If you're talking about energy and power, you need to be exact about units! Otherwise you're talking gibberish.

• energy is expressed in Joules or derivatives like kWh (kiloWatt x hours, which is 1000 J/s * 3600 s = 3.6 MJ)
• power is expressed in Watts (Joules/second) or derivatives.

If you are using an average 1.5 kW of power, you're using 1.5*24 = 36 kWh or 129.6 MJ of energy per day.

On the other hand, if you use 1.5 kWh (5.4 MJ) of energy per day, you average power usage is 1500/24 = 62.5 W. (1.5 kWh => 5.4 MJ. 1 day => 86400 seconds. 5.4e6/86400 = 62.5)

There is a factor of 24 between them!

(don't mean to be rude, but it's late and this irks me to no end! )

I just assumed we were talking about 1.5 kWh of energy per day. Honestly, despite having had the basic physics that tells me they're different, I use the terms "energy" and "power" interchangeably in common conversation. I suspect that they have probably reached that level in English, much the way "theory" has been downgraded from being a really well-tested hypothesis that's a kissing cousin to a fact to just being a simple hypothesis. Granted, I can see why you'd get annoyed by that downgrade --- I get really annoyed by "theory" being thrown around so blithely, mostly because it gives non-scientists an easy way to argue against evolution --- but it's a losing battle to fight modern semantics.

Or maybe I'm on the completely wrong track?

Maybe I'm just too headstrong, but I do fight "modern semantics", or at least point out the error in perception!

As for people who misuse the word theory, especially evolution denialists;

(With thanks to plognark for coming up with this brilliant sketch!)

No sense getting irked by a simple omission of one word (hours) that merely occurred because of oversight. I was speaking of kwh, not our rate of usage, and for most people, I'm sure my omission did not create any confusion.

But, to address the question at hand, "how do we make it work?":

We use CFL bulbs throughout the house, one per fixture and do not leave unnecessary lights on, day or night. We no longer use our electric stove. We cook on and bake in our wood cookstove which also heats our home during the winter. We're setting up a summer kitchen with a small wood cook stove. If this doesn't work, we'll use a propane stove. We do not run a freezer anymore. We converted one chest freezer into a refrigerator with an external thermostat. This uses about 1/10 the power per day of a regular refrigerator/freezer (~10 watts per hour). We have a front loading washing machine that uses 75 to 250 watts per load, depending on how full and which cycle is used (generally, it's set to a wash cycle which consumes ~100 watts per load). We have a propane dryer that we can use but seldom do since we hang the clothes to dry, inside on racks during the winter and outside on lines when it's warm. We got rid of our electric water heater and installed an on-demand propane heater. We primarily use this for showers as we use water heated on the wood stove in the reservoir for washing dishes and wash our clothes in cold water. We bought a laptop because it uses less power than a desktop (20 - 50 watts per hour) and because we wanted a laptop. We only occasionally use our small TV and DVD player, but it's power consumption is low. We have ceiling fans which we will use when it gets warmer, but they consume less than 50 watts per hour on high and less on medium or low. My wife uses her sewing machine on occasion, but it doesn't consume great amounts of electricity, either. We also have a small grain mill that we use to make flour or corn meal, but it's rate of power use is not much either (I do not remember the numbers for these last two appliances). I heat water for coffee on the wood stove and use an Aeropress. Basically, we don't use items with heating elements (stove, water heater, iron, coffee maker, etc.) because they consume too much power.

On sunny days when the batteries are already charged, we are able to produce more power than we can use off of our 1.25 kilowatts of panels. At those times, our level of consumption could increase. I don't think we'll be able to use all of the power potentially available, and we likely won't even try. I designed our system so that we would have at least three days reserve from our batteries (actually more because we still harvest power on cloudy days) and with the ability to provide for our 1.5 Kwh per day expected usage.

You can read about our system in my posts about our solar power system on my blog if you're interested.

A very wise choice not to use electricity for heating! And a good idea to have a display that shows actual energy reserves and usage.

What is your estimate your ratio of energy consumption for electricity and heating?

In my appartment in the Netherlands last year I used 7400 MJ of electrical energy (basically appliances, computer, lights), and 28000 MJ (in natural gas) for heating, hot water and cooking. So only about 20% of the energy I used was electricity, most of it was natural gas for heating. My appartment building is around 40 years old, and while the windows have been updated to double glass, the double brick wall doesn't have insulation since that only came into usage later.

Interesting to see your solar power setup. And a very cute cat.

How long do you reckon it will take to recoup your investment? Now matter how often I look at the numbers, I cannot make a solar installation pay for itself in less than the lifetime of the components.

I really don't know how to estimate the energy we consume for home and water heating. This winter has been colder than previous ones, resulting in our using more wood burned to provide heat. We currently live in a early 1970s mobile home while I build our new house which will be more energy efficient (straw bale walls -- my house blog). Our usual use of wood has been about three cords per year. This amount varies depending on the weather and the wood. We use trees that are already down or dead, ones that others want removed from their property, or tops left after a neighboring property has been logged. We have enough trees for heating and cooking available in this area and enough on our property, but I'd rather use what would be waste on others' properties (with their permission) before using my own. We use less than 200 pounds of propane per year for our dryer and hot water heater (estimating since the water heater has only been installed for a little over a month). We use a lot less electric energy than energy for heating, I'm sure.

