Generator/ Charger Questions

adam1984 · February 2010

So heres what I am wondering. I calculated to offset 100% of electricity, I would need an 8.27kW system. This is the array size only. Now... if i would install a XW6048 inverter/charger, and a 3000i honda generator, since i wouldn't need the batteries to charge solely off the panels, would this help lower the system? If so would it be huge or small? I know it depends on do i want the generator running 24/7.... but lets say the generator shouldn't run more than 4 hours a day, and the charger... well lets kill two birds with one stone. In an off grid system, how were is the charger getting power to charge the batteries? Confused about that. Lets say the batteries are charged by the panels, and they run out of power at night. Where exactly is the charger getting power from?? So anyway with that question answered would a generator 3000i and a 100amp charger take away some of the required panels?

Kamala · February 2010

Re: Generator/ Charger Questions

adam1984 wrote: »

... Lets say the batteries are charged by the panels, and they run out of power at night. Where exactly is the charger getting power from??

The battery bank is sized (according to expected loads) large enough to prevent this from happening.

adam1984 wrote: »

So anyway with that question answered would a generator 3000i and a 100amp charger take away some of the required panels?

This is a little out of my league but I will try to add some perspective. You can see what components are in my system by reading my signature line below. I have only 225 watts (not Kwatts) of panels. This system cost ~ $5000.

This is not my full time residence.

Good luck!

K

BB. · February 2010

Re: Generator/ Charger Questions

Adam,

You are kind of looking at the system from the middle and then changing things around the edges... It gets real confusing real fast exactly what your questions are (at least to me).

First--conservation. It is almost always cheaper to spend money on conservation than it is to generate the power. I.e., you conserve 1kWhr per day--that is 365 kWhrs per year, and 7,300 kWhrs per 20 years...

That is a lot of energy never needed--and, in this case, a 12% or so smaller battery bank, 12% less battery replacement battery banks every 8 years or so, 12% less solar panels, 12% less generator fuel, etc...

So--double pane windows, lots of ceiling and wall insulation, weather stripping, high efficiency heat pumps (if electric heat/cooling/hot water), laptop computers instead of desktop systems, energy star appliances, no extra beer fridge in the garage, etc...

So... Using PV Watts program for Newark NJ. 8.27 kWatts of panels, assume 0.77 for GT, (0.77*0.90 inv eff=) 0.69 hybrid eff, and (0.69*0.80 batt eff=) 0.55 efficiency when cycling the batteries...

So, the PV Watts output for 0.77:

"Station Identification"
"City:","Newark"
"State:","New_Jersey"
"Lat (deg N):", 40.70
"Long (deg W):", 74.17
"Elev (m): ", 9
"PV System Specifications"
"DC Rating:"," 8.0 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 6.2 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 40.7"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:","11.2 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.36, 662, 74.14
2, 4.05, 715, 80.08
3, 4.58, 867, 97.10
4, 4.84, 848, 94.98
5, 5.30, 934, 104.61
6, 5.33, 881, 98.67
7, 5.27, 890, 99.68
8, 5.25, 880, 98.56
9, 5.06, 854, 95.65
10, 4.46, 804, 90.05
11, 3.15, 574, 64.29
12, 2.87, 554, 62.05
"Year", 4.46, 9464, 1059.97

For 0.69 (XW GT inverter):

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.36, 591, 66.19
2, 4.05, 639, 71.57
3, 4.58, 775, 86.80
4, 4.84, 757, 84.78
5, 5.30, 833, 93.30
6, 5.33, 784, 87.81
7, 5.27, 792, 88.70
8, 5.25, 785, 87.92
9, 5.06, 762, 85.34
10, 4.46, 718, 80.42
11, 3.15, 511, 57.23
12, 2.87, 493, 55.22
"Year", 4.46, 8439, 945.17

And the XW GT inverter in Off-Grid mode 0.55:

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.36, 463, 51.86
2, 4.05, 502, 56.22
3, 4.58, 611, 68.43
4, 4.84, 594, 66.53
5, 5.30, 654, 73.25
6, 5.33, 614, 68.77
7, 5.27, 621, 69.55
8, 5.25, 615, 68.88
9, 5.06, 599, 67.09
10, 4.46, 563, 63.06
11, 3.15, 399, 44.69
12, 2.87, 385, 43.12
"Year", 4.46, 6621, 741.55

So, depending on which what type of inverter and which mode your XW is running in--we get quite a wide range of estimated outputs...

