Upgrading from 12 to 48 volt system?

Re: Upgrading from 12 to 48 volt system?

alberta wrote: »

So I have gone through a load calculator at ABS Alaskan website. Daily usage is about 4700 watts (Watt*Hours? -Bill B.) with no air conditioning.

Just to be clear--You are probably talking about Watt*Hours here... I.e., 100 watt light running for 5 hours would be 100w*5h=500WH.

That is a goodly amount of power... I like to talk about a 100 kWH per month/3.3 kWH per day as just about enough power for a very energy efficient off grid cabin/small home to live a pretty normal life (with propane and other fuels/heat sources).

Your 4.7 kWH per day (or ~141 kWH per month) is not too far off of that "ideal" number.

That chart recommends increasing the wattage by 1.15% for inverter losses, although some of the inverters I have looked at say they are 93% efficient. Should a conversion factor for AC loads still be 15%, or do you use 7%?

I like to use 85% as the inverter efficiency number. A bit on the conservative side and accounts for times when you may be running small loads on a large inverter (usually inverters are much less efficient when lightly loaded) and the few times you may be using very heavy loads (again, efficiency tends to fall off a bit under heavy loads).

It is a "good enough" to start number--but if you have an "unbalanced" setup (say a 3kW inverter that carries mostly 100 watts or so and only occasional heavy loads like a well pump--lots of "tare" losses as the inverter idles).

You can address some of those issues with, perhaps two inverters. Once small one for your computer, few lights, battery/cell chargers that you leave on most of the time. And a second larger inverter that only turns on when you need to support the water pump, washing machine, etc.

There are some nice inverters out there that remote On/Off switches and/or "standby modes" where they use much less power (turn on for a cycle every couple of seconds looking for a >6 watt AC load that was just turned on).

The ABS chart then uses an additional load correction factor for other inefficiencies in the system. In my case, this would mean 4700 watt hours per day, times 1.15% for inverter loss, or 5405 watt-hours, and then increased further for the 70% efficiency factor, to 7,721 watt hours per day.

For a first cut rule of thumb--I start with ~0.52 derating from solar panel to charge controller to battery bank to AC inverter. And you have to tell us how many hours of day of "full sun" you get/want to plan for.... Say 4 hours, then the equation for the solar array would look like:

• 4,700 WH per day * 1/0.52 system derating * 1/4 hours of sun minimum per day = 2,260 Watt minimum solar array

Of course, if you use much of your power during the day, you will have less battery losses. In sub freezing temperatures with a MPPT charge controller and snow on the ground--you can get some pretty high panel efficiencies, etc.

Using PV Watts for Calgary Canada, a 2,260 Watt fixed array would produce around (and 0.52 derating, enter 2.26 kW in array size):

"Station Identification"
"City:","Calgary"
"State:","AL"
"Lat (deg N):", 51.12
"Long (deg W):", 114.02
"Elev (m): ", 1077
"Weather Data:","CWEC"

"PV System Specifications"
"DC Rating:"," 2.3 kW"
"DC to AC Derate Factor:"," 0.520"
"AC Rating:"," 1.2 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 51.1"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 0.1 CanB/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value (CanB)"
1, 3.72, 142, 0.12
2, 4.39, 148, 0.13
3, 5.24, 193, 0.17
4, 5.38, 182, 0.16
5, 5.49, 185, 0.16
6, 5.59, 176, 0.15
7, 6.24, 201, 0.17
8, 5.90, 193, 0.17
9, 5.26, 168, 0.14
10, 5.09, 178, 0.15
11, 3.41, 123, 0.11
12, 2.79, 106, 0.09
"Year", 4.88, 1995, 1.72

So a 2.26kW array would meet your demand for ~9 months of the year--and use the genset the other three as needed.

As I have a generator to fall back on, and since the system is used mainly in the summer, I think would be OK having 2 days reserve in the battery bank. Using a 50% dod, I would need a battery bank of 643 amp hr at 48 volts, or 1286 amp hrs at 24 volts.

Hmm... My simple calculations. Assuming 0.85 efficient inverter, 2 days of "no sun" and 50% maximum discharge for a 48 volt battery bank:

• 4,700 WH * 1/0.85 inverter eff * 1/48 volt batt * 2 days no sun * 1/0.50 max discharge = 461 AH @ 48 volt minimum battery bank

Do these calculations seem right?

That is still a pretty good sized battery bank.

And, I do not like paralleled strings of batteries. I would prefer larger AH rated cells (6 volt, or even 4 volt or 2 volt cells) in series (I would suggest no more than 2-3 parallel strings of batteries).

Paralleled batteries have more cells to check for water, more wiring/current sharing problems, and a shorted cell can take down an entire battery bank.

Inverter
Peak usage appears to be about 3000 watts, being when the fridge is starting, a saw is starting, and some miscellaneous loads are running.

You may want to split this into two inverters--A smaller (ideally) TSW inverter sized to run your home appliances (1,500-2,000 watt or so), and a second, possible MSW (less expensive) to run the hand tools.

I think I could use either a 24 volt or 48 volt system, with the inverter putting out at least 3000 watts? I prefer the Magnasine so I could run 220 in from the generator, and then 220 out to the panel for my 220 loads(water and sewage pump)

You are close to the 24 / 48 volt decision point... If you can use 24 volt DC appliances (truck appliances, 24 volt DC water pumps, etc. with relatively short 24 VDC wiring runs)--You might stick with 24 volt.

If you have only AC equipment and/or longer wiring runs for your well pump, etc... Then running from 48 volts is probably better (keep battery strings down, and reduce DC current/wire gauge). Assuming you can find a 48 volt inverter that meets your needs and price point.

Batteries
In terms of batteries, I have boat access only to my cabin, and have to haul the batteries up a hill, so I would prefer limiting each battery to about 120 lbs, so that eliminates forklift-type batteries.

Some battery vendors are offering some large AH batteries in 4 volt and 2 volt cells. That will allow you to have high AH cells to string together in series and keep the weight down somewhat.

• Trojan has some 120 lb 6 volt and 2 volt batteries in higher AH ratings.
• Crown has some 2 volt 500 AH batteries that only weigh ~76 lbs.

What brand/model of battery to get--First size out your system / battery bank (with a few options), then see what is available in your area.

There are lots of battery vendors and lots of opinions about battery vendors.

I don't have an off-grid system, so I am not the guy to really recommend specific ones.

-Bill