Tim’s Tiny Home Project Notes 4

An ongoing series.  Part three here.

Gambling on Solar

Solar Gambles

Minimum and Maximum Wagers

Maximum: (600 WATT HOURS/DAY)  

Based on real world experience with a modern high-output 10 watt LED lightbulb, Tim’s Tiny Home’s total energy budget for LED lighting is being set at 100 watts energy draw per hour.   That seems weighted far enough into the “overkill zone” to cover even my most fanciful ambiance illumination ruminations. Running all 100 watts of LED lighting simultaneously for six hours requires 600 watt/hours of daily energy capacity.

Minimum: (100 WATT HOURS/DAY)

In a minimalist configuration, the light from my current 10 watt LED bulb is quite sufficient to adequately light my little corner of Europe.  Running that solitary, yet powerful high-output bulb (or a combination of smaller bulbs totaling ten watts) a full six hours every night would require a mere 60 watt/hours of daily energy generation and collection!  In an all-out survival mode I could switch from a single ten watt bulb to a five watt (or less!) bulb, cutting the total tiny home solar illumination budget to a mere 20 watt hours/day.  For expediency, I’m going to use the 60 watt hour per day minimalist figure and round that up to 100 watt/hours per day.  Adding “excess” capacity covers unavoidable innate system inefficiencies which I suspect might be more pronounced in a system design so close to the margin.

TV illumination

In addition to the dedicated 12V solar lighting system in Tim’s Tiny Home there will also be a parallel solar system of much larger capacity designed to run 110 volt electrical appliances that will be fleshed out in the near future.  One of those appliances will be an LED TV/monitor (or LED DLP projector!!) which will cast a good dose of light into the living room on it’s own accord.

Budgetary Considerations

Twelve Hundred Dollar Bargain

This light can be yours for just under $1200!

In order to keep my overall tiny home budget somewhat under the construction cost of the Taj Mahal, my target budget for lighting is 1000 dollars.  Setting a thousand dollars as a limit constrains possible system permutations to a manageable amount.

The various parts of any total system must work together in harmony no matter what budget you set for your solar design project.

There’s something internally satisfying about being able to say that my entire home lighting system, top to bottom, costs less than a good chandelier for the entry foyer of an average yuppie McMansion!

Solar Exposure by Region

US Solar Exposure Map

This chart shows the average hours of peak solar illumination per region.  It is a rough estimate and it is heavily weighted towards the minimum.  The thing to note is that no matter what zone you look at, there’s at least four hours of “peak” sunlight in all but the most brutal zone (Zone 6), and even there, it’s still three and a half hours.  The “peak hours” of sunlight numbers don’t account for the total hours of sun exposure per day.  The need to know peak sun hours is because until very recently, older generation solar controllers didn’t operate with any real efficiency outside that limited range.  Things have changed in regard to the solar tech, both in the efficiency of the panels, as well as the electronics necessary to harness the sun and properly manage the condition of the battery bank.

Getting the Sun Under Control

The Solar Controller

MPTT Solar Controller

Maximum: A good MPPT controller.  Older style solar controllers are a lot less expensive than the same sized (20A) MPPT controller shown above. These new controllers are known as maximum power point tracking (MPPT) units.  The newer tech is a bit more expensive than the older style pulse width modulation (PWM) units, but the increase in system efficiency.  The older and cheaper style PWM controllers, as noted above, are really only effective at peak solar hours, while the newer MPPT style controllers allow electricity to be generated much more efficiently at off-peak hours and in cloudy conditions.

Another big advantage of the MPPT units is the ability to handle a higher voltage solar array while still outputting at twelve volts to charge the batteries.  Simply put, solar panels can be strung together in series for a higher charge controller input voltage. This allows for thinner (cheaper) wiring between the solar panels and the charge controller, which may be a good distance away from each other. MPPT controllers generally have computerized innards that allow for all manner of customization, including selection for the type of battery being charged as well as computerized network setup and monitoring options.  The Renogy unit appears to be a well ranked representative of this category for the amperage necessary to charge the battery to run my tiny home lighting.

Renogy MPPT Remote Controller Display

In my opinion it is not worth considering a minimum (cheaper) option on this critical piece of solar infrastructure.  Total price factored below includes price of the MT-5 remote control display/monitor ($35)

Total solar controller cost: $163

Sizing up Solar Panels

 Panel Variety

Maximum Collection Target: (600 watt/hours per day)  Any solar project needs to take into account unanticipated inefficiencies as well as leave a little room for the batteries to ‘breathe’.  I’ll work up a system that can collect and deliver around a thousand watt/hours per day to cover my anticipated maximum usage of 600 watt/hours per day.  The added collection efficiency of an MPPT solar charge controller should allow for upping the anticipated average peak collection hours shown in the solar chart above by fifty percent, to around six hours of average sunlight per day.  With solar, a good bit of excess capacity is always better, so for starters, I’m accounting for a pair of 100 watt panels for a total output of 200 watts. This should allow an average of a kilowatt hour of solar collection per day based on conservative peak solar numbers.

100 Watt Renogy Solar

1 KW/h per day of solar collection per pair.

Maximum total solar panel cost: $300

Minimum: Collection Target: (100 watt/hours per day) Given a five hour daily sun diet I’d only need a panel that outputs about twenty watts/hour but from my perspective, it makes more sense to start with a slightly larger panel since the difference in price between a fifty watt and hundred watt panel is only sixty dollars.  Therefore, my minimum configuration will include a single 100 watt solar panel of the same type as shown above in the maximized system.