In general, the minimum battery bank for a 6kW XW inverter is recommended at 600 AH @ 48 volts... If you are going to cycle it every day--need a lot more.

The Honda eu3000i is a nice little genset--but it is no prime source... Only good for emergency power--not multi-year power source...

So--I am confused.

-Bill

adam1984 · February 2010

Re: Generator/ Charger Questions

Well now that i have confused everyone, the simple question is, if I need a 5kw system to offset my energy, would using a generator and charger mean that I only need a 4.5kw system since the batteries arent charging solely off the panels.

dwaller · February 2010

Re: Generator/ Charger Questions

adam1984 wrote: »

Well now that i have confused everyone, the simple question is, if I need a 5kw system to offset my energy, would using a generator and charger mean that I only need a 4.5kw system since the batteries arent charging solely off the panels.

You mean that you need to offset 5Kwh/day? If so, you will need approximately 20, 200W panels to completely offset your usage during the month of December, which is your month of lowest daylight hours/production.

I used the following math (stolen from BB when he helped me figure this out):

45,000 Wh per mnth * 1/1,000 watt of panels * 1/30 days per month * 20 panels * 200 watts = 5940 WH per day

I don't know how to calculate how much of a charge you can expect from a genset, someone else will have to step in and tell you how much you can scale down your array rating based on how often you want to run the generator and how big of a generator you would use for the offset.

audredger · February 2010

Re: Generator/ Charger Questions

adam1984, If you need 5kw and run a .3kw gen 24/7 then, you would need 4.7kw to make up the difference.

I think the confusion lies in the generator part, alternate energy refers to an alternate to fossil fuel. The cost of, a generator every 6 months (they are NOT designed to run 24/7), oil and fuel (either delivered or sleped yourself) will cost way more than grid power!

In an off grid system, we all use a generators to offset peak short term loads (I run the big diesel gen when I weld or use the plasma torch) and to charge the batteries in bad weather but, using one offset a "normal" load does not make $cents.

B.B. is correct, it is far, far, far cheaper to conserve than to generate (read generate via pv, wind, hydro, steam or fossil or neuclear fuel).

If you need 5kw, then build a 5kw system. Better yet build a 6 or 7kw system and not run on the raged edge all the time. Lots of good advice in this forum and good people to help you design it right the first time. (far cheaper to do it right the first time than to redo it!!!)

It is also cheaper to build a grid tied system than an off grid system.

BB. · February 2010

Re: Generator/ Charger Questions

Adam,

Are you looking for Grid Tied, Hybrid (GT with Off-Grid for power failures), or 100% Off-Grid?

In the end, the power has to be generated, stored (if hybrid or off grid), and inverted back to 120/240 VAC (if GT, Hybrid, or off grid)...

A generator can provide from ~5 kWhrs to ~10 kWhrs per gallon of fuel when operated in its efficient range (depending on fuel type, generator, output load, etc.).

Conversions cost power--A battery charger is ~80-90% efficient, an inverter is approximately 80-95% efficient, a lead acid battery bank is ~80-90% efficient, a MPPT charge controller is ~90-95% efficient, solar panels are ~15% efficient wrt sunlight and are 80% efficient with respect to their "factory sticker rating", running a genset at 25% rated load is 1/2 as efficient as one loaded to 50%-100% rated load...

Every time you go through a conversion step, there will be loses.

Take a genset powering your AC loads:

5.5 kWhr per gallon (efficient small gasoline genset):

5.5 kWhrs/gallon direct to AC load = 5.5 kWhr per gallon (direct gen to load)
5.5 kWhrs/gal * 0.80 battery charger * 0.80 batt * 0.85 invrtr = 3.0 kWhr/gal (charging battery for later use)

So--just connecting a genset to the load vs charging a battery bank during the day to use the power at night from the bank/inverter costs you almost 2x the fuel usage for the same amount of useful energy. If you have heavy loads and need to use the genset (pumping water, A/C, etc.)--you are generally better off running both the genset and heavy loads at the same time and use the battery bank at night when the loads are light and variable--running a genset then tends to be fuel inefficient.

I still am not understanding exactly what your question is... Power used has to come from somewhere. You have power that is only available under certain conditions (sun is up, generator is running, generator is full of fuel, battery bank is charged, utility bill paid for and power lines not down, etc.).

If you have mixed sources (solar and genset) feeding loads (battery for storage; and "real" loads like washer, lights, radio, computer, etc.)--how you use them is a function of your needs and physics.

