs of a separate system can be a pain.

You can get inverters with "search" mode--Basically, they turn on for a couple cycles every second or two--looking for AC loads greater than ~6 watts. If you have a few automatic loads (like a fridge/freezer), that can save a fair amount of standby power. However, as the system grows in size, the "tare losses" become less significant and just become rounding error.

With modern AC fridge and freezers these days--Many/most have auto defrost timers (turn on internal heaters to melt the ice)--Over all they are still quite energy efficient--But turning off the power resets the (these days) electronic timer--forcing a defrost cycle every time the the power is "restored".

A search mode inverter would work well with a manual defrost chest freezer (and some folks--typically of the male persuasion) will use a refrigerator thermostat with a chest freezer to make a very efficient refrigerator.

Many of the Off Grid Electric vs Propane issues depend on how the home/cabin is occupied (and the cost of propane in your area). The break even point is probably a spring/summer/fall occupied home will justify the cost of adding panels+batteries+inverter for a very efficient (energy stary type) electric fridge/freezer vs propane. Winter power collection for many folks is not that great and usually need a backup genset anyway.

Yes, you can add an AC charger to your genset and probably not spend that much extra for fuel. Most gensets take ~50% fuel flow for loads from zero to ~50% rated output anyway.

I am a big believer in matching loads to to power sources. If you can use a smaller genset for pumping/backup battery charging, you may safe significantly on fuel costs (use the bigger genset for backup and shop power).

In the end, conservation is going to be an important way for you to save money. Both on cost of equipment and for ongoing maintenance/fuel costs. Refrigerators and computers are very power intensive. Washing machine and (efficient) well pumps typically use much less power.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Banned Posts: 17,615 ✭✭
Re: Starting calculations
Minnow wrote: »
Hi Bill,

You posted while I was responding to Cariboocoot. Thanks.

I have a Samlex PS-1000-12 inverter we're using that's true sine wave. It handles the computers, modem, router, etc. but I know it's not big enough to handle a lot more stuff. On switching lights to AC: the inverter draws power while it's running...I'm wondering if that loss is a significant factor if we just want to run the cabin lights for 3 or four hours in the evening?

We do have a generator that we use for pumping up the water tanks. It was what allowed us to run the computers up there this year. Though I haven't gotten to the charging side of things--since the generator produces far more power than is needed by the water pump, I've thought about getting something like an IOTA charger to take the excess power and put it into whatever battery bank I end up with.

Assuming you mean the Samlex PST-100S-12, it uses about 10 Watts running. So if it is on 24 hours a day that's 240 Watt hours or roughly 20 Amp hours @ 12 Volts. It does not have a search/standby mode. A large inverter-charger like an Outback FX2012T which does have a search mode will use 6 Watts in 'search' but 20 Watts when 'running'. In that respect it is better to stay with the Samlex and just shut it off when not needed. The bigger inverter-chargers really don't make sense unless you need a lot of power on tap all the time, like for running a refrigerator. Otherwise, as you can see from that example, you're using 2X the power on the inverter to supply the same amount of AC.

As has been so often said, scaling up an off-grid system is difficult.
Re: Starting calculations

For folks with a 12 volt system... The MorningStar 300 watt TSW 12 VDC inverter is tough to beat... Both a "search mode" and a remote on/off switch (both draw very little power).

You can probably even justify two inverters--the Morningstar on "seach mode" 24x7 running lights, laptops, etc., and a second inverter that only turns on when more power is needed.

There is nothing like the Morningstar in the 24/48 volt systems until you are spending some serious money (and serious power).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 15
Re: Starting calculations

You are both being a great help. Thank you.

