My down and dirty math... Is it even close?

Re: My down and dirty math... Is it even close?

As a PS to the previous:

All load calcs should be done in watt HOURS (or KWHs) Watts mean little, whs is what you actually consume, and need to recharge. Also realize, that in simple terms, for every watt hour you consume, you are going to have to put 1.2 wh (or more) back into the battery because batteries are not one for one so to speak. FLA batteries take at least 20% more power to charge than they can deliver.

Also keep in mind that there are loses everywhere, most of which cannot be overcome, and indeed need to be included in the calcs.

For example, in addition to the 20% for the battery, you need to include inverter loses that, depending on the invert ef and loading can be 5-50%. Charge controller loses, wiring loses also should be figured in. (which is why my /2 rule works pretty well as a simple, yet not 100% accurate calc).

Also, as loading gets bigger say over 1 kwh/day or peak loads over say 500 watts ~40 amps 12vdc, going to a 24 or even a 48 vdc system has big advantages. Bottom line, in the design stage, do everything you can to estimate the loads, but at the same time, do everything to limit them. CFLs, gas cooking, water heat etc, limited appliances etc.
I use the genny for all the really big loads, since they are rare, it is much cheaper to build a solar system for average peak loads (and daily loads) instead of building it to run the skill saw or vacuum once in a while).

Since you will need a genny any way, plan on using it for the big, occasional loads, and wire the house accordingly. Also, consider your water pumping requirement carefully. A large, fast (high powered) well pump, might well be suited to run on the genny into a big tank run once a day ( or once a week) instead of trying to run it off the PV system. You could also consider a small (slower) less powerful pump that might run more, but in the net, consume about the same amount of power, but not require a big, inefficient inverter for example. DC water pumping may be an advantage.

Once again, hope this helps,

T

Re: My down and dirty math... Is it even close?

My living with 5-800 wh is not really a trick. It should be noted that we use a propane fridge. If I was to start over, I would consider a conventional fridge, effectively doubling the PV and batteries. The point is the new fridges are very efficient, and reasonably priced. That said, propane is the better choice the less a cabin is used. For week ends or the occasional week, a lp fridge wins, but for full time the conventional wins nowdays. When I started, a conventional might use 2 kwh/day, and propane was $.30/gallon. and PV cost $10/watt. Nowdays, a new fridge might use .5 kwh, propane is close to $5 and Pv is $$1.50/watt.

As for the other ways is really pretty simple. We have a very small house, we pump water directly out of the lake, and we have no TV. That said, we are well lit, enjoy radio through a good 12 vdc car stereo and good speakers. We have Sat radio, listen to internet radio using a Ipod touch system. Our ISP Sat modem draws 15 watts, that plus the router are on for most of the day. We use lap tops (and now Ipads) and we time shift the loads so that we generally charge them during the day, run them on the batteries at night. I am excited to discover that at long last, we will be able to access the net via a HSPA cell system with a monster Yagi antenna. That system will involve a mobil hotspot that will only draw 15 watts max, and eliminate the need for the router, making the total internet power draw,, 15 watts, down from over 30, plus the inverter loss,, which isn't much. So we will increase our speed 5 fold, and reduce our load by half,,, gotta love it!

We also heat with wood, we have a couple of paddle fans, and we cook with propane. We also don't use any electrically heated devices, no toasters, hair dryers, no micro etc. (I do use a soldering iron, but also use a butane one instead) I have also built a propane toaster that uses little bottles that I refill.

We also run the shop tools with the gennies. A few gallons of fuel per year,, maybe 30-50 total is much cheaper than trying to build a system to power saws, routers, planers etc.

As I noted before, you have the advantage of starting with a clean slate, making your choices rational. Please do remember my rules however,, don't over estimate your solar potential, don't underestimate your loads, and realize that those loads WILL grow with time. Just to give you an example, we started with a simple car battery, and a couple of window mounted battery tenders, all to run the radio. Then we realized that having a 12 vdc light over the bed made a great reading light instead of the Kerosene lamp, then we added another over the cookstove. Then we realized we needed a real panel, and ran with a 53 watt Solarex for a number of years. That was followed by a couple of computer fans to get the heat moving, then by a Sat phone,, an inverter to power the lap top in the old days (no ISP in those days). Somewhere alone the line we bought a couple of T-105s and a real 120 vac charger, followed by a better SW inverter, followed by a few more lights, a paddle fan, then another, a fridge light!, water pump, Sat internet, cell phone, laptops, Ipads sat radio, internet radio etc.

Like I suggested before, do your load calcs, and build the system around those calcs, building in a good size buffer.

Tony

Re: My down and dirty math... Is it even close?

nnsharda wrote: »

I don't understand how people run their houses on solar.

I looked at my Nevada Power bill for September. I used 52 KW per day! That is 100 times what Icarus is using, and I am NOT wasteful of power. Heck, air conditioning at 83 degrees, gas stove, high efficiency lights and I am out of house all day working! Only pool pumps going during day. I don't even watch television, mostly work on computer at night like tonight....

I live in a 3340 square foot house so I guess it's a little bigger than average, but that should not really change total power usage, as I only have one refrigerator (LG high efficiency side by side) and only do two loads of laundry a week....

Am I missing something? I wish there was a way to do actual calculation of house power consumption rather than blindly accepting the power company calculation.

I am with BB on the measure routine. Know your loads and you can start to make decisions on where to attack the problem. We attacked the problem backwards and added a large solar system first.

