Need to power 14w 24/7 thought i needed only a ~58ah battery but need 201.6ah?

Cerot

Hi, this past week i have been reading a lot about solar & batteries. I am hooked!

My newbie project is to bring 24/7 wifi internet access to an off grid cabin. The signal will come form a house 1.1km away.

I need to power:
3w- Directional antenna with a radio (for a 5ghz wireless bridge)
5w- 1 router/access point (2.4ghz wifi signal to the cabin)
6w- 2 IP Cameras (for remote security and maybe monitor batteries voltage)
14w- Total

336 watt-hour (14w * 24) - If i want to charge the occasional laptop and cellphone. How many watts-hour should i add?

With a Solar Insolation 5.5 i need a minimum of 61w (336wh/5.5) of Solar. I plan to buy a 100w Monocrystalline

According to this page: http://www.batterystuff.com/kb/tools/calculator-sizing-a-battery-to-a-load.html#Note1
I need a 58ah battery (1.2a load 24/7)

But then found this page. http://www.wholesalesolar.com/battery_sizing.html
28ah/day
84ah (for 3 day operation)
84ah / 0.5 depth of discharge = 168ah
168ah * 1.20 ambient temp multiplier = 201.6ah
You can find temps average of the cabin location here: http://www.weatherbase.com/weather/weather.php3?s=982031&cityname=La-Rumorosa-Baja-California-Mexico&units=metric

According to that page i would need a 201.6ah battery

I really need that much? Of course i want the batteries to last several years, but investing that much on battery for just 14w seems overkill.
If i use 70ah (two 35ah) would i risk them too much on winter or when is cloudy a few days in a row?
Should i buy a higher wattage panel?

niel

Re: Need to power 14w 24/7 thought i needed only a ~58ah battery but need 201.6ah?

i'll make a few comments here. the 3w transmitter is rf power and not the dc draw to run the transmitter. figure on doubling that if you don't know for sure the dc draw on it. the transmitter will need a separate pv, cc, and battery being it is 1.1km away. you also forgot about the 5.8ghz receiver unless it's built into the router/wifi arrangement.

aiming those antennas will not be as easy as you think.

as to accounting for days when the pvs don't deliver (cloudy days) this depleats your battery capacity and when the pvs do deliver again you are only accounting for your loads and not recouping the lost capacity in the batteries necessitating your pv system bigger than you first thought needed to bring up the charge in the batteries. i think you are also confusing yourself to a degree here on the capacity needed after figuring for the daily load requirements. it goes total load draw in ah x 2 for minimum daily battery capacity. then for the number of days you wish to be able to operate then multiply the daily needed by the number of days of operation. always try to up the pv capacity and battery capacity farther than you feel you need to to cover unforeseen bases and future expansions to the system.

there is also a hidden danger in undercharging your batteries when making the capacity larger for more days of autonomy as the pvs must meet a minimum of a 5% charge and it may be better at 10% for most batteries with possibly even more depending upon the battery type and manufacturer. remember that you will have loads drawing away from the charging of the batteries so if you were to base it at say for example a 5% minimal charge rate that loads on it will put it under that minimum.

westbranch

Re: Need to power 14w 24/7 thought i needed only a ~58ah battery but need 201.6ah?

I did not see any mention of the voltages to be used, assume you are talking all at 12 volt, BUT are there any pieces that use 120V? If so you will need an inverter.

jcheil

Re: Need to power 14w 24/7 thought i needed only a ~58ah battery but need 201.6ah?

westbranch wrote: »

I did not see any mention of the voltages to be used, assume you are talking all at 12 volt, BUT are there any pieces that use 120V? If so you will need an inverter.

And just another thought. I know all of those devices you are running technically run on 12v thru those 120v AC Block type wall transformers. Don't bother trying to eliminate the AC transformers and run them directly from the 12v.

Your 12v battery bank can sometimes be 11v and sometimes 15v and the 15v can damage some of that equipment and the 11v may be too low for it to work at all (or properly). Those electronic devices are designed for a constant 12v that is provided by the block transformer. I know this from experience of a neighbor who thought it would be better to just connect them to the 12v and ended up damaging the devices.

So as was just suggested above, plan for an inverter.