Just some context in case it helps frame the discussion:

* We currently use the cabin 3-5 weeks a year. If/when we retire, that would go to an absolute max of 10 weeks, but probably not even that high.
* Use of lights and use of computers are largely mutually-exclusive: we tend to telecommute during the day and read at night.
* Yep, that's the inverter...sorry for the mistaken designation: Samlex PST-100S-12.
* The other equipment already in place for a decade is a Morningstar ProStar PS-15 charge controller, two 220ah 6v batteries (they are newer), two 75w panels.
* We don't pump off the inverter, we use the generator for that. I thought the startup draw from the pump when the tank has some pressure might be more than the inverter we have could bear. The generator is a Honda EM3500SX I no longer needed at home and brought up rather than buy something new for the cabin. It's definitely more than I need but I only burn 5 gallons of gas a year in it so I haven't wanted to buy a replacement
* We use more propane per week at \$1/lb. than I'd like but, at 3-5 weeks a year of cabin use, the same sort of argument applies...is it worth switching to something else?
* Burying lines doesn't work as we are on an exposed granite island. Winds are a factor with strung wires. For the short run from my rack with 2 panels to the cabin, I ran the wire inside some hard pipe to protect it.

So, for lighting I have four choices to think about:

1) Leave it exactly like it is now and build a new, parallel system that is 24v for larger loads.
2) Leave lights DC, switch system to 24v, use DC to DC converter for lights.
3) Switch lights to AC, leave system 12v, use Morningstar inverter when only lights are needed.
4) Switch lights to AC, switch system to 24v, accept the fact that inverter will use more power when just lights are on.

If switching, is 48v even better than 24v? Or does cost scale up in a non-linear fashion?

Let's assume for the moment (until I can measure next May) that 3.3 kW-hour is a better number. Let's also assume 24v for the moment. Am I correct that I would then calculate batteries as:

3300 (kW hour) / 24 (volts) * 2 (50% battery discharge) = 275 amp-hours of battery to store one day of use? I.e., two strings of 4 220ah batteries would give me about a day and half of power with no sun or generator charging?
• Banned Posts: 17,615 ✭✭
Re: Starting calculations

Let's see what we can analyze here.
Minnow wrote: »
* The other equipment already in place for a decade is a Morningstar ProStar PS-15 charge controller, two 220ah 6v batteries (they are newer), two 75w panels.

Okay, there's a problem: 220 Amp hours @ 12 Volts is not going to recharge properly from 150 Watts of panel. You need more like 350 Watts. As it is you maybe see 8 Amps of current and perhaps 500 Watt hours per day max.
* We don't pump off the inverter, we use the generator for that. I thought the startup draw from the pump when the tank has some pressure might be more than the inverter we have could bear. The generator is a Honda EM3500SX I no longer needed at home and brought up rather than buy something new for the cabin. It's definitely more than I need but I only burn 5 gallons of gas a year in it so I haven't wanted to buy a replacement.

Pumping water from an inverter takes a big inverter if it is a standard centrifugal pump. Other options include using a small pump like this 12 Volt Shurflo: http://www.solar-electric.com/2088-443-144.html
* We use more propane per week at \$1/lb. than I'd like but, at 3-5 weeks a year of cabin use, the same sort of argument applies...is it worth switching to something else?

Probably not. The short duration of your stay, even if expanded to 10 weeks a year, means investing thousands of dollars in a large solar electric system is not worth it. We are here usually May through October about 90% of the time. This makes the electric more economical than using propane and a lot of gasoline.
So, for lighting I have four choices to think about:

1) Leave it exactly like it is now and build a new, parallel system that is 24v for larger loads.
2) Leave lights DC, switch system to 24v, use DC to DC converter for lights.
3) Switch lights to AC, leave system 12v, use Morningstar inverter when only lights are needed.
4) Switch lights to AC, switch system to 24v, accept the fact that inverter will use more power when just lights are on.

If switching, is 48v even better than 24v? Or does cost scale up in a non-linear fashion?

