'Growing' loads.

icarusicarus Solar Expert Posts: 5,400 ✭✭✭✭
I have always advised folks that when they are designing and building a system to realize that loads will ALWAYS grow. I am finding it to be very true in my own case (not surprisingly) as well.

We built a new house a couple of years ago and designed and built the Pv system, and the electrical system we thought we needed. A close monitoring of our usage reveals that since we moved in, our electrical use has gone up by about 25-30%. Still within the capacity of the system, but getting perilously close. The 200 watts of Pv have grown to 300, (and soon may be a few more!) the 25 amp CC is pushed to it's limit on cold days with edge of cloud events, and we draw the batteries down almost 10% on a daily basis, up from ~7%. Not enough to worry about but interesting non the less.

The reasons for this are varied and subtle. For example, because we have an excess of Pv, leaving the 20 watt modem and router on for more of the day just because we can. We leave the telephone on 24/7 even though it burns 3 watts. Susan wished a new light in the kitchen to give better light at the stove, 15 watts for a few minutes. 2 Fridge fans, 5 watts each going ~8 hours. Ipod, satellite radio each burning 1/2 amp. Very warm earlier this summer, so a new paddle fan in the bedroom, drawing 15-30 watts over night some nights. We have a wood fired hot tub that we fill with a fire fighting pump. If it needs to be cooled off because it is too hot we used to pour buckets in from the lake. Now a hose connected to the house water runs 15 gallons in to cool it off, all water that has to be pumped back in.

So, if you are planning on building an smallish off grid system, do all your load calcs and then add a significant fudge factor, of say 30-50%. It is much cheaper to size stuff up front than to add later.



  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: 'Growing' loads.

    Wise words indeed! :D

    We have experienced the same "creeping growth" here on the other side of Canada. Because you have electricity readily available, you start to use it more. If you have to start a generator for power - you think twice.

    This year we added a microwave (saves propane. No, really) and a second computer workstation. That one is in use only occasionally, but it's still a big 'ouch' on power. So is the microwave. Meanwhile the batteries suffered a catastrophe so we're doing a lot of load shifting and careful monitoring - and running of the generator.

    There is still plenty of room to expand. The MX60 is only at about 1/2 capacity, so more panels can be added. The batteries now must be replaced, so we might as well go with extra capacity there.

    Trouble is, you know that once you expand the system the usage will creep up again and soon you have to expand the system again.

    "Rinse and repeat."
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: 'Growing' loads.

    it's human nature i think that we keep going for more power. seeing as how it doesn't come as easily with pv for many reasons, that growth is not like one would see with people on the grid. there are obviously exceptions as some can do do without no matter what. i guess you could say power diets can be like food diets to some.
  • MangasMangas Solar Expert Posts: 547 ✭✭✭✭
    Re: 'Growing' loads.

    As Tony says, I believe the 30-50% reserve capacity planning number is about right net of making important and cheaper conservation investments.

    You'll probably use all the excess capacity net of a little bump from future HVAC/appliance/home electronics efficiency improvements. I figure maybe 15% over 20 years. After three years, we're probably using about 35% of our planned 40% excess capacity (i.e. 26% excess capacity available)

    Initially, we included the complete system's infrastructure in our reserve calcs. By that I mean siting the array field with expansion capacity, civil work, larger wiring sizes, oversized solar mechanical room, larger A/C breaker boxes, etc.

    Since we don't have the grid to fall back upon, we justified the additional 30%-40% excess capacity incremental cost outlay to assure our real time power requirements would support our chosen lifestyle without relying on frequent generator cycles. Based on the system's capacity, we estimated the genny would run about 125 hours a year with tests. Real time it's currently running about 30-70 hours. Not bad.

    In my opinion, a good 1800 rpm standby generator sized beyond the battery bank's requirements is mandatory for off gridders providing emergency/reserve boot to the batteries when required. Cheap insurance when you amortize the genny's cost over say, 25 years.

    In my opinion, while solar is an expensive way to produce power the impending Cap and Trade tax, under charged rural utility rates, new alternative energy tax credits and improving solar hardware efficiencies will help moderate, not justify, the cost of living on off grid solar. So far, I think the curve seems to be going in the right direction.
    Ranch Off Grid System & Custom Home: 2 x pair stacked Schneider XW 5548+ Plus inverters (4), 2 x Schneider MPPT 80-600 Charge Controllers, 2 Xanbus AGS Generator Start and Air Extraction System Controllers, 64 Trojan L16 REB 6v 375 AH Flooded Cel Batteries w/Water Miser Caps, 44 x 185 Sharp Solar Panels, Cummins Onan RS20 KW Propane Water Cooled Genset, ICF Custom House Construction, all appliances, Central A/C, 2 x High Efficiency Variable Speed three ton Central A/C 220v compressors, 2 x Propane furnaces, 2 x Variable Speed Air Handlers, 2 x HD WiFi HVAC Zoned System Controllers
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