# Competing Designs

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Solar Expert Posts: 252 ✭✭
Gentlemen,

I was wondering, given your druthers and money was not an issue, what you would recommend for on off-grid, battery based system with a normal load of 150W, high draw (650W for five minutes, 20 minutes cycle, but only for 12 hours) with 48hour energy reserve?

I would even be interested in recommended manufacturers for MPPT, PV, INV/CHGR, battery type/voltage/size, etc.

I hope you all jump in with your thoughts, could be very educational for us Newbies.

Thanks

• Banned Posts: 17,615 ✭✭✭
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Re: Competing Designs

Let's see if I understand this.

150 Watts constant? For 24 hours? That would be 3600 Watt hours right there.
650 Watts for 5 minutes every 20 minutes for 12 hours? (20 off, 5 on = 25 total?) 28.8 cycles, the 5 minutes @ 650 Watts per cycle total 312 Watt hours (less 150 allotted in the other part of the calculation) 240 Watt hours.

Total: 3840 Watt hours (unless I understood part of that wrong or slipped a digit on the calculation).

This would be a case of design for 4kW hours. If that is AC it would be probably ~4500-4800 DC or 100 Amp hours @ 48 Volts used.

Now it gets interesting because the load demand is always <1kW but the best storage is at 48 Volts; try finding a small 48 Volt inverter. So we slip down to 24 Volts and at least 400 Amp hours (50% DOD). But you want forty-eight hours battery capacity? Whoops! Multiply by two at least: 800 Amp hours @ 24 Volts. There you are maxing out an 80 Amp controller.

Why not use a 48 Volt system? Because of the inverter issue: larger inverter = more tare loss as a rule. But you have also spec'd an inverter-charger, which puts the choice back up to the big units as the tare on a 24 Volt I-C is the same on a 48 Volt one.

So minimum battery bank: 440 Amp hours @ 48 Volts. That's two parallel strings of GC2's if you want to go cheap. Array to recharge it: 2743 Watts minimum (round up). If you want to limit the DOD of the battery bank to 25%, double it. You could still almost get away with this on a single 80 Amp MPPT charger (FM80, Classic 150). Might have to do a little EQ charge now and again. Could alternately choose slightly smaller battery bank: something between 400 and 800.

Smallest quality 48 Volt inverter-charger is probably the OB FX3048T.

That's the general. To be more specific we'd need more info on what is being done and why.
• Super Moderators, Administrators Posts: 33,541 admin
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Re: Competing Designs

How much sun? Alaska in winter--May be tough with pure solar.

Obviously, my first question would be the power draw. If you could get it below 300 Watts, the MorningStar 300 Watt 12 VDC TSW inverter with Search Mode/Remote On/Off would be sweet. Otherwise, just finding a larger (TSW recommend) AC inverter would be the next choice (and the 10-20 Watt Tare load--Can be a killer for small systems).
• 150 Watts * 12 hours = 1,800 WH continuous
• 400 Watt peak * 5min/60min * 12 hours = 400 WH peak load
• 15 Watt Inverter Tare * 24 hours = 360 WH inverter running 24*7
=====================================================
• 2,560 WH per day
Battery bank, at least 24 volt recommended:
• 2,560 WH * 1/24 volts * 1/0.85 inverter eff * 2 days * 1/0.50 max discharge = 502 AH@ 24 volt bank
Array based on bank size. 5% to 13% typical recommendation, for full time, 10-13% better--I would go with 13% as a start:
• 502 AH * 29 volts charging * 1/0.77 panel+controller derating = 2,458 Watt array minimum (based on battery size and day time loads)
And based on daily loads and amount of sun... You pick the numbers here... If 9 month a year system, typically 4.0 hours of sun. If winter, could be 2.0 hours or less (again depending on location):
• 2,560 WH per day * 1/0.52 system eff * 1/4 hours of sun per day = 1,231 Watt array minimum (4 hours of sun minimum)
• 2,560 WH per day * 1/0.52 system eff * 1/2 hours of sun per day = 2,462 Watt array minimum (2 hours of sun minimum)
And then there is the question of what happens with bad weather. Genset? Auto Start Genset? Run the battery bank at 10% to 20% rate of charge. Typical gensets that are ready for autostart from the factory (including auto-choke) are probably 4kVA minimum (roughly).
• 502 AH * 29 volts charging * 1/0.80 charger eff * 1/0.80 genset derating * 0.10 rate of charge = 2,275 kVA minimum (real rough numbers)
• 502 AH * 29 volts charging * 1/0.80 charger eff * 1/0.80 genset derating * 0.20 rate of charge = 4,549 kVA minimum (real rough numbers)
So, you could justify a ~4kWatt/kVA backup genset pretty easily.

If the genset was for emergency use only (failed inverter, manual start), I would be very likely to pick a Honda eu2000i (or even a pair) with an extended run setup (gas cap with hose to draw fuel from a larger fuel tank). Much more fuel efficient and less bells and whistles to automate the whole (unattended) off grid power system.

Unattended systems (auto start, fail over, remote diagnostics, auto switch between A and B power systems, etc.) are expensive and quickly become overly complex--Especially if the site is either attended or can suffer outages until somebody comes by for a look-see.

Selecting hardware... Choosing a "cheap" inverter or a full function inverter-charger--Not sure until you validate (or repudiate) my assumptions above.

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 252 ✭✭
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Re: Competing Designs

Thank you gentlemen.

The parameters are from our needs. We don't do the auto start, it's easy enough to turn a key when we need to charge. When we sell this place the installed components are going with it, so I wanted to get some information on the best way to set-up if and when we relocate. I got real lucky with what I bought the first time around, it works fairly well for us, but I want to do it right the next time.

I wish I had known this site existed before..........

The cyclical draw is from the Toyo Heater when run when not at home to feed the woodstove.
• Solar Expert Posts: 140 ✭✭
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Re: Competing Designs

<<snip>>

Now it gets interesting because the load demand is always <1kW but the best storage is at 48 Volts; try finding a small 48 Volt inverter.

<<snip>>

Looking at our host's data sheet for the Exeltech XP series (PDF alert), it appears that Exeltech does offer their 1100W inverter in a 48VDC version. A quick Web search for the appropriate model number (XPK-1-4-6) turned up a few for sale online (our host only lists the 12VDC and 24VDC versions), but the least expensive one I saw had a price tag of \$855.93. That vendor's Web site did not indicate whether the vendor offered the X2 low no-load option (10W versus the standard 20W, both under no load).

Of course, this option would require a separate charger for when the genset was in use.