Were it me, I would upgrade the existing 12 Volt system because it can handle your lights and your computer set-up. There isn't much sense in putting in \$8,000 worth of 24 VDC based inverter & equipment for that short stay and low power use. The other investment you might find profitable is an inverter-generator like the Honda EU2000i. That's about \$1,200 but it will use less fuel than the gen you have now.
Let's assume for the moment (until I can measure next May) that 3.3 kW-hour is a better number. Let's also assume 24v for the moment. Am I correct that I would then calculate batteries as:

3300 (kW hour) / 24 (volts) * 2 (50% battery discharge) = 275 amp-hours of battery to store one day of use? I.e., two strings of 4 220ah batteries would give me about a day and half of power with no sun or generator charging?

Well I like to do it step-by-step to lessen the risk of confusion: 3300 Watt hours / 24 Volts = 137.5 Amp hours. Times 2 for 50% DOD so a minimum 275 Amp hour battery bank. You got that number right.

Four 220 Amp hour 6 Volts would give you 220 Amp hours @ 24 Volts (you need all four in series to get 24 Volt, so the Amp hours do not increase at all). That is less than the calculated amount. You'd need to use eight to get 440 Amp hours @ 24 Volts (up to 5kW hours). But remember that your AC loads require a bit more power due to conversion loss and you have to 'feed the inverter'. As long as those considerations are included in your 3.3 kW hours you're all right.

I have 232 Amp hours @ 24 Volts and run around 2.8 kW hours per day. The secret is making use of the power available from the panels once the batteries are fully charged. Basically the battery capacity only has to carry through the 'nighttime' (between charge periods) which still gives me 100 Amp hours * 24 Volts or 2.4 kW hours DC.

The ideal formula is to try for 25% DOD. This gives you two days no sun automatically. On the third day you start the generator. Given your short stay, though, it is probably more economically feasible to run the batteries down to 50% instead of having even more capacity that sits unused most of the time.
• Registered Users Posts: 15
Re: Starting calculations

there's a problem: 220 Amp hours @ 12 Volts is not going to recharge properly from 150 Watts of panel

Yes. When I bought that stuff, I didn't size anything. My sole goal was not turning on a propane light in the heat. It's why I'm a beginner now. The batteries were the type the store carried and they only had 2 panels in the shop although they said I should come back for more. But, when I tried them, my lights ran with never a single problem, even after a couple rainy days, so what did I care? *smile*

Now that stuff starts to matter, I need to educate myself.

Four 220 Amp hour 6 Volts would give you 220 Amp hours

I probably didn't word it well. When I said "two strings of 4 220ah batteries" I meant 2x4...8 batteries in two strings of 4 batteries each. My bad for not being clearer. The 440 amp hours would be about 1.6 the 275 needed, assuming that 3.3 kWh was correct.

You've both given me a lot to think about although I'm sure I'll be back with more questions, especially since I haven't even started figuring out anything about panels.

Thanks.
• Solar Expert Posts: 5,433 ✭✭✭✭
Re: Starting calculations

'Coot has th numbers dialed for the ISP stuff. sat modem, about 15 watts, router a few less, VoIP phone box about 15. We are blessed nowadays to have HSPA cell, JUST close enough to get pretty good service. 3 times the speed of the SAT service, at 1/3 the price, and it included the phone with no airtime charge. All burning just 13 watts, including the built in router!

(if anybody's interested in a bunch of Xplornet/Wildblue equipment, including TRIA's Modems, dishes etc, I have. Bunch for the taking!)

Tony

PS. Why did this morph front eh other thread thqt was started this morning?

T
• Registered Users Posts: 15
Re: Starting calculations

Mostly because the other was a general question and, while waiting for answers, I was trying some calculations myself and wanted to check them.
• Registered Users Posts: 15
Re: Starting calculations

After thinking about this for a bit I'm swayed by Cariboocoot's observation that, given the small amount of annual usage, it may not make sense to upgrade to a 24v system. With either 12v or 24v I will need new panels but the latter would mean a new inverter (~500 for an equivalent model), plus automatically new batteries (~\$600 I think?). If I stay with 12v the existing batteries may be adequate if I had more charging capacity.

