Sizing inverter

bgu1982

I have a 600 watt solar array hooked up to 4 135ah deepcycle batteries. What size inverter would be right for my system? 

bgu1982

And does brand and price really matter a whole lot because I'm seeing big price differences on similar sized inverters.

706jim

With that size array I would avoid a big cheap inverter. Also, depends on what you intend to power with it. You can run all sorts of things with a 600 watt inverter but at the same time you will be making a list of what it WON'T run. I have used a 1500 watt Trace for many years. 1500 watts will run anything that you could normally plug into a wall outlet. Larger inverters have larger idle current that will tend to drain your battery bank.

bgu1982

Ya that's kind of what is was wondering about. As far as the idle draw. I have a generator for high power stuff like power tools ect. The inverter would be for t.v. computers ECT.

BB.

Ok... Like that is not an "open question"...

My Questions:

• Where is the system installed (nearest major city, full sun)?
• Needs--What are your average and peak Watts and Watt*Hours per day
• Any "special loads" (lots of hours LED lighting, TV, laptop computer, or Microwave, well pump)?
• What seasons (non-winter, in winter)?
• Full time living, or weekend/sunny weather system.

Based on what I know "right now":

Assuming full time off grid--For flooded cell lead acid batteries, plan on using 25% of battery bank per day for 2 days (no sun), and 50% planned maximum discharge (for longer battery life and another day of backup power if needed).

==============Wrong battery assumption on my part. Not 6 volt, but 12 volt batteries. Update in #9 below=============

Based on the battery bank full time off grid system:

• 4x 135 AH * 6 volt (?) batteries * 0.85 AC inverter eff * 0.25 daily usage = 688.6 WH per day
• Say 5 hours of usage (evening time LED lighting, TV, laptop computer)
• 688.6 WH per day / 5 hours per day = 138 Watt average load
• Based on average load--Suggest a 300 Watt AC inverter would be nice

Then there is based on an 8 hour discharge rate (4 hours to 50% loading)--Max suggested AC inverter for that battery bank:

• 4x 6 volt @ 135 AH = 270 AH @ 12 volt battery bank
• 270 AH / 8 hour discharge rate = 33.75 Amp max suggested sustained draw:
• 33.75 amps * 12 volts * 0.85 AC inverter eff = 405 Watt max inverter suggested

Solar charging:

• 600 Watt array * 0.77 panel+controller derating * 1/14.5 volts charging = 31.9 Amps max available charging current
• 31.9 Amps charging / 270 AH charging = 0.118 = 12% rate of charge
• That is a good rate of solar charging (10% to 13%+ recommended for full time off grid)

Hours of sun per day (need to know your location):

• Say 3.0+ hours of sun per day is a good amount for most areas, none-winter time
• 600 Watts * 0.52 off grid system AC system eff * 3.0 hours sun = 936 Watt*Hours per day "break even" spring/fall

500 to 1,000 WH per day is a "good amount" for an off grid cabin with LED lighting, RV water pump, small laptop/tablet computer, and cell phone charging.

You can use this link to estimate hours of sun by month for your location:

http://www.solarelectricityhandbook.com/solar-irradiance.html

My guesses....

-Bill

Photowhit

Usually, you build a system based on loads, NOT the other way around.

I've heard the rule of thumb, an inverter no larger than your array... That sort of works.

Based on wanting to run electronics, TV and computers. I'd suggest a pure sine wave inverter. Modified sine wave inverters tend to cause a bit more heat in electronics which shortens their lives and may cause interference. If you intend to expand, a slightly higher wattage than 600 watt might be in order. If you hope to run a fridge someday, might shoot for a minimum of a quality 11-1200watt up to a 1500 watt.

Assuming the batteries are 135 amps at 12 volts, and all are in parallel, you might look at cautions on having so many batteries in parallel.

bgu1982

I understand you start with load first. But we are at the extent of how far we are going to expand our solar array. Sounds like I should use a pure sine 500 watt. It's only going to be for t.v.,laptop,charging divices like tablet and phones and led lights.

bgu1982

What do you mean about looking at cautions for my 4 12v 135ah batteries in parallel? Is it bad to have them this way? That one I never heard about being bad.

