Energy, power, and management on a tiny house scale

Energy has been a difficult thing to manage in an unusually small space such as the 72 sq. ft. of space I live in. It isn’t that I consume large amounts of energy, but many of the ways I use energy are not efficient. I got through the winter with a “blueflame” propane gas heater, thermostatically controlled and supposedly efficient. From the perspective of cost, it was very efficient. I used a 20lb tank, which I think holds around 5 gallons, and used it for 3 months through the winter without having to swap tanks. At less than $2 a gallon at most stations in my area and one station being as low as $1.49, It was a no brainer to use gas to keep myself from freezing.

Before I switched to propane I was using a 1500 watt oil-filled radiator heater, powered from the grid. I like it because it is also thermostatically controlled, and has the added benefit of splitting the 1500 watts into 900 and 600 watt settings, which can be used together or independently.  So you get an effective high, medium and low, with the thermostat to keep it where you want it.  But I am striving for independence and redundancy of life critical functions of the shelter.  Now, before you say it I am well aware that I don’t and can’t produce propane to fuel the fire of my heater.  But I can look towards harvesting methane (natural gas) in the not so distant future, and both gases are cleaner in terms of emission when compared to burning wood or other biomaterials.

I have a secret weapon in my energy quest, a Watt-meter.  It plugs into a wall socket and logs usage and provides a readout of energy usage currently and over time as a cumulative usage.

Watt meter displaying draw from a “transformer powered” incandescent light when not in use.


This meter is displaying 3.5 for watts consumed by a transformer powered desk lamp in the “off” position.  Some devices draw a standby current to keep devices ready to use, and the transformer in this vintage desk lamp, which I love for no reason other than it being mine, consumes energy even when not in use.  The nerdy reason is because the energy must be converted from 120v ac to 12v dc to power the bulb, and transformers made of coiled wire are incredibly robust and durable, but not efficient compared to modern power modulation circuits.  More on that in another post at a later time.  I may be able to get a much more qualified Ben to chime in on that one.

So to make improvements one must examine the brevity of the situation and identify places where changes will make a positive impact.  I started logging consumption of various electronics as I use them daily.  The output in cumulative kWh is a helpful starting place for designing an energy production and storage system, and the ability to monitor current draw of each individual product as I currently use it adds information to consider.  Energy stored in 12v batteries, like the one in your car, uses lead and acid to move ions into a potential action state (charged), and thanks to a series of “cells” linked by a cathode and an anode, it can add up 6qty 2volt cells to 12volt and start your car, or power the radio even when the engine is not running.  It uses stored energy to do so.  Your car has what is called a cranking battery, which is designed to provide a large amount of current (big bada boom) to the starter to get the process of internal combustion started.  After the engine is running, the alternator, which is driven by a belt off of the drive system, generates electricity thanks to electromagnetic fields created by winding wire in a pattern that interacts with magnets located on a rotating stator, an axle of sorts.

Now this is all very neat Ben, but I don’t really get it and how does it apply to energy in a tiny house?  Well, I’m glad you asked.  In the internal combustion engine energy must be stored for later use, or everyone would be cranking a rod to start cars, or parking on hills to push start them.  They are dependent on siphoning energy (with belts) from the primary system of transferring power to the roadway.  In much that same way, I need to create and store more power than I utilize on a continuum.  Solar photo-voltaic panels collect solar radiation and rather than burn their non-existent skin, the panels transfer the energy to a load (such as a light) or a storage device.

Your cell phone goes dead in the middle of the day and you can plug it in somewhere, as most of us now carry chargers, extra batteries, battery re-charge packs and the like.  If I rely on solar energy I am at the mercy of the gods…errr, the apophenia of natural processes.  The same is true of wind energy, which operates nearly identically to that of the car alternator, outputting power through electromagnetic discharge created during physical movement of magnets interacting with wire windings of a specified pattern.  So, much like you may find yourself at the mercy of a bartender or an unlocked power outlet in a public place to plug in your cell phone, I find myself at the mercy of unpredictable circumstances.  Hench the careful planning (grab your car charger!)

