New small Off-Grid System Design and Evaluation

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

Hello all,

I massaged my sys design based on your input, more of my own load and component evaluation and have come up with the following:

(4) SunXtender 224Ah, 6V batteries wired in series for a 224Ah, 24V battery bank
(3) Solarworld 250W PV Panels wired in series - 750 watt total array
(1) Midnite Solar Classic Lite 200V MPPT charge controller
(1) Trimetric TM-2025-A Battery Monitor
(1) Deltec 500A, 50mV current shunt
(1) Mini-DC disconnect Power Center w/ 250 amp breaker
(2) Midnite Solar MNEPV 60A breakers
(1) Samlex 1500W, 24V sine wave Inverter (.85A idle, 3000W surge < 1 sec, 85% eff)

My calculated loads total 1200 AC watt/hours per typical day.
I have also designed around an "air-conditioner emergency" load of 648W 5.4A AC load for short duration (no other loads connected).

I get alot of good sun in AZ averaging 6.45 sun hours (5.06 low - 7.12 high).

So, am I in the ballpark?
Any experience with a Samlex Inverter?
I figured my battery bank charge rate at @ 9% - is this ok for AGM sealed ?

Thanks a bunch,
-lockwhip

Home: Gridtied - (30) Mage 235W panels w/enphase micro-inverters, eu2000i tri-fuel gen
Cabin: see above sys design - hope to install soon...

niel · February 2013

Re: New small Off-Grid System Design and Evaluation

you don't need the classic 200 as the classic 150 will handle this just fine for 3 solarworld 250s in series. if you were to plan a 4th pv in series then the cl200 would be needed.

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

niel wrote: »

you don't need the classic 200 as the classic 150 will handle this just fine for 3 solarworld 250s in series. if you were to plan a 4th pv in series then the cl200 would be needed.

Hi Neil,
I chose the 200V cuz it was the same price as the 150V thinking I might add that 4th panel. Any drawbacks with the 200V if I don't add the 4th panel for awhile?
Thanks a bunch - lockwhip

BB. · February 2013

Re: New small Off-Grid System Design and Evaluation

I believe the higher voltage controllers have a lower maximum rated output current to the battery bank.

-Bill

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

BB. wrote: »

I believe the higher voltage controllers have a lower maximum rated output current to the battery bank.

-Bill

Hi Bill,

Right you are, the Midnite Solar Lite 150V is rated at max current output of 96A, the 200V at 79A.
So... will the 150V CC charge my 224Ah 24V battery bank faster than the 200V CC (with just the (3) 250W panels)?
I'm assuming it will...

Thanks a bunch -lockwhip

BB. · February 2013

Re: New small Off-Grid System Design and Evaluation

I don't know the details of the Midnite product line--So you might want to try their array sizing/controller selection tool:

http://www.solar-electric.com/misoclcoto.html

There are a lot of interrelated configuration issues... Output current limit based on max voltage rating, actual battery bank voltage, etc...

-Bill

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

BB. wrote: »

I don't know the details of the Midnite product line--So you might want to try their array sizing/controller selection tool:

http://www.solar-electric.com/misoclcoto.html

There are a lot of interrelated configuration issues... Output current limit based on max voltage rating, actual battery bank voltage, etc...

-Bill

WOW! What a great array sizing/controller selection tool...
So I'm marginal on the Hypervoc with (4) 250W panels on the 150V CC but OK on the 200V CC - all other parameters OK.
Think I'll opt for the 200V CC.

-lockwhip

westbranch · February 2013

Re: New small Off-Grid System Design and Evaluation

your answer might be on this page http://www.midnitesolar.com/pdfs/classicPowerChart.pdf

vtmaps · February 2013

Re: New small Off-Grid System Design and Evaluation

lockwhip wrote: »

WOW! What a great array sizing/controller selection tool...
So I'm marginal on the Hypervoc with (4) 250W panels on the 150V CC but OK on the 200V CC - all other parameters OK.
Think I'll opt for the 200V CC.

I think that is a mistake. Go for the Classic 150. When you get a fourth panel, wire them up in two strings of two panels per string. All of the Classics work more efficiently (makes less heat and lasts longer) when the input voltage is not so much greater than the output voltage.

I don't recall, did you mention how far your combiner is from your controller? The only good reason to use a classic with very high input voltages is if there is extreme distance between combiner and controller. And if that is your situation you should have a 48 volt system... its easier for your classic to down convert a high voltage to 48 volts than to 24 volts.