My suggestion--pick the "heart" of your energy system... If Off-Grid--that sort of is the battery bank. Panels "charge" the battery bank. Loads "drain" the battery bank.

-Bill

adam1984 · February 2010

Re: Generator/ Charger Questions

So BB, am I correct to say that you are paying for the power one way or another, so rather than relying more on a generator and charger, I would just get a bigger system? And it would be better to use the sun rather than gallons upon gallons of gas to accomplish the same task? And rather than worrying about how to get more power, perhaps my time would be better spent figuring out ways to reduce my power usage? If i confused you more im sorry.

dwaller · February 2010

Re: Generator/ Charger Questions

First, you need to answer these questions:

1. Are you going off-grid completely (and do you understand what that entails)?
2. Do you have grid power available?
3. What is your daily usage?

adam1984 · February 2010

Re: Generator/ Charger Questions

dwaller yes off grid completely/ yes power available/ and 23kwh/day. BB i only asked for that clarification to ensure i got the main point of your post and really respect your opinion, so i didn't know if that is what i should be looking at (2 posts before this one).

BB. · February 2010

Re: Generator/ Charger Questions

Adam,

Thank you--but I am human (and a guy)--so it is very possible that I have screwed something up somewhere. :roll:

My points--if there are any:

Conserve electrical usage. Look into transferring electrical loads to alternative fuels (wood, propane, natural gas, etc.).
Conserve
Conserve (ongoing joke here--first three rules of solar power)
Look at Solar Thermal for DYI hot water / home heating projects. Much less expensive to install. Downside is a lot of plumbing and pumps--so can be a "maintenance experience" for years down the road.
Qualify your loads--by day, month, season. Note that some loads (well pumps to holding tanks/ponds--can work well with solar panels and no batteries--just pump during the day. For presurizing the house/cabin--use a small DC or AC pressure pump off of your battery bank for indoor plumbing). For many people, there larger loads tend to be in sunny weather (irrigation pumping, A/C)--so limits the absolute "winter size" of PV array/Battery bank.
Quality your site. If you are in the far north/located in coastal regions, etc.--your local weather conditions may dramatically impact your average output by season.
Qualify your site for shading... If you have shading/partial shading (power lines, tree branches, buildings, etc.) that you have no control over (you can't cut down city/neighbor trees)--Solar arrays do not work well in partial shade--need full sun.
Now compare your needs (loads by day/month/season to the various options out there.
In areas with, for example, poor winter production due to weather, may choose to size your system for 100% solar for 9 months of the year, and make up "missing solar production" with a fuel powered genset. Sometimes it is just not worth the solar panels + battery banks to attempt 100% solar 100% of the year. You can always try the 9 month solution--and add solar panels later if you decide to offset more generator use.

In the old days ($10 per watt solar panels)--A good number for off grid power system is around 100 kWhrs per month (3.3 kWhrs per day).

And even today--that is still a good break point for Utility Power vs Off-Grid system. As the system sizes increase, their complexity and absolute costs go up too... Not many people want to pay $100,000-$250,000 up front costs for a 20 kWh per day off-grid system for home use.

Don't get me wrong--some people do want that size of off-grid system. Power usage is very personal and we attempt to be "non-partisan" about personal power use. We do recommend Conservation^3 --- just because most of us are "CHEAP"

.

-Bill

crewzer · February 2010

Re: Generator/ Charger Questions

adam1984 wrote: »

yes power available/

How about using the grid as a backup power source? You can do this with the XW inverter/charger.

Mangas · February 2010

Re: Generator/ Charger Questions

I think Bill's summary is right on the mark for off grid planning.

I've got a large hybrid system. I planned it to be a comfortable off grid compromise using alternative fuel for heating but splurged on two zone central A/C and providing more than enough power (30% actual useage excess capacity) for everything else.

Even though it's a larger system, we planned extensive conservation measures i.e. RASTRA building materials, triple pane windows, insulated architectural metal roof, variable speed high efficiency HVAC compressors and air handlers, aluminum retractable shutters on all doors and windows, high insulation construction values and so on. You just can't do enough conservation.

Rather than add even more panels and batteries to a very robust capacity, we back up the system with a 15 KW water cooled 1800 rpm propane genset to cover three months in the winter. In our application that means maybe running 2 hours a day during December-February inclement weather days for a total run time of only 60 hours or so a year. Still cheaper than adding more solar hardware.

I do not recommend building a larger off grid hybrid system without a properly sized and maintained genset standing in reserve. Peace of mind.

Generator/ Charger Questions

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