The following is all theoretical pending getting to the cabin next May and actually measuring all the loads.

I'm a little uncertain about how one calculates panel needs. Is the following on the right track?

* I have 220 amp hours of battery, don't want to discharge more than 50%, so I need 110 amp hours of charging.
* I can expect about 5 hours of sunlight (thanks, Bill, for the link to PV Watts) so: 110 / 5 = 22 amps per hour
* 22a * 12v = 264 watts of charging, however, there is a 77% efficiency factor so this becomes: 264 / .77 = 343 watts
* I have 150 watts of panel, so I need ~200w more

12v panels don't seem to come in that size, so I'd need to buy two of something like the Kyocera KD140GX. This would exceed the capacity of the ProStar PS-15 controller but Morningstar says they can be paralleled, so I could buy a PS-30 for the new panels. If the situation changed and a 24v system became desirable, only the new controller is wasted money since the panels could be put in series. Does this make sense?

Btw, what is the difference between the LPU and the LFBS models of those panels?
• Banned Posts: 17,615 ✭✭
Re: Starting calculations

Quickest way to get an array "sized right" is to use the peak current shortcut formula:

220 Amp hours of 12 Volt battery. Peak current of 10% capacity = 22 Amps @ 12 Volts = 264 Watts. Typical efficiency derating of 77% = 342 Watt array. Round up to the nearest available panel size (it never hurts to have a bit of extra Watts on the roof).

Second test: compare the resulting array size to the daily Watt hour consumption using the Icarus formula:
342 Watts * 5 hours equivalent good sun * 0.52 (over-all system efficiency) = 889 Watt hours AC.

Don't try to do it "the other way" because charge rate is not constant and batteries do not recharge on a linear power curve. In other words the panels will not put out 'X' Amps all day long and the batteries do not take in 'Y' Amps * time of charging. You start out with low Voltage and high current and as the process progresses the Voltage comes up and the current goes down. Towards the end you are putting in a very small amount of current and the higher Absorb Voltage for a fairly long amount of time.

You are just below the "threshold" of making an MPPT type controller worth the extra money (all other things being equal) so you will probably choose the less expensive PWM type controller. This means you should be adding up the panels' Imps to see if they match the peak charge current and basically ignoring the Watts.

Electrically the two panels are the same. I believe the LFBS has additional bracing on it for different mounting configuration. If you contact NAWS they can tell you specifically.
• Registered Users Posts: 15
Re: Starting calculations
220 Amp hours of 12 Volt battery. Peak current of 10% capacity = 22 Amps @ 12 Volts = 264 Watts. Typical efficiency derating of 77% = 342 Watt array. Round up to the nearest available panel size (it never hurts to have a bit of extra Watts on the roof)

My number was right but my approach to it was wrong...got it. Thanks for the "good" formula.
Second test: compare the resulting array size to the daily Watt hour consumption using the Icarus formula:
342 Watts * 5 hours equivalent good sun * 0.52 (over-all system efficiency) = 889 Watt hours AC.

All right. This part of the test will have to wait until I can get up there and measure all the loads and know exactly what I need.
You are just below the "threshold" of making an MPPT type controller worth the extra money (all other things being equal) so you will probably choose the less expensive PWM type controller. This means you should be adding up the panels' Imps to see if they match the peak charge current and basically ignoring the Watts.

The Kyoceras say Impp=7.91a. The panels I already have say 4.3a. So, (2 x 7.91) + (2 x 4.3) = 24.42 amps, so I'm good if I need 22 amps.