In terms of recouping the costs of the solar electric system, it will take many years and some of the components will need to be replaced in that time. I invested approximately $7,000 in our system using money set aside for our new house since we'll move the system to it when it's finished. Electricity is cheap here in KY. If I base calculations upon our previous usage (because there's less motivation to conserve when connected to the grid) at the low end of $35/month with no change in rates and not figuring in replacing components of our system, it would take 16.6 years to pay for itself. But after the 30% tax credit we're offered by the gov't, it would take 11.6 years. At our current, off-grid usage, we would pay the electric company about $17/month. It would take 34 years to recoup the whole initial investment or 24 years after the tax credit. Since electric rates are bound to increase, the numbers would look better. However, there will be components of the system that will need replaced periodically, especially the batteries. I would estimate that with changes in rates and replacing components, I can figure it will have paid for itself within 20 years, leaving 5 or 10 years of free production based upon the expected life of the panels.

We went off grid not for purely economic reasons, because I too would have a hard time justifying it on solely that basis. There are moral reasons. We wish to contribute less to the harmful effects associated with coal powered electricity, both in the mining of coal and the generation of electricity. We also like the independence that we are provided. I worked hard to keep the costs down on our system; I could easily have spent twice as much for basically the same capability. I'm happy with what we've put together.

Someone else who lives in a 1970s era mobile home --- how exciting! (Well, I think ours is actually from the 60s, but it's pretty close.) Do you actually have your wood stove in the trailer? People told me that was massively unsafe, so we went with the exterior furnace, but sometimes I wonder if we should have ignored them. I suspect that if we'd had it, we would have gone through four cords of wood this winter, partly due to the power outage and extremely cold weather, but also partly due to the exterior wood stove being less efficient than one actually in the house.

I'm impressed by the relatively low cost of your solar system. It sounds like you did everything right, making sure you scaled back your system enough that it made sense. (And, of course, I understand going off grid for moral reasons!)

Roland --- 80% on heating! Wow! Although, that probably just means that you're very efficient on your electricity use.

Yep, we have our wood stove in our mobile home. It's really no more dangerous than a regular home, IMO, especially with an older mobile home. When we moved here, the previous owners had already had a wood stove in the place. So, there was already a chimney. I replaced it when we bought our cook stove -- it goes straight up through the roof. Our home is UL listed in 1972. It is 12x60 with two bedrooms. The smaller bedroom was taken out, enlarging the kitchen. It is in this area that we have our wood stove. There are enough air leaks around windows that we don't need to worry about asphyxiation, and there isn't anything more combustible in this home than in other 'regular' homes. It's really nice having it inside the house and no requirement for extra electricity to keep it warm in here. We find that the heat circulates pretty well through the house.

I built our solar power system on a budget. I went over that budget, but that allowed me to install some 'top of the line' components, and once we apply the 30% tax credit, we'll be back in budget. I learned a lot in the process, and I hope to someday be able to help others build their own systems.

Daddy --- selling electricity back to the power company is the holy grail, but I wonder whether even that would make a system fiscally responsible. After all, you have to buy a much larger system to sell electricity back, which means much higher startup costs. On the other hand, electricity prices are rising, so we may hit a crossover point pretty soon.

DP --- I'm glad to hear it can be done quite safely! Your mobile home's bigger than ours, which means that we'd probably use even less wood with an interior stove. On the other hand, now that our new shed is built, it might increase our efficiency back up to a good range. I really appreciate you sharing all of your solar experiences

DP - I can understand your reasons for not wanting to use coal-based power!

But I've been wrestling with the real costs of solar panels. The seimconductors that make up solar cells are not trivial to make. The ones based on silicon wafers require ultra-pure silicon which is expensive and energy intensive to make. Most of that material is wasted when sawing the boule into thin wafers. Those then have to be "doped" to create a semicondoctor, and metal grids have to be applied to both sides of the cell to get the current out, using basically the same processes as used for other semiconductors like computer chips. The plant needed to do this is called a "fab", and costs billions of dollars. They also require a lot of very clean water, and I'm guessing quite a lot of power to operate.

Hmm, according to an Ecogeek article, solar cells can deliver about five times the energy that was needed to make them over their lifetime. But fabs also tend to use strong acids for etching, and other stuff for masking and removing masks. So I'n not sure an energy balance is the whole story.

Anne - you might be interested in this compact hydrogenerator. At $3000 it's not what you'd call cheap, and it needs four feet of water depth. But it can deliver 500 Watts.

Using wood to heat is much more polluting than electricity. Switching from the grid to propane switches completely to fossil fuel. An increasing amount of grid electricity is generated using renewable, low-pollution sources of energy. You might be cutting your personal electric bill at the cost of sending more pollutants into the air for your family and neighbors.

If you're going to be off the grid, set up solar energy collectors, geothermal collectors, water power, etc. Sure, one family burning the forest to heat their home isn't so bad, but if tens of thousands do it then there go the forests in literal clouds of smoke.