Cerot

Re: Need to power 14w 24/7 thought i needed only a ~58ah battery but need 201.6ah?

niel wrote: »

i'll make a few comments here. the 3w transmitter is rf power and not the dc draw to run the transmitter. figure on doubling that if you don't know for sure the dc draw on it. the transmitter will need a separate pv, cc, and battery being it is 1.1km away. you also forgot about the 5.8ghz receiver unless it's built into the router/wifi arrangement.
Sorry i forgot to mention, the other house does have grid energy.
I only need to power the radio in my side, datasheet show max power consumption is 3 watts. http://dl.ubnt.com/ag5_datasheet.pdf

aiming those antennas will not be as easy as you think. I know

as to accounting for days when the pvs don't deliver (cloudy days) this depleats your battery capacity and when the pvs do deliver again you are only accounting for your loads and not recouping the lost capacity in the batteries necessitating your pv system bigger than you first thought needed to bring up the charge in the batteries. i think you are also confusing yourself to a degree here on the capacity needed after figuring for the daily load requirements. it goes total load draw in ah x 2 for minimum daily battery capacity. then for the number of days you wish to be able to operate then multiply the daily needed by the number of days of operation. always try to up the pv capacity and battery capacity farther than you feel you need to to cover unforeseen bases and future expansions to the system.
336wh / 12v = 28ah battery * 2 = 56ah * 2 days = 112ah Total Battery. I am correct here?


there is also a hidden danger in undercharging your batteries when making the capacity larger for more days of autonomy as the pvs must meet a minimum of a 5% charge and it may be better at 10% for most batteries with possibly even more depending upon the battery type and manufacturer. remember that you will have loads drawing away from the charging of the batteries so if you were to base it at say for example a 5% minimal charge rate that loads on it will put it under that minimum.

With a 100w panel and 5.5 insolation you think i will be able to charge the 112ah battery?

Thanks for your help

Cerot

Re: Need to power 14w 24/7 thought i needed only a ~58ah battery but need 201.6ah?

westbranch wrote: »

I did not see any mention of the voltages to be used, assume you are talking all at 12 volt, BUT are there any pieces that use 120V? If so you will need an inverter.

Yes, i plan to use an inverter, 12v to 110v.
I forgot to add the inverter losses to my load. So if am using 14w for a 80% efficient inverter i would need 17.5w right?
I was looking to this inverter in amazon: http://www.amazon.com/emergency-appliances-converters-generator-MRI10011-1/dp/B007SLDDHQ/ref=wl_it_dp_o_pC_nS_nC?ie=UTF8&colid=3P7KOIM15JWEB&coliid=I1W4OG1LWOHUMD

Is 1000w inverter overkill for my load needs? What wattage you recommend?
Thanks for the help.

jcheil wrote: »

And just another thought. I know all of those devices you are running technically run on 12v thru those 120v AC Block type wall transformers. Don't bother trying to eliminate the AC transformers and run them directly from the 12v.

Your 12v battery bank can sometimes be 11v and sometimes 15v and the 15v can damage some of that equipment and the 11v may be too low for it to work at all (or properly). Those electronic devices are designed for a constant 12v that is provided by the block transformer. I know this from experience of a neighbor who thought it would be better to just connect them to the 12v and ended up damaging the devices.

So as was just suggested above, plan for an inverter.

Thanks for the tip.

westbranch

Re: Need to power 14w 24/7 thought i needed only a ~58ah battery but need 201.6ah?

our sponsor has this Samlex 300W for ~$150 http://www.solar-electric.com/sa300wa12vos.html

as well as a smaller 150W model

BB.

Re: Need to power 14w 24/7 thought i needed only a ~58ah battery but need 201.6ah?

In general, you want the AC inverter sized to your load... The good quality True or Pure Sine Wave inverters seem to run around a minimum of 6-7 watts just truend on and no loads. The bigger guys can run 10-20-40 watts just turned on.

So--Part of the research is to look for the minimum sized AC inverter that will meet your needs. With small AC loads, just running the inverter is darn near the same as the rest of your loads.

Another issue is the minimum cutoff voltage for the inverter... Many will run to 10.5 or even 9.5 volts--That takes a battery dead--And pretty much will kill the battery for any further use. A good minimum is around 11.5 volts--if the inverter can be programmed for that. Another thing that can hurt is the maximum operating voltage for the inverter--Many deep cycle batteries will need near 15.0 volts to fully/quickly charge. And many inverters will shutdown at 15 volts (some will shut down at 16 volts). If your battery bank gets really cold (below freezing), the high charging voltage can be an issue.

So, several calculations. I will make them (more or less) worse case--So you can see the math. If you wish to make different assumptions, be my guest.

