Load shaving with Xantrex 4548

System · June 2011

I am installing a Grid Tie Solar System with Battery Backup Power using (2) Xantrex PIXW-4548 inverters & (16) AGM 12v 245AH LTP Deka batteries.

As I first understood it, the batteries would kick in only if the grid went down.

Now I see that with load shaving I might be able to set up the Xantrex system so that the batteries are used every day (say in the evening when no solar is being generated).

As I understand it, I’d still be connected to the grid and not have to manually throw some switch. Is that correct?

How do I configure the system settings to do that?

Thanks for any replies!

mike95490 · June 2011

Re: Load shaving with Xantrex 4548

Each time you use your batteries, you take some life off them, and then consume more power, at another time, to recharge them. So if you don't have a power source (solar/hydro/wind) feeding battery power back to the grid, costs you $.

System2 · June 2011

Re: Load shaving with Xantrex 4548

Sure. I have (36) Sharp PV Panels @ 235w. So the batteries should charge during the day.

As far as reducing the battery life, I wonder if I'd save enough on energy costs (from the grid) to replace the batteries when needed? I don't know how to calculate that!

BB. · June 2011

Re: Load shaving with Xantrex 4548

You can estimate your "costs" of using batteries to load shift... Sort of Worst Case assumptions:

Flooded Cells 80% efficient
Inverter 90% efficient
Charger 90% efficient
Cycle Life (from battery faq):

depth of discharge 50%=1,000 cycles; depth of discharge 25%=2,000 cycles

So batteries that would last ~8-10 years in "float" service vs 1k-2k cycles (or 2.7 to 5.4 years in cycling duty).

So your power costs go up by 1/0.8*0.9*0.9 = 1.54x (yes, this is a worst case assumption using "standard" equipment and older batteries).

To offset 1/2 the load of an "average home" with 1,000 kWH per month power usage; use a 48 volt battery bank with 50% depth of discharge:

500 kWH per month * 1/30 days per month * 1 day of use * 1/0.50 maximum discharge * 1/48 volt bank = 0.79 kAH = 790 AH @ 48 volts

Assume 10 year life for inverter/charger XW (Xantrex/Schneider) hardware. Assume 2.7 year battery life using Trojan batteries. From NAWS:

$3,400 for XW 6kW inverter/charger
$5,280 for 740 AH 48v of 16x Trojan L16RE-B 370 AH Deep Cycle Battery
$2,000 for misc hardware/labor costs

10 year life costs:

$3,400 + $5,280 * 10 yr/2.7 yr life + $2,000 misc hardware/permits/etc.
= $21,559 (yea--just an estimate)

kWH costs for cycling based on equipment life (ignoring labor and inflation/cost of money over ten years + labor to change out batteries ~3-4 times):

$21,559 ten year costs / (500 kWH per month * 12 months * 10 years) = $0.355 per kWH hardware costs

Assuming Northern California power costs... worst case power costs (PG&E PDF): Buy $0.30 per k/Wh and "offset" $0.52 per kWH.

estimated cost of HW + Power = $0.35 HW costs + $0.30*1.54 system losses using Grid Power = $0.81 per kWH
estimated cost of "peak summer TOU power" = $0.52 per kWH
estimated "loss" = -$0.29 per kWH to "time shift" power

Anyway--a very quick back of the envelope estimate for a "time shifting" Power system giving it the worst case assumptions (cheapest summer off-peak power vs most expensive summer peak power costs). And some realistic estimates of hardware and battery life.

Feel free to use the above with different assumptions (industrial batteries, different inverter/charger hardware, different life cycles, etc.).

-Bill

Eric L · June 2011

Re: Load shaving with Xantrex 4548

That's a nice back of the envelope BB. I've played around a bit with this too, and never got it to make economic sense to try to load shift with batteries, using actual peak/off-peak rates.

However, I made some different assumptions that did get load shifting closer. They may or may not be correct, but here's where I differed from what BB did:

- I did not factor in the full inverter cost into the cost of load shifting, since you need an inverter to grid tie. However, I did factor in that a Xantrex XW costs more than most comparable GT inverters. This basically means that the equipment cost for load shifting relative to pure grid-tie is slightly lower, since you're looking at battery replacement costs + extra cost of XW inverter and some extra DC wiring and fuses. I would thus guestimate the 10 year hardware costs to be lower.

- I did not try to compute "estimated cost of HW + Power" in the same way, but was also unsure how to compute it. BB's computation seems to assume that all AC power is put through the entire DC side, including batteries. It's possible the XWs would force you to do this, but from what I've been able to tell (and I don't own one), they don't; they can pass through some AC power from the grid, I think.

- Some of the peak rates I looked at are less that 12 hours/day (50%); often 8 hours/day (33%). This means the DC side could be made a bit smaller, or the DOD on the batteries lesser (prolonging life).

If I'm right with these assumptions, they might make the case for load shifting look a bit better. But I'd be surprised if it actually made financial sense even so.

(Edit: thinking about my last point it's probably a wash since if the peak rates are shorter the savings from load-shifting will be proportionately lower.)

Load shaving with Xantrex 4548

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