Inverters: 1 large PSW or two small PSW/MSW

levsmith

I am ready to buy an inverter for my battery backup/future solar system and have come to a halt. Right now I am just planning on running lights, a tv and maybe a sump pump during the summer depending on the weather when the power goes out. I would like to buy a small MSW right now (~1000w) and maybe a small PSW later on down the road for fridge, freezer (again~1000w) when the budget allows. I started looking into it, and now I'm not sure whether to go that route or buy one large PSW (~2000w). The 1000w MSW is 66 bucks and the 1000w PSW is 260 so that would be $326 total and the 2000w PSW is $400 so there really isn't much of a price difference.

Are there any benefits of having two small inverters rather than one large one or should I bite the bullet and get the large PSW? Thanks for any opinions!

BB.

Re: Inverters: 1 large PSW or two small PSW/MSW

It always comes done to loads and hours per day of run time...

A full sized energy start type refrigerator probably needs a 1,200 to 1,500 watt inverter minimum. That is right at the 12/24 volt divide for the battery bank voltage.

For pure emergency, you may be able to get away with an MSW inverter. However, for refrigerators and many "modern" electronic devices, you are probably better off with a PSW type... It is the old 80/20 rule--80% of your stuff will work fine, 10% will probably overheat and fail, and another 10% is hard to predict.

Running induction motors on MSW will dump ~20% of MSW energy into the motor itself (PSW does not do this)--So motors run hotter. If you are looking for a reliable backup power installation and want to run more than a few appliances that your are willing to "test"--Go with PSW type inverters.

You can get a small PSW type inverter to recharge cell phones, battery chargers (AA, battery packs for tools), and laptop computers--And get a larger MSW for other loads (sump pump, tools, etc.). Smaller inverter waste less power--As soon as you go larger inverter + 24 hour per day power (such as running your refrigerator)--The system has to be much larger to account for all the "new" losses too.

It is very difficult to "grow" off grid power systems... A small system that will keep you in lights, run a small sump pump, cell phone charging, small TV/laptop/etc. is probably 1,000 WH per day (1 kWH per day).

As soon as you add a full size refrigerator and a bit more loads, you are getting closer to 3.3 kWH per day system. Using a typical 2 day / 50% discharge and 12 volt battery bank:

• 1,000 WH per day * 1/12 volt battery bank * 1/0.85 inverter eff * 2 days storage * 1/0.50 max discharge = 392 AH @ 12 volt battery bank
• 3,300 WH per day * 1/12 volt battery bank * 1/0.85 inverter eff * 2 days storage * 1/0.50 max discharge = 1,294 AH @ 12 volt battery bank
• 3,300 WH per day * 1/24 volt battery bank * 1/0.85 inverter eff * 2 days storage * 1/0.50 max discharge = 647 @ 24 volt battery bank

The 1 kWH per day system--4x 6 volt ~200 AH Golf Cart Batteries will do nicely.

The 3.3 kWH per day system--To big for golf cart batteries at 12 volts (6-7 parallel strings) and would require 2 large MPPT controllers to recharge at 10% rate of charge (~130 amps).

Go to 24 volts, then you are looking at 65 amps charging (one mppt charge controller) and ~3 parallel golf cart battery strings (4 batteries per 24 volt string * 3 parallel strings = 12 batteries--Still not a small bank). For this large of bank, I would suggest a larger AH capacity cell/battery to get down to 1-2 strings (you could start with GC batteries for 3-5 years and see how it goes).

From a solar array point of view, 5% to 13% is the usual sweet spot for a solar array when recharging a battery bank. Based on 10% rate of charge (a good "off grid" nominal system for most people):

• 392 AH * 14.5 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 738 Watt array nominal for 1 kWH per day system
• 647 AH * 29 volts charging * 1/0.77 panel+controller deratings * 0.10 rate of charge = 2,437 Watt array nominal for a 3.3 kWH per day system

You can see these system are pretty different in scale.

Don't get into buying hardware yet... Measure your loads using a Kill-a-Watt type meter and figure out what you would need/want from your installation.

Another option is to run a small Honda eu2000i genset for 6 hours in the morning and 6 hours in the evening on ~2 gallons of gasoline per day... 20 gallons of stored fuel (siphon from car, etc.) will keep you going for 10 days pretty comfortably.

If you still want 24x7 AC power available, install a 1kWH per day system for lights (and a ~300 watt AC PSW inverter), cell phone/battery charger, and laptop+efficient TV. Use solar and/or genset to keep the smaller battery bank charged during outages.

There is are differences between a $400 and a $1,600 PSW inverter--For $400 and emergency usage--You might try the less expensive inverter and see how it works for you (run it for at least a few days in good weather to make sure it will work when it is really needed for your loads).

-Bill