Load shaving with Xantrex 4548

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! :D


  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    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 $.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • System2
    System2 Posts: 6,290 admin
    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.
    BB. Super Moderators, Administrators Posts: 33,474 admin
    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:
    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.).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Eric L
    Eric L Solar Expert Posts: 262 ✭✭
    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.)