Academic power calculation

Cariboocoot

This came out of a conversation I had yesterday with someone while discussing the differences between an off-grid system and a standard utility service.

Take the Amp capacity of your service and multiply by 240 to get the Watts.
Multiply that by 24 hours to get the maximum Watt hours per day.
Multiply by the days in the billing cycle.
Multiply by the cost of kW hours.
Compare to your actual cost.

I get:
100 * 240 = 24kW
24 * 24 = 576kW hours
576 * 60 = 34,560kW hours
34,560 * $0.10 = $3,456
Actual 60 day charge for kW hours: ~$96

In other words we use less than 3% of the service potential.

solarix

Re: Academic power calculation

so what? Your inverter doesn't run full tilt except for a small fraction of the time either.

Cariboocoot

Re: Academic power calculation

solarix wrote: »

so what? Your inverter doesn't run full tilt except for a small fraction of the time either.

That is, in fact, the point.
People wonder how we can live off grid with "only" a 3.5kW inverter. The answer is that most of the time you don't use that much power at home anyway.
The exception to this is an all-electric house, which off grid is just foolish. Electricity is not the best way to provide heat of any kind. This has been proven repeatedly, yet people still say "it's efficient". Well not when you have to try and store it up for later use it isn't.

If I could afford it I'd dump the electric hot water heater and put in a gas on-demand. For two people it would be more efficient. But the payback time is rather long with ten cent electric.

jonr

Re: Academic power calculation

I'd differentiate between producing heat (resistance) and moving heat (ie, a heat pump). The latter is pretty efficient, especially when combined with thermal storage (to avoid the use of batteries).

Cariboocoot

Re: Academic power calculation

jonr wrote: »

I'd differentiate between producing heat (resistance) and moving heat (ie, a heat pump). The latter is pretty efficient, especially when combined with thermal storage (to avoid the use of batteries).

Depends on where you are of course. Up here a heat pump is pretty useless most of the time.

South Africa

Re: Academic power calculation

Cariboocoot,

To heat water for 2 people whom are actively saving every step of the way, is very easy. I see that with my parents water heating. As with you, grid is by far the cheapest for them.


For the rest us, based on where we are in the world, and how high or low, weather patterns etc, this is what I have been able to put together re. heating water for sunny areas are that:
- EV (evacuated) tubes are very efficient, especially if you have some cloud cover at times. Effective for a 3-4 people household.
- Flat panels are ok and are cheaper than EV tubes, but does not like any cloud cover. Also 3-4 people households.
Learned recently, if you do go for either of the above, get a larger geyser to hold more hot water. Bigger is more effective.
- Heat pumps are better for 5 plus households. Flat panel and EV tubes are then becoming to expensive.
- Gas for smaller households beats the pants of all the above, if managed properly.

But, one has to start off by actively managing the geyser (hot water cylinder) by reducing the temp to +-60 degrees celsius, adding a timer, shorten shower times and use flow reducing shower heads to save hot water, as well as water. This alone will make a rather huge difference.

Only after the above has been done does one consider what works best in your area.

As a result, if we push, we can be 100% off grid power in summer with EV panels for heating the geyser. In winter we can get away with 2 hours max grid to top up after a rainy day. EV tubes, if you have the right ones, work exceptionally well for me.

Cariboocoot

Re: Academic power calculation

Solar water heating would work great here - in Summer.
For our seven months of Winter, not so great.
As such investing in it would be a waste of money compared to just paying for the cheap electric.

Hurrah for -40 degrees!

Chris

Re: Academic power calculation

Hey Coot,

Have you considered a DIY solar water heater for your cabin? of course only for summer use.

I'm just putting the finishing touches on a solar "batch" water heater for the lake (RV's have tiny water tanks)
The wife is pretty excited to have a "real" shower wile out there LOL

Cariboocoot

Re: Academic power calculation

Chris wrote: »

Hey Coot,

Have you considered a DIY solar water heater for your cabin? of course only for summer use.

I'm just putting the finishing touches on a solar "batch" water heater for the lake (RV's have tiny water tanks)
The wife is pretty excited to have a "real" shower wile out there LOL

I have considered it. Unfortunately the cost of material involved will buy a lot of propane. We use about 20 lbs. per month as it is, and that's not a lot.

For direct solar heating I'd have to put in a tank, a lot of tubing, collector, and a pump because someone would object if I did it the easy way and put it on the lawn.

One of the worst things about the place is that the 'ideal' location for just about anything is all in the same spot!

Chris

Re: Academic power calculation

Wow, 20 pounds a month isn't bad....Last summer we were averaging 30-40 mind you we have 3 kids (teenage daughter who loves her showers)

I guess the wifey would not be impressed with me if I set my contraption in the middle of the lawn, luckily there is a bank with a natural shelf right behind our trailer to set up.

So far I've spent a whopping $60 on the project most of that was plywood and reflectix. Almost all of the materials are "re-claimed"

Cariboocoot

Re: Academic power calculation

Chris wrote: »

Wow, 20 pounds a month isn't bad....Last summer we were averaging 30-40 mind you we have 3 kids (teenage daughter who loves her showers)

I guess the wifey would not be impressed with me if I set my contraption in the middle of the lawn, luckily there is a bank with a natural shelf right behind our trailer to set up.

So far I've spent a whopping $60 on the project most of that was plywood and reflectix. Almost all of the materials are "re-claimed"

That's good indeed! You know what copper costs around here.
I'd like to build a collector just for experimental purposes but there are already too many demands on my dwindling resources of money, time, and energy.

jcheil

Re: Academic power calculation

Sometimes I forget how lucky we have it down here in Florida where we only "might" get a "hard freeze" overnight 1 day every couple of years.
I rigged up 20 pieces of thin-wall 1" PVC into the shape of a radiator, painted it black and plumbed it into a standard water heater. Even without a pump (the tank was about 1' higher than the collector) I was getting 120-130 degree water in the summer and 100-ish in the winter. And that wasn't even with any glass over it, etc.

It was originally done just as a test/hobby, but it has lasted almost 9 years now and I recently added a differential temp controller and tiny 12v pump ($100 total) to increase the production significantly.

So if you are in an area where it won't freeze, it's a cheap option/idea.

jonr

Re: Academic power calculation

jcheil wrote: »

And that wasn't even with any glass over it, etc.

Glass might be expensive, but would a sheet of polyethylene plastic improve performance?

BB.

Re: Academic power calculation

This company uses plastic panels (a version of plastic panels used in green houses?):

http://solarroofs.com/specifications.html

.236" (6.0mm) Twinwall Polycarbonate UV Treated.

10 year life and reasonably cheap/easy to replace (one of our posters here, Solar Guppy, has said nice things about the system).

-Bill

jcheil

Re: Academic power calculation

jonr wrote: »

Glass might be expensive, but would a sheet of polyethylene plastic improve performance?

Interesting idea, I might try it. Can't really hurt other than the cost of a 10'x10' piece of plastic.

BB.

Re: Academic power calculation

I would look at what is available for greenhouses:

http://h2othouse.com/html/greenhouse_glazing.html

Seems to have mfg. and other information.

-Bill