BB.

If these are 12 volt batteries in parallel--I have to update my numbers. I have assumed they were 6 volt batteries (I should have realized they were 12 volt batteries).

Generally, I like to limit placing "a lot" of batteries in parallel. Suggest 1-3 strings of batteries... If >3 strings needed, suggest looking at larger AH batteries and/or higher battery bus voltage.

To parallel batteries, this website has a nice discussion:

http://www.smartgauge.co.uk/batt_con.html

===========Redo above post based on 12 volt @ 135 AH battery bank instead==============

Based on the battery bank full time off grid system:

• 4x 135 AH * 12 batteries * 0.85 AC inverter eff * 0.25 daily usage = 1,377 WH per day
• Say 5 hours of usage (evening time LED lighting, TV, laptop computer)
• 1,377 WH per day / 5 hours per day = 275 Watt average load
• Based on average load--Suggest a 500-600 Watt AC inverter would be nice

Then there is based on an 8 hour discharge rate (4 hours to 50% loading)--Max suggested AC inverter for that battery bank:

• 4x 12 volt @ 135 AH = 540 AH @ 12 volt battery bank
• 540 AH / 8 hour discharge rate = 67.5 Amp max suggested sustained draw:
• 67.5 amps * 12 volts * 0.85 AC inverter eff = 688.5 Watt max inverter suggested

Solar charging:

• 600 Watt array * 0.77 panel+controller derating * 1/14.5 volts charging = 31.9 Amps max available charging current
• 31.9 Amps charging / 540 AH charging = 0.07 = 7% rate of charge
• That is a not a high rate of solar charging (5%+ for summer/weekend sunny weather use. Otherwise 10% to 13%+ recommended for full time off grid)

Hours of sun per day (need to know your location):

• Say 3.0+ hours of sun per day is a good amount for most areas, none-winter time
• 600 Watts * 0.52 off grid system AC system eff * 3.0 hours sun = 936 Watt*Hours per day "break even" spring/fall

500 to 1,000 WH per day is a "good amount" for an off grid cabin with LED lighting, RV water pump, small laptop/tablet computer, and cell phone charging.

Still within the "ballpark" for an off grid system... But if full time off grid, would not hurt to upgrade the solar array:

• 540 AH battery bank * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,017 Watt array @ 10% charge
• 540 AH battery bank * 14.5 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,322 Watt array @ 13% charge

Still don't know where the system is located (hours of sun per day by month) or your usage (non-winter, part time or full time off grid, etc.).

Batteries are "historically" expensive these days, and solar panels are historically "cheap" (pricing is pretty volatile). Keeping the batteries "happy" with enough solar (or even over paneled to a degree) can help extend the life of your batteries.

-Bill

bgu1982

Sorry McAlester, OK. Unfortunately I bought a  HQST
600 watt 40 amp charge controller thinking that would be adequate. I know you should start with load first but I didn't know this. But theoretically I could get another CC of the same brand and size and connect simaler solar array size and connect to the same battery bank safely, correct?

Photowhit

bgu1982 said:

Sorry McAlester, OK. Unfortunately I bought a  HQST
600 watt 40 amp charge controller thinking that would be adequate. I know you should start with load first but I didn't know this. But theoretically I could get another CC of the same brand and size and connect simaler solar array size and connect to the same battery bank safely, correct?

Loads grow, if you are planning on adding, I might go with a 1100 watt - 1200 watt inverter. Samlex makes one that I particularly like. Northern Arizona Wind and Sun doesn't list it, but I bet they can get it. 

It's 1200 watt Inverter Charger and it is designed to be hard wired, and reasonable for what it is. 20 watt idle and >8 watt on stand by. 




Link to the GFI version;
https://www.donrowe.com/samlex-evo-1212f-pure-sine-inverter-charger-p/evo-1212f.htm

mike95490

I'm partial to the morningstar 300w Suresine inverter.  it's as rugged as a tank.  600w surge
   our host's store:
https://www.solar-electric.com/morningstar-si-300-115v-ul-inverter.html
  mfg's page:
https://www.morningstarcorp.com/products/suresine/