Referring back to something you said in an earlier post: "The secret is making use of the power available from the panels once the batteries are fully charged." If I took all the things that charge up--laptops, iPods, my son's GameBoy, etc.--could I put them on a socket that only turned on when the batteries were fully charged? If I understand the description of something like Voltage Controlled Switch, it does that and could control current flow to a socket. iPods and GameBoys are easy in this regard because they have cigarette lighter chargers. I guess I could get a car charger for the laptop or a very small inverter.
• Banned Posts: 17,615 ✭✭
Re: Starting calculations

Yes you can do that. It's called "load shifting"; making use of the 'extra' power by turning on controllable loads when the power is there and leaving them off when it isn't.

Using a VCS is one way of doing it automatically. You could also just flick the switch on the powerbar at the right time if you are there. High-end charge controllers like the Outbacks have a programmable AUX function that can control a relay to turn things on when the batteries reach Absorb or Float stage, for example.
• Registered Users Posts: 15
Re: Starting calculations

Okay...I guess that's about all I can do at the moment.

While I'm here, can I confirm that I wired up the existing system correctly and am not obviously going to fry something?

1) A wire from Panel #1 to the junction box on Panel #2, where they connect in parallel to #6 wire running to a fusible (15A) disconnect switch and then to the charge controller.
2) From the charge controller #6 wire (largest the terminals on controller will take) to bus bars I had a friend make out of 1/4" copper stock.
3) From batteries #1/0 wire to fusible (110A) disconnect switch and then to bus
4) From inverter #1/0 wire to bus through some Anderson connectors, positive wire has 110A fuse
5) From "Load" terminals on charge controller, #6 wire to small breaker box, Square D QO breakers, then out to DC lights in cabin

If I add the parallel charge controller described above, I'd just duplicate #1 for it, though use a larger fuse.

I'm a little concerned about grounding. There's a small spike in one of the cement supports for cabin. We're on bare granite out in the boonies and I've been hunting for someone with a rock drill to put a spike down into bedrock. May have to bite the bullet and rent one...it's not the money, it's the 2 hour drive each way to the rental store.
• Banned Posts: 17,615 ✭✭
Re: Starting calculations

I don't see a fuse/breaker in between the charge controller and bus bar.
The amount of power batteries can put into the controller (if something goes wrong in it) would be substantially more than the 6 AWG can handle, causing it to go up in flames. Or at least smoke.

Ideally the wires from the two panels would be equidistant to a common connection point. If this is a short distance between the two we won't quibble over daisy-chaining them.

A parallel controller would be attached to the bus bar the same as the existing one, with appropriate fuse/breaker.

The small spike in the concrete is probably not sufficient for grounding. There are other ways of grounding rather than drilling a hole in solid rock (which tends not to be a good conductor). In simple terms, if you can't go deep go long: spreading the ground contact over a larger are is acceptable. They even make grounding plates for this: http://www.homedepot.ca/product/galvanized-ground-plate-c-w-connector/910158 You may need multiple plates to meet code for your soil conditions/area. Some inspectors would no doubt just say "no" even though they are approved. As deep as possible in the wettest, loamiest soil you can get. If you have a lake nearby there are ways of grounding to it too.
• Registered Users Posts: 15
Re: Starting calculations
I don't see a fuse/breaker in between the charge controller and bus bar.
The amount of power batteries can put into the controller (if something goes wrong in it) would be substantially more than the 6 AWG can handle, causing it to go up in flames. Or at least smoke.

Good point. I'll add one as soon as I can get back up there.
Ideally the wires from the two panels would be equidistant to a common connection point. If this is a short distance between the two we won't quibble over daisy-chaining them.

The two panels are side-by-side on a rack. Looking at the data sheet, say 21" extra wire for Panel #1.
The small spike in the concrete is probably not sufficient for grounding. There are other ways of grounding rather than drilling a hole in solid rock (which tends not to be a good conductor). In simple terms, if you can't go deep go long: spreading the ground contact over a larger are is acceptable. They even make grounding plates for this: http://www.homedepot.ca/product/galvanized-ground-plate-c-w-connector/910158 You may need multiple plates to meet code for your soil conditions/area. Some inspectors would no doubt just say "no" even though they are approved. As deep as possible in the wettest, loamiest soil you can get. If you have a lake nearby there are ways of grounding to it too.