--vtMaps

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

Hello Neil, BB, Westbranch, vtmaps:

I think I am starting to get a handle on a design thanks to all your input; thanks so much.

Since my off-grid solar system design is to provide only 1200 Wh/day, I started looking at system component "self-consumption", idle current, "tare" etc.

BB- you used a 0.52 system derating factor - would this "cover" the component self-consumption?

The Midnight 150 Lite consumes @ 3-5 watts from what I've read.
And a 1500 watt inverter runs 20 watts and up! For instance, the Samlex 1500W, 24V inverter has an idle current rating of 0.85A!

I'm surprised at how much the system self-consumes!@#@!

Any advice or suggestions on minimizing "self-consumption" is appreciated.
Note: I will not be powering anything at night (sleeping). Should I, can I just turn the whole system off at night?

Thanks a bunch,
-lockwhip

BB. · February 2013

Re: New small Off-Grid System Design and Evaluation

The 0.52 derating comes from:

0.81 solar panel derating * 0.95 charge controller losses * 0.85 inverter losses * 0.80 flooded cell battery losses = 0.52

So, you can look at your overall inverter efficiency... But, basically, the inverter is around 0.85 efficient when loaded. If the inverter is "on" but not loaded for 20 hours per day, then you should add the "tare losses" to your total power needs.

If the inverter is supplying loads 24x7 at ~85% efficiency (or only "turned on" when loads are present), then you do not need to add any tare losses.

-Bill

westbranch · February 2013

Re: New small Off-Grid System Design and Evaluation

have you considered a 2 inverter system? a small ` 300-500 W for most items and a larger one to switch on for big loads.

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

BB-

Thanks for the info on adding "tare losses" to my power requirements.
Since I'll have no loads at night, should I just shut this puppy down for the night to "save" some battery bank Ahs?

-lockwhip

lockwhip · February 2013

Re: New small Off-Grid System Design and Evaluation

Hey westbranch-

Yep, (2) Inverters... I really think that's the way I should go. Sizing my system for that occassional large load makes less and less sense...
The "tare current" on an inverter to handle an emergency load is a killer. Thinking more about using my EU2000i when I need to...

Thanks a bunch,
-lockwhip

BB. · February 2013

Re: New small Off-Grid System Design and Evaluation

Many of the larger inverters have a "search" mode... They basically turn on one cycle every second looking for a >6 watt AC load. Can save a fair amount of energy.

If you stay small, the 12 volt TSW 300 watt inverter from MorningStar (600 watt for 10 minutes) is difficult to beat. Has both a search mode and a remote on/off switch (12 vdc signal level).

You usually need to go to a much larger/more expensive inverter to get that sort of functionality. Turning off the DC to an inverter is not cheap (need high current switch/breaker/relay) for most inverters.

-Bill

vtmaps · February 2013

Re: New small Off-Grid System Design and Evaluation

BB. wrote: »

If you stay small, the 12 volt TSW 300 watt inverter from MorningStar (600 watt for 10 minutes) is difficult to beat. Has both a search mode and a remote on/off switch (12 vdc signal level).

But the OP is building a 24 volt system. There are not a lot of choices in low power 24 volt inverters. Exeltech inverters have a good reputation and their 250 watt unit has a tare loss of 6 watts and their 600 watt unit has a tare loss of 8 watts.

Victron also has a good reputation, and their Phoenix line of inverters has a 700 watt unit with a tare loss of 5 watts :cool: Problem with the phoenix is that it has outlets and is listed for marine use... I would prefer a unit (like the exeltech) which is meant to be hard-wired into a home.

And then there is the Midnite vaporware inverter... the engineers at Midnite have indicated (on their forum) that they are working on an inverter... it might be worth waiting to see what they come up with.

You can start out with only a large inverter (as I've done) and at a later time add a small inverter to the system as part of your ongoing energy efficiency upgrades. I have two circuit breaker panels for my house. One has two 20 amp circuits going to a total of five outlets. This panel will power laundry, vacuum, kitchen appliances, and other large loads. The second panel has a dozen 15 amp breakers and goes to all other lights and outlets in the house. Right now both panels are connected to my big inverter, but it will be very easy to add a small inverter and wire it to the small loads panel.

--vtMaps

New small Off-Grid System Design and Evaluation

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