Minimum total solar panel cost: $150

Expansion Options 

The Full Enchilada

The Renogy MPPT solar controller will handle up to four twelve-volt panels of 100 watts each so future expansion is easy.  Adding additional panels in the future is not going to be an issue using this Renogy controller.

Sizing Up Storage Capacity

Variety of AGM Batteries

The key to sizing for solar storage batteries is to default to as much extra capacity as you can, to be able to handle several days of backup power without immediately falling into an energy deficient state that leaves you relying on candlelight like some (godforsaken) fifteenth century monk.

It also pays to oversize your battery bank so you won’t be discharging them too deeply between recharges.  This GREATLY extends their lifespan. There’s a good variety in battery types one can choose to store solar energy.  AGM (absorbed glass mat) deep cycle lead-acid batteries cost a bit more than the ugliest option (cheap six volt golf cart batteries) but AGM type batteries are my minimum requirement for a battery bank that will be enclosed inside my tiny home. They need extremely little outside venting as the gas produced during cycling the battery (hydrogen) is mostly contained within the sealed and leakproof no-maintenace design. The correct amount of ampere/hours reserve storage needed must be calculated ahead of their purchase.

Fun With Math

Fun with math?  Might as well smile through the pain.  It’s simple stuff.

Screen shot from Wholesale Solar’s battery worksheet for those who want to play along at home.

Solar Worksheet

Direct link to solar planning worksheet PDF.

Calculating Raw Need

Divide the watt hours necessary per day to run your lighting load by the chosen battery bank voltage.  For my maximized system, six hundred watt hours per day running off a 12 volt system requires at least 50 ampere hours of storage capacity. For the minimalist configuration, 100 watt hours per day divided by 12 volts rounds up close enough to 10 ampere hours per day.

Calculating Reserve Storage Capacity


The free spirited enviro-hippy chick that lives inside my head is engaged in a risqué escapade with the gruff looking project manager who lives there as well.  They are getting emotionally bonded over the anticipated shift of project money from extra batteries to extra windows.

Whether your raw daily need is fifty amp hours or only ten, your daily raw amp hour figure must be multiplied by the number of days you want the battery system to be able to work with no sun available for recharging.  By settling on only two days of backup power, I’m in the class of either optimist or fool in comparison to the norm.  I’m counting on that high dollar MPPT controller to work a little magic on even the cloudiest of days.  Batteries are a major expense and environmental nightmare as wel.  Battery size (maximum and minimum configuration) for two days backup runtime:  Max: 100 a/h,  Min: 20 a/h.

Depth of Battery Discharge Calculation/Adjustment

Fuel Gauge

Anything below eighty percent depth of discharge will seriously affect battery longevity.  I’m going to factor in a depth of discharge no greater than fifty percent.  To get the depth of discharge properly dialed into the battery equation, you divide the number of ampere/hours you need by the percentage depth of discharge you’re willing to accept in normal usage.  Wholesale Solar suggests a fifty percent depth of discharge.  This makes the math easy as you simply double the battery capacity already calculated.

Battery size requirements (factoring in discharge depth of fifty percent): Maximum: 200 a/h, Minimum: 40 a/h

Temperature Compensation

Cold Winter Chill

My battery box will be housed in a custom-built enclosure and be kept at room temperature inside my tiny home.  This should alleviate any extreme variances in temperature and keep the batteries warm and operating at near peak efficiently.  Battery storage expansion options much be considered in the planning stage of a tiny home build.  I’ll be planning ahead for battery expansion while at the same time hoping that I never need to.   Final battery storage requirements in amp hours (no temp adjustment) for the Tim’s Tiny Home:

Maximum: 200 a/h, Minimum: 40 a/h

Getting Amped Up

Amped Up

Wholesale Solar worksheet spits out a requirement for 200 amp/hours of deep cycle battery capacity on the maxed lighting option and a 40 amp/hour battery for the minimal configuration. Overall storage capacity can be increased by paralleling another battery post-build, so the system can be field tested using a smaller, lower capacity battery at first, and then collecting real world usage data to factor future adjustments..

I expect that the maximum usage figure of 600 watt hours per day set at the outset,  is inflated by at least a factor of two or three, since it assumes every bulb running simultaneously six hours every night

Therefore, I’m limiting my initial battery purchase to 125 amp/hour capacity.   I can’t imagine needing more than thirty watts worth of lighting inside my tiny home at any given time and that’s less than a three amp/hour drain per hour.  In other words, a 125 amp/hour battery can do backup service for a good three days without over-discharging the battery.

If I find I need additional capacity I’ll add it later.

Minimum Battery Requirement

60 A/H AGM battery60 a/h  Cost: $185

Lowest Maximum Battery Requirement

125 A/H AGM battery

125 a/h  Cost: $265

The Results – Let There Be Light

Putting the pieces in place.Putting the Pieces Together

Adding up the bits for the maximized setup (solar panels, controller and battery) the maximized system is still under $750 ($300 + $163 + $265).

This leaves over $250 leeway for purchasing proper fusing, wiring, LED bulbs and fixtures, while still keeping the entire lighting system under the one thousand dollar target!  I’ll be adding more details on the cost and selection process for the LED lighting fixtures chosen as well as the associated wiring costs after developing the floor plan on my tiny home build.

By dropping to a single 100 watt panel and using the smaller battery option, the minimal 60 watt/hours per day configuration comes in just under $500 ($150 + $163 + $185).

Next up?  Designing a larger capacity solar system for running 110 volt appliances.



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