Code is largely absent...we're on an island out in the middle of northern Ontario. The grounding plates are a possibility though there is only about 6" of soil anywhere near the cabin and it's very well-drained...i.e., not wet. Island=plenty of water within 25 feet of cabin. What's the approach here?...long cable dropping down into the water? Is there any risk from electricity coming back up the cable since there are lightning strikes on the water every year?
• Banned Posts: 17,615 ✭✭
Re: Starting calculations

Grounding to the lake is possible. Experts disagree on how best to do it.

My opinion (worth exactly what you're paying for it ) is that you could run ground cable below or at ground level to and into the lake and to the bottom if possible. Put a plate there. If there's any sandy/gravely beach around go there. It needs to be in deep, because lake currents and freeze cycles can shift amazing amounts of weight. Get it low enough and you minimize the action against the ground sink.

Lightning shouldn't be a problem as the lake is one big Voltage sink - that's why the bolts hit it. Can be interesting to watch.

Sorry but it's hard to be more specific without seeing the exact layout and I don't want anyone taking my generalized advice on this as gospel for that reason.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Starting calculations

icarus has grounded to his lake for similar reasons. not sure if he changed it at all. grounding to a lake is far better than to soil in my opinion as the water would present a lower resistance than the soil. what you might need to watch is anything that could possibly break the wire on its way to the lake and in the lake. you could trench it that 6in with bare #6 and when it gets to the lake you may want extra protections as high/low lake levels coupled with freezing (ice) could bend and break the wire. if the trenched wire could be accidentally breached then you could lay rocks over the top to prevent access by tools. make the path to the lake as straight as possible and go as deep as you can without getting ridiculous.
• Banned Posts: 17,615 ✭✭
Re: Starting calculations

If it can be dug in that's best. if it's solid rock you could anchor the wire to the rock with a bit of common technology: hammer-drill, concrete anchors, and wire clips.

• Registered Users Posts: 15
Re: Starting calculations

Cariboocoot: Not to worry. I take all advice simply as suggestions, and accept that anything I do was done by me and I'm responsible.

Niel & Cariboocoot: Running below ground/trenching is not really possible. We really are on bare granite. I can find some inches of soil back toward the middle of the island but not heading toward the water. Hammer drills are a possibility, though many hours of work...the granite is hard!

We have a small pump house down near the water so that the pump pushes instead of pulls. The electrical line down to it runs through a corrugated conduit similar to Flex-Cor, which is then inside some heavy-duty ABS pipe for extra protection; that's the standard solution in the area when running across the bare rock. You can step on it and nothing happens. I could do the same for a ground wire, I guess. There'd be no earth contact during that part of the run, but I can get 40' deep of water within 6 feet of shore.

Ice is a major problem as we are right at the mouth of a river coming off the lake and, when the ice breaks in the spring, it takes everything downriver that isn't bolted down...and some things that are. Our docks are heavily bolted into bedrock but we pull our water system and floating docks when we leave. My inclination would be to put a haul rope on the plate, then use flexible/heavy wire for grounding and pull the plate when we leave.
• Solar Expert Posts: 10,300 ✭✭✭✭
Re: Starting calculations

maybe go with that protection and have it extend below what the lowest water level would be to protect it from ice or debris and just leave it there. it would not be worth the trouble to keep dragging it up and the plate's weight could break or weaken the wire. if you go with stranded keep in mind that this could corrode through faster than one large solid wire. if possible you should go in and inspect the wire condition at least once a year if at all possible.
• Registered Users Posts: 15
Re: Starting calculations

I'll talk to some folks who live up there about that possibility. The ice melts out from the solid shore so, if the line was close enough, it may free before the pack ice starts moving.

Inspecting is no problem...I'm in swimming every day when we're up there.