One item that might be a concern is the pool pump. How big is it? How long does it run? A 2 hp pump can draw lots of power, I replaced mine with a VFD pump with a controller that is much more efficient and can be programed to run full speed for a short time to in floor clean and a reduced speed to stir and filter.

Summer loads from AC can be huge. My 2500 sq ft house was built in 1997 and had the required 10 seer units on it. After adding the TED it was obvious that this was a major load, over the last 2 years both my 3 ton ground mount units were replaced with Trane high efficiency 2 speed units and added attic insulation to R50. That cut my consumption by at least 1/3, maybe more.

Our system on a good day in May can generate 70+ kWh a day. With our utility net metering system we bank kWh early in the year to use in the major portion of the cooling season. That being said this summer our peak day was over 120 kWh yet we paid nothing to the utility until our Aug bill and then it was only $85.

The key to all that is to understand your TOU rates and your net metering tariffs at your utility. Our utility will buy back our excess generation at year end for a credit on the account for the following year. This covered all our off peak usage, fees and taxes until Aug this year and our solar covered all our on peak usage. We study our usage often and look for ways to minimize usage and load shift as needed to control our costs.

That being said we determine that we need a bit more solar as we added some new loads this year, a pair of Chevy Volt EREV cars. They can use up to 13kWh per charge. I found a lease deal for $1 a watt up front for 5 year term with a $99 residual payment at lease end. Lots cheaper than buying gasoline.

Re: My down and dirty math... Is it even close?

NN,

you must realize that my situation is atypical. First, my house is ~350 sq ft., about 1/10 the size of yours. In addition, we use a genny for some occasional loads that are not included in that calc, as well as propane for the fridge. We have no pool pump, which is a big deal, and no A/C which is a much bigger deal.

As has been suggested, look at your line item numbers and I would posit that with a modicum of effort, and some investment you could save a bunch of KWH/day. For example, I would guess in your climate solar hot water is a no brainer. Perhaps more important would be a ground source or hot water (for pool or DHW heat) recapture heat pump system. Such a system gives two benefits. The first is it allows your A/C to work less hard, and it gives you hot water. Next, as has been suggested, look at the pool pump. Size, run time etc will have a huge effect on KWH load. Then one could look at insulation, especially on the roof, roof colour, shade trees, retractable awnings etc, all might have an effect on your A/C load.

Wash in cold water, use a clothes line. (Why for example, would anyone in a desert climate use power to dry clothes when they dry on a line very fast?) Lighting, motion/heat sensors to turn off unused lights, CFLS (which are now very good light sources both in light quality as well as design shapes, unlike the ice cream cone lights of old!) High efficiency appliances, lap tops vs desk tops, turing off phantom loads etc. (a cable box or sat box might draw 25-100 watts even when "off", (600-1200wh/day!) power bricks at ~5 watts each, and many houses might have a dozen or more, another ~1kwh.

Bottom line is all the little things add up, and making the small changes can have a small but significant effect on the whole, but generally cost little or nothing. Making the bigger changes have a bigger effect, but come at a greater up front cost.

Good luck, and search out the ways to save.

Tony

PS, as Dave suggests, if you have Time of day metering (TOU) solar can work wonders in your costs. If you are paying top tier rates at the height of sun, then you can sell back at a premium and buy back cheaper. With some time shifting/load shifting you can game the system to your advantage, for example over cooling when the power is cheap, turning it warmer during the day when the power is expensive.

Re: My down and dirty math... Is it even close?

nnsharda wrote: »

I don't understand how people run their houses on solar.

I looked at my Nevada Power bill for September. I used 52 KW per day! That is 100 times what Icarus is using, and I am NOT wasteful of power. Heck, air conditioning at 83 degrees, gas stove, high efficiency lights and I am out of house all day working! Only pool pumps going during day. I don't even watch television, mostly work on computer at night like tonight....

I live in a 3340 square foot house so I guess it's a little bigger than average, but that should not really change total power usage, as I only have one refrigerator (LG high efficiency side by side) and only do two loads of laundry a week....

Am I missing something? I wish there was a way to do actual calculation of house power consumption rather than blindly accepting the power company calculation.

actually, i can understand as your home is large compared to the average and you are in a hot climate. when you see 103 degrees i might see 93 degrees. given the exact same house in my area would mean it could be put down to 73 degrees to get the same kwh used. i'll be more comfortable than you though. most homes are 2000sq ft or less, at least around here and 2000sq ft is not small so your area is 50% to 100% larger than that of others so you are cooling like a house and a half to 2 houses with your home comparatively. maybe limit cooling to only occupied rooms would help or getting a swamp cooler.

side by side refrigs also generally consume far more energy than other refrigs of the same size even if they say it is energy star. why this is i'm not sure, but i suspect the newer french door style is even worse as every new style can't be compared to other styles and are not applied to all refrigs on a cubic ft basis. this allows manufacturers to set standards in a strange sense.

now in general i can say 52kwh in a day is high, but maybe not for your area and home size. other efforts seem to be in order like going with windows that have multi-panes. if you have 2 panes now then start going to 3 and put awnings or shutters into use to block solar collection. add insulation wherever possible, including the walls. it's possible your air conditioner is poor on its eer or has become defective by a coolant leak. there are so many things and factors that come into this that it would be difficult for me to pin it on just a few things. even all of you taking long hot showers rather than the preferred cooler short ones can greatly impact your cooling bill in a bad way. you also have a year round clothes dryer in solar that people in more colder climates can't do in the winter months and i hope you are taking advantage of it.