First sizing the battery. 1-3 days of storage and a maximum of 50% discharge for normal operation. Pick 2 days as a good nominal design:

• (14 watt load + 6 watt inverter) * 1/0.85 inverter eff * 24 hours per day * 2 days of storage * 1/0.50 max discharge * 1/12 volt bank = 188 AH battery bank @ 12 volts

Get a pair of 6 volt @ 220 AH golf cart batteries (cheap, pretty rugged, and will last 3-5 years unless there is an "oops"--And there is almost always an oops).

To charge the batteries from solar power, first the rate of charge of 5% to 13%:

• 220 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 207 Watt array minimum
• 220 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 414 Watt array nominal
• 220 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 539 Watt array cost effective maximum

To charge the battery bank based on daily use and 5.5 hours of sun:

• (14w+6w) * 24 hours per day * 1/0.52 end to end system eff * 1/5.5 hours of sun = 168 Watt minimum array

So, in your case, you have lots of sun and the minimum array would be based on the battery bank's minimum charging current of 207 Watts... Of course, you could go as high as 539 watts with a pair of golf cart batteries--Probably only useful if you have stretches of cloudy weather and don't want to run a genset or shut down your loads after a couple of days--Or if you will have other loads later.

So, any 150 to 300 watt inverter would be fine--And if you can find an efficient/smaller inverter, that meets your needs--That would be good too.

Looks "doable". The above calculations are pretty conservative. But you should never plan on running 100% of your predicted power (i.e., don't expect a 20 watt load on a 5.5 hour sun day)... you should have some "extra headroom" to allow for bad weather/growing loads.

-Bill

Cerot

Re: Need to power 14w 24/7 thought i needed only a ~58ah battery but need 201.6ah?

BB. wrote: »

In general, you want the AC inverter sized to your load... The good quality True or Pure Sine Wave inverters seem to run around a minimum of 6-7 watts just truend on and no loads. The bigger guys can run 10-20-40 watts just turned on.
Looking at this 150w inverter specs: http://gpelectric.com/files/gpelectric/Docs/Specs/Go_Power_SPC_GP-SW150-300-600.pdf
It says No Load draw: 0.2a - 2.4watts (.2a * 12v)
No sure how true this will be as looking at other 150w inverter specs the no load draw is <0.5a


So--Part of the research is to look for the minimum sized AC inverter that will meet your needs. With small AC loads, just running the inverter is darn near the same as the rest of your loads.

Another issue is the minimum cutoff voltage for the inverter... Many will run to 10.5 or even 9.5 volts--That takes a battery dead--And pretty much will kill the battery for any further use. A good minimum is around 11.5 volts--if the inverter can be programmed for that. Another thing that can hurt is the maximum operating voltage for the inverter--Many deep cycle batteries will need near 15.0 volts to fully/quickly charge. And many inverters will shutdown at 15 volts (some will shut down at 16 volts). If your battery bank gets really cold (below freezing), the high charging voltage can be an issue.
I looked a few 150w and others < 1000w inverters and most have the cutoff voltage at 10v. Most will shutdown voltage at >15v
With a proper sized bank and solar panel is this cutoff voltage really necessary? Is there other (automatic) ways of cutting energy to the inverter when voltage is <11.5v?


So, several calculations. I will make them (more or less) worse case--So you can see the math. If you wish to make different assumptions, be my guest.

First sizing the battery. 1-3 days of storage and a maximum of 50% discharge for normal operation. Pick 2 days as a good nominal design:

• (14 watt load + 6 watt inverter) * 1/0.85 inverter eff * 24 hours per day * 2 days of storage * 1/0.50 max discharge * 1/12 volt bank = 188 AH battery bank @ 12 volts

I see, so let's say an inverter draws 3w at no load. It will draw more when there's load or that loss is accounted on the efficiency %?

Get a pair of 6 volt @ 220 AH golf cart batteries (cheap, pretty rugged, and will last 3-5 years unless there is an "oops"--And there is almost always an oops).
Sorry for the dumb question, but you mean two 6v battery @ 110ah each to make them 12v in series right?
The price of flooded batteries is great (compared to sealed). But do you think maintaining flooded batteries will be a problem if the cabin is not visited sometimes up to a month?