Back to me…hi.  How does one plan for the unknown?  By overcompensating, naturally.  But hold-up, wait a minute, my house is on wheels, which means it will be moved and may become too heavy to be easily towed and relocated.  And the typical battery chosen for such an application as mine weighs in at almost 70 lbs, and four of them would be necessary, while two is the bare minimum to make 12v of power.  And with that 12v, 230Ah (we can talk about amp hours later), I will be converting most of the energy utilized into 120v AC, like you get from a household outlet.  Even the concept of converting (or inverting in this case) 12 volts direct current to 120 volts alternating current would take a whole entire post, due to decisions between square-wave inverters and sinewave inverters, LF (low frequency) and HF (high frequency) inverters, name brand or changlish manual equipped import. Moving on  VVVVVVVVVVVVVVVVVVV

The battery has a capacity rating of 230Ah.  The watt-meter image demonstrates that I use 18.4kWh in about a month.  So divide 18,400(Wh) by 30 days and you get 613.33 watt hours a day, average.  This is peculiar because my air popper alone uses 1400 watts, but only for 5-6 minutes a day most days to roast a batch of coffee beans in the morning and make a popcorn snack.  The tricky part about battery capacity and power consumption is the conversion of energy and the efficiency of the system as a whole.  A 230Ah rating implies that during testing the battery can supply 230 amps of energy over a constant draw in a 20 hour time period, or roughly 11.5 amps an hour. What runs on 11.5 amps an hour?  My 15ft, 300led light strips consume 20 watts each at full tilt, which divided by 12 (volts) equals 1.6666666repeating amps.  So if left on for an the entire hour the lights would draw 1.67 amps in an efficient system.  But systems become inefficient really quickly.IMG_6771

For example, the air popper uses 1400 watts of 120v AC power.  I use it for lets say 8 minutes.  The efficiency of a typical inverter is around 85%, so to get the energy from my batteries to my popcorn kernals, I must consume 1647 watts (I think) due to system efficiency.  1647 watts drawn from a 12 volt power source requires 137.25 amps from the batteries.  For comparison, the same device uses only 10-12 amps in a 120v system like a household outlet supplied with power from “the grid.”  And the amperage dictates how thick the wires in a power cable need to be depending on a given length, and insufficient wire thickness, will create resistance in the system.  Politically speaking, resistance can be productive to combat oppressive leaders, but in an electrical system it means that power is being consumed in the transmission between source and load in the form of heat.  Have you ever noticed how hot your vacuum cord gets after cleaning the entire house?  The length of those cords coupled with the high current draw will create heat while traveling to the vacuum motor.  Not too big a deal on something that gets used 10-20 minutes every week or so on the power grid, but a real big deal for a minimalist sustainable energy system.

Back to batteries and the 20 hour discharge rating.  There are four (presented) confounds with this mathematical equation:

1) Energy usage exceeding the 11.33 amp an hour measure we found earlier will negatively alter the overall capacity of the batteries, at a nearly exponential rate.

2) Lead Acid batteries are temperature sensitive, and deliver less voltage and power potential at low temperature.  Remember that time the battery in your car was fine until the first frosty morning of winter?  It’s that problem.

capacity_temperature trojan

3) And a big confound is that although the battery can supply 11.33 amps for 20 hours straight, it would be discharged 100% after that 20 hours, and that is bad.  When batteries dip below a certain voltage, around 12v if I recall correctly, they start a process known as sulfation.  The electrolyte salts bind with the lead plates, creating a film or coating on the plates, thus reducing conductance, vis-à-vis energy potential.  Damn! Isn’t there anything you can do?



Yes, I suppose solutions are to be had. But a further consideration before shifting gears is that of

4) battery lifespan measured in charge cycles, relative to state of discharge measured in volts but represented here as percentage.  The rule of thumb is 50% depth of discharge (DOD), so you can only use half of your super heavy battery system at any given time if you want it to last as long as intended by design.

DOD trojan

Because…while batteries are marketed with ratings based on full discharge cycles, they are designed for a typical DOD of 50%.  That being said, 20% DOD between cycles (retaining 80% capacity) increases the number of potential charge cycles by double compared to 50% DOD.  So if you overcompensate, the longevity and potential backup power in a system increases by at least one magnitude.