To charge the batteries from solar power, first the rate of charge of 5% to 13%:

• 220 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 207 Watt array minimum
• 220 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 414 Watt array nominal
• 220 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 539 Watt array cost effective maximum

To charge the battery bank based on daily use and 5.5 hours of sun:

• (14w+6w) * 24 hours per day * 1/0.52 end to end system eff * 1/5.5 hours of sun = 168 Watt minimum array

What "1/0.52 end to end system eff "means?
I am confused here, so i need a minimum of 168w (or the 207w?) array to be able to charge the 220ah bank every 24hr in a day with 5.5 insolation?


So, in your case, you have lots of sun and the minimum array would be based on the battery bank's minimum charging current of 207 Watts... Of course, you could go as high as 539 watts with a pair of golf cart batteries--Probably only useful if you have stretches of cloudy weather and don't want to run a genset or shut down your loads after a couple of days--Or if you will have other loads later.

So, any 150 to 300 watt inverter would be fine--And if you can find an efficient/smaller inverter, that meets your needs--That would be good too.
Finding a cost effective small inverter seems hard. Now thinking adding an irrigation system to make the loads higher

Looks "doable". The above calculations are pretty conservative. But you should never plan on running 100% of your predicted power (i.e., don't expect a 20 watt load on a 5.5 hour sun day)... you should have some "extra headroom" to allow for bad weather/growing loads.

-Bill

Thanks for your help Bill!

BB.

Re: Need to power 14w 24/7 thought i needed only a ~58ah battery but need 201.6ah?

Cerot wrote: »

Looking at this 150w inverter specs: http://gpelectric.com/files/gpelectr...50-300-600.pdf
It says No Load draw: 0.2a - 2.4watts (.2a * 12v)
No sure how true this will be as looking at other 150w inverter specs the no load draw is <0.5a

Off grid power is a series of trade off between conservation and building out a lager system... Conservation is key to a cost effective system (in my humble opinion).

I looked a few 150w and others < 1000w inverters and most have the cutoff voltage at 10v. Most will shutdown voltage at >15v
With a proper sized bank and solar panel is this cutoff voltage really necessary? Is there other (automatic) ways of cutting energy to the inverter when voltage is <11.5v?

It depends... If you will be monitoring the system (say you add an Internet Capable voltage meter to your setup)--You can always shut the system down/start a small genset to keep the battery from discharging too deeply.

This is the bain of designing a remote power system... Adding ability to operate automatically adds costs (warn user, start backup genset, turn self off before system goes dead, add battery monitor to log AH/battery storage levels, etc. Vs accepting that the battery bank may "go dead" on occasion and reduce battery life if you don't go and check the system every few days--especially during poor weather.

I see, so let's say an inverter draws 3w at no load. It will draw more when there's load or that loss is accounted on the efficiency %?

At low power, you probably need to model the Tare load (on and no load) plus losses from the load--Probably something like:

• 3 watt + (14w*1/0.85 inverter eff) = 19.5 watt average DC load from inverter input

I try to be conservative in my estimates... Could you use 90% or 95% efficiency in the above equation--probably yes.

• 3 watt + (14w*/0.95 invt eff) = 17.7 Watt load with "better model"

Did it make a large difference--Not really. More or less--Doing the paper power calculations (and guestimate on hours of sun per day)--If you are within ~10% of the "real numbers"--That is pretty much "exact" for solar power system work.

You should only use around 66% to 75% of the systems estimate daily power... Gives you some "wiggle room" for mistakes in power estimates and a bit of room for growth in power usage over time.

Sorry for the dumb question, but you mean two 6v battery @ 110ah each to make them 12v in series right?
The price of flooded batteries is great (compared to sealed). But do you think maintaining flooded batteries will be a problem if the cabin is not visited sometimes up to a month?

Yes and no... Yes, two 6 volt batteries in series gives you 12 volt battery bus.

No in that when you add batteries in series, the voltage increases, not the Amp*Hours. So, when you take two 6 volt @ 220 AH batteries in series, you get a 12 volt @ 220 AH battery bank.

If I had two 12 volt @ 110 AH batteries and connected them in parallel, then I would have a 12 volt @ 220 AH battery bank.

The price of the batteries and size/weight of the two banks would be the same. The advantage of two 6 volt batteries in series is that you will only have 6 total cells to check water levels in. And only one string of current flow--No problems with parallel current paths with multiple parallel strings (bad connection will "knock" battery string out of the circuit--but you will have a hard time seeing the failed electical connection). A failed series connection, the system stops working before the batteries are damaged.

With a good battery charger, you can check battery water levels about once a month--And add water about every two months (assuming everything is working OK).

When you first bring up the system--You will need to check it every couple of days, then every week until you are sure everything is OK... And probably every few days if you have heavy cloud layers (during winter storms) to make sure the system does not over discharge the batteries and kill the bank.

To charge the batteries from solar power, first the rate of charge of 5% to 13%:

• 220 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.05 rate of charge = 207 Watt array minimum
• 220 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 414 Watt array nominal
• 220 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.13 rate of charge = 539 Watt array cost effective maximum

To charge the battery bank based on daily use and 5.5 hours of sun:

• (14w+6w) * 24 hours per day * 1/0.52 end to end system eff * 1/5.5 hours of sun = 168 Watt minimum array

What "1/0.52 end to end system eff "means?
I am confused here, so i need a minimum of 168w (or the 207w?) array to be able to charge the 220ah bank every 24hr in a day with 5.5 insolation?

More or less, if you put a 100 watt panel on your system, hit it with 5 hours of sun per day, you can get about 52% of that power out your 120 VAC inverter as "usable energy". The details:

• Solar panels average around 81% of rated output in warm weather and/or with a PWM type charge controller
• 95% efficient charge controller
• 80% efficient lead acid battery
• 85% efficient AC inverter
• 0.81 * 0.95 * 0.80 * 0.85 = 0.52 end to end energy conversion/storage/inverting efficiency

Is it exact, no--But close enough for solar power system designing.

168 Watt panel will run your system, on average. However, the battery bank will not like the level of charging current... Batteries need around 5% minimum rated charging current (~207 watts in the above calculations--Should add your ~20 watt*1/0.77=26 watts of solar array) load and loses to be exact) to properly "stir" the electrolyte (bubbling gases), open the pores in the lead plates, get the battery "quickly recharged", and allow for a bit of battery "self discharge" (which gets worse as the battery ages).

Just one of those rules of thumbs where saving a few bucks on solar panel size ends up with a shorter battery life as the downside (and generally less happy with system performance).

Finding a cost effective small inverter seems hard. Now thinking adding an irrigation system to make the loads higher

Yep--Welcome to the life of a design engineer/off grid power systems designer... Everything is 2-3x larger than you think it should be due to "losses" and inefficiencies--And costs 2-3x as much too.

And why we really push the paper design first before you buy the hardware--Buying without looking at the details first--Almost always will be less than ideal solution (many times we have people return/sell pieces of hardware once they do the "paper designs".

-Bill

Cerot

Re: Need to power 14w 24/7 thought i needed only a ~58ah battery but need 201.6ah?

Hi Bill, thanks a lot for your thoughtful replies. I am learning a lot with your responses here and in other threads.

BB. wrote: »

It depends... If you will be monitoring the system (say you add an Internet Capable voltage meter to your setup)--You can always shut the system down/start a small genset to keep the battery from discharging too deeply.

I read that the LVD in most inverters are actually to protect the AC loads (or the inverter itself?) from damage when the voltage gets too low. I don't know how true this is, but makes sense since the LVD in most is too low to protect the batteries. Then i found out some charge controllers have LVD too, but with good parameters to protect the batteries compared to inverters.

Definitely i will need something automatic that disconnects the inverter (no DC loads in my batteries) from the batteries when the voltage is low (50% soc) and reconnect when the bank is full again. I know voltage only can't tell the true state of the batteries, but if my AC loads are constant is good enough?

I found a product on an UK store that supposedly can do this (can i post the link here or is not allowed?). Of course i think the best approach is to find an inverter with good LVD settings.

168 Watt panel will run your system, on average. However, the battery bank will not like the level of charging current... Batteries need around 5% minimum rated charging current (~207 watts in the above calculations--Should add your ~20 watt*1/0.77=26 watts of solar array) load and loses to be exact) to properly "stir" the electrolyte (bubbling gases), open the pores in the lead plates, get the battery "quickly recharged", and allow for a bit of battery "self discharge" (which gets worse as the battery ages).

This still a little confusing for me ..

I need to add the 26 watts of solar array to the 207 watts?

Yep--Welcome to the life of a design engineer/off grid power systems designer... Everything is 2-3x larger than you think it should be due to "losses" and inefficiencies--And costs 2-3x as much too.
-Bill

Yes, this is actually very discouraging for solar newbies. In my opinion solar panels should be sold with more information like for example "This Panel of 100w will give you an estimated of 260watt-hour/day in a location with 5hr of solar insolation per day" But i know that leaves out a lot of variables

BB.