Designing the system with these variables, and more that I won’t get into, allows for energy security. I started small with a 30 watt solar panel and an 18ah sealed lead acid battery. Sealed lead acid is cool because flooded lead acid, like that in your car, emits hydrogen during the charging process.  Hydrogen is flammable, remember the Hindenburg?  So having a sealed system with a hydrogen re-circulator built in is neat and safe, and these batteries can be used in any orientation where as a flooded battery would leak out it’s electrolyte.  Back to the panel, It cost me around $40 on ebay, china special, along with the $30 battery and the $15 charge controller, and don’t forget the $15 marine grade 12v/USB outlets on my bucket.  I suppose if you want to get technical, the bucket and hardware cost me about $20, but that is because it is designed to hang on the rack of my bicycle.

So for around $100, give or take $15, I can harvest the suns rays and store the power or use it on the fly.  But do I really get 30 watts out of the panel constantly?  Absolutely not.  Previously unspoken confound #5 with charging batteries using solar:

5) The complication with solar is that when you need it most to heat and light those long and cold winter nights, you have the least available energy input.  The sun makes about 6 hours of usable power during the winter here in Southern Oregon, and that is with the off chance that the sun is unhindered by clouds and other atmospheric influences, and the panels are tracking the optimal path of the sun, which is variable by latitude and season.  At optimal levels, the sun outputs 1300 watts per square meter.  Solar panels harvest energy at rates between 10% and 20% efficiency. So if you had a highly efficient panel which measured 3.28 feet, by 3.28 feet you could hypothetically harvest 260 watts an hour.  The square footage of my roof is 90 square feet, so I could potentially fit 2,080 watts of solar collectors.  In reality, I can’t use every square inch, and efficiency of panels I will be capable of purchasing will limit my roof output to around 1500 watts, remember this number.  Of course there will be loss in the system due to resistance in the wiring, and the management of transferring the energy through the charge controller to the battery.  My charge controller claims a 92% charge efficiency, but English language was not the design firms strong point so hopefully numbers are consistent.

So I have loss between solar radiation and panel efficiency for collection, as well as loss to the controller in the form of line resistance (can calculate, but I will spare you the trouble), then the controller takes at best 92% of that potential 1500 watts (at peak solar exposure) and puts it into the battery, which involves a storage potential of around 85% of energy input to the system, so another 15% loss from the net input.

Graph shows relationship between c ratings (hours to full discharge) and potential capacity/output.  Notice the C3 rating and how much lower is starts and ends compared to even the C5 rate.  More power demand=less power potential

To add fuel to the solar house-fire, it must be noted that photo-voltaic panels create lower voltage and current as temperatures increase.  Conveniently enough, the one job panels have, collecting the suns energy, has an inverse relationship with the efficiency of the system.  Batteries become more efficient at higher temperatures and less efficient at low temperatures, so perhaps they equal each other out…but it’s unlikely.  So what is the moral of the story Ben?

Hire a professional.  Call someone out to do an energy assessment and suggest system possibilities.  I can’t explore that option because my desired system is substantially smaller than any professional installer can cost effectively install and make a profit.  And if you can’t pony up $20-30K for a typical household system, financing is available in some situations, but not if you live in anything other than a stick built home on a foundation.  No manufactured home or mobile domicile love in the solar financing world.  But I don’t mind learning and trouble-shooting.  Mathematics isn’t that interesting until you find yourself calculating power curves using trigonometry and calculus.  It’s a puzzle, and a better use of time than chuzzle.  And I don’t claim that the 30-watt system does anything more than supply 1-2 amps an hour to my stereo system, at 12v with no conversion from the battery to the device, for 5-6 hours a day.  And even that equation is sun and audio volume variable.

My attention is waning, and I feel like I must have confused anyone who took the time to read this.  Energy is complicated, but so intriguing in it’s dynamic nature.  The fact that we can capture, store, and transform energy to extend the daylight and toast our bread really is incredible.  The fact that losses occur when playing with it, moving it around and trying to hold onto it for later should be no surprise. If anything is a complex system with layers of patterns and variables, it’s electricity.  If only Nikola Tesla could see us now, he would be more upset with our handling of energy than when he was alive, Haha!  teslaAs far as the propane heater that I started this thread with, it is cost effective but increases humidity of air that it heats (combustion), and creates a heat differentiation of warm thick air near the ceiling (which is only 6ft6in) and cold air at floor level.  It works, but it’s not nearly as comfortable as the electric radiant heater.  Hence the effort to understand electricity.

When I get more data about the 120w panel I use to charge the large flooded lead acid batteries I will continue this line of thinking and explore the possibilities of being energy independent-ish.


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