Re: Need to power 14w 24/7 thought i needed only a ~58ah battery but need 201.6ah?

Cerot wrote: »

I read that the LVD in most inverters are actually to protect the AC loads (or the inverter itself?) from damage when the voltage gets too low. I don't know how true this is, but makes sense since the LVD in most is too low to protect the batteries. Then i found out some charge controllers have LVD too, but with good parameters to protect the batteries compared to inverters.

Yes, some charge controllers have an output port. You have to check the spec's... Some of the LVD controlled ports support fairly small current levels--As long as your loads are small--it can work.

Definitely i will need something automatic that disconnects the inverter (no DC loads in my batteries) from the batteries when the voltage is low (50% soc) and reconnect when the bank is full again. I know voltage only can't tell the true state of the batteries, but if my AC loads are constant is good enough?

Your loads may be constant, but the amount of sun is not...

There are Battery Monitors that have programmable outputs (Victron is one). Program it to "turn on" at 50% and turn off at 80% SOC. And get a small/efficient AC inverter (like the MorningStar 12 volt 300 Watt TSW inverter) which has a remote on/off input. The MorningStar also has a dip switch that can set the LVD to 11.5V and Reconnect to 12.6V (factory default).

11.5 volts as minimum battery voltage under load/cutoff is not too bad.

I found a product on an UK store that supposedly can do this (can i post the link here or is not allowed?). Of course i think the best approach is to find an inverter with good LVD settings.

Adding links that help move the discussion forward are fine...

This still a little confusing for me ..

I need to add the 26 watts of solar array to the 207 watts?

Basically, what I was trying to say--If you are trying to charge the battery bank with a 5% minimum charge, and you have a constant load, you need the solar panels to not only supply the 5% load (207 watts), you need the panels to supply the 24x7 load too--In this case another 26 watts, or 233 Watt minimum panel.

It is just one of the issue as if you try to run the minimum panel needed to keep the battery bank charged at the 5% minimum rate, you should account for the running loads too.

Yes, this is actually very discouraging for solar newbies. In my opinion solar panels should be sold with more information like for example "This Panel of 100w will give you an estimated of 260watt-hour/day in a location with 5hr of solar insolation per day" But i know that leaves out a lot of variables

In general, if your numbers are within 10% of what I have predicted here--You are doing well. The amount of variation in sun from day to day, weather, etc... Let alone trying to make power measurements that are better than 5% accuracy.

-Bill

Cerot

Re: Need to power 14w 24/7 thought i needed only a ~58ah battery but need 201.6ah?

BB. wrote: »

There are Battery Monitors that have programmable outputs (Victron is one). Program it to "turn on" at 50% and turn off at 80% SOC. And get a small/efficient AC inverter (like the MorningStar 12 volt 300 Watt TSW inverter) which has a remote on/off input. The MorningStar also has a dip switch that can set the LVD to 11.5V and Reconnect to 12.6V (factory default).

I looked at the Victron battery monitor manual, your recommendation is to connect the monitor relay output to the remote on/off input of the MorningStar?

But if using an inverter without this feature (remote on/off) could i use a latching relay connected on the cable from the battery to the inverter and control this relay with the relay of the battery monitor?

11.5 volts as minimum battery voltage under load/cutoff is not too bad.

Yes, but i would like to have the LVD option apart in the case of adding other inverter in the future or replacing for other model. The 20w load is not yet set in stone, we are thinking in more loads like a small tv, satellite receiver, led lights, small fan and irrigation system.

Adding links that help move the discussion forward are fine...

This is the products http://www.reuk.co.uk/buy-REUK-PROGRAMMABLE-LOW-VOLTAGE-DISCONNECT-LVD.htm
It seems ok, but of course an actual battery monitor approach would be better.

Basically, what I was trying to say--If you are trying to charge the battery bank with a 5% minimum charge, and you have a constant load, you need the solar panels to not only supply the 5% load (207 watts), you need the panels to supply the 24x7 load too--In this case another 26 watts, or 233 Watt minimum panel.

It is just one of the issue as if you try to run the minimum panel needed to keep the battery bank charged at the 5% minimum rate, you should account for the running loads too.

-Bill

Got it, thanks

If i end buying a 250w panel like this one http://www.solar-electric.com/solarworld-sunmodule-sw250-polycrystalline-solar-panel.html
Voc: 37.6 Volts Vmp: 30.5 Volts Isc: 8.81 Amps Imp: 8.27 Amps

Using a PWM controller i will need a 24v battery bank?