Description:

Description

Simple Off-Grid Living with Solar

Outside our tinyhouse in rural PA overlooking a pond and garden

Design Specifications

Location: Allentown, PA

Sq Ft: ~300, 14.5 x 12 (2 floors)

Foundation: Existing goat shed foundation, reconstructed

Construction: 2x4 wall construction (Recommend 2x6 but space was limited)

Insulation: R13 walls (would be 19 with 2x6 walls), R27 ceiling 2x8 rafters, R38 Floor 2x10 joists.

Windows: Reclaimed Victorian windows from the 1880's, Single pane with added storm windows.

Door: Reclaimed beveled glass Victorian door from the 1890's.  Storm door to be added later.

Siding: Real cedar for low maintenance.

Heat/AC: Mini-Split unit with SEER rating of 23.  Recommend SEER of 30 or hybred Solar / Grid mini-split. Recommend a vented propane heater for backup with insufficient sun runs the Powerwall down.

Hot water: Not added yet but solar electric with backup propane anticipated.

Cooking: Countertop induction hotplate.  Could install a small propane stovetop if you are installing propane for backup heat and hot water.

Refrigeration: Efficient mini-fridge or mid size fridge.

Off-Grid Inverter / Battery System: Tesla Powerwall 2 with 13.5kw of usable storage.

Cost: $25,000, doing everything yourself. (Cost of well and septic not included)

Powerwall: $6500

Off-Grid Power Discussion

We have made a number of attempts over the last 6 years but had not been able to find a cost-effective off-grid system to provide our customers.  This has finally changed with the introduction of the Tesla Powerwall 2.  This is a true breakthrough in living off-grid comfortably and cost effectively.

The Basic Problem: Batteries cost a lot and store comparitively very little power.  This is true of all types of lead acid batteries as well as the newer Lithium batteries. In fact the lithium batteries cost much more per kwh stored but have a longer lifespan (in the end lithium is still usually more costly that lead). A smaller secondary issue is that the equipment for off-grid is more elaborate than on-grid so costs more to purchase initially.

An Example of the best we could do before the Powerwall 2: We have worked out an example system.  This is a 3600 watt system so it is moderate in size and has decent capacity for small to medium applications.  Importantly, this is a simple, preconfigured system so it is possible for a DIYer to install.  Equipment includes: inverter, charge controller, breakers, enclosures, distribution panels, control panel).  However, we've combined it into a single unit so you just need to connect grid-power(if available), solar panels, batteries, load panel connection.  Estimated cost of the equipment is $5000(not including the solar system itself).

A 3600w system can provide 80% of it's rated maximum capacity without shortening the life of the equipment so that's 3600w * 80% = 2.9kw maximum draw.    

This will need solar panels to charge it and this system makes use of 3600w of panels which is 12, 310w panels (Total 3760). 

Now the batteries: A set of four 105AH lead-acid sealed batteries will cost $1400 including the necessary cables.  This does not include the shipping cost of the 300lbs of batteries. If instead, we choose high maintenance wet cell batteries then the cost is considerably lower at $800 for a set of four.  Wet cells need monthly toping up of distilled water and also can vent hydrogen so need a vented enclosure. Wet cells are high maintenance and you would need to learn some chemistry to maintain them correctly.

What do four 105AH lead-acid 12volt batteries provide?  The basic formula is Amps * Volts = Watts.  So one 105AH battery at 12 Volts = 1260Watt Hours or 1.2kwh.  Since there are four batteries this is 5kwh.  There is a small conversion loss but we'll ignore that for now. See this calculator.

Next to consider is how much you plan to drain the batteries.  Draining them 80% will give only 300 charge cycles before they wear out (about a year of full use).  40% will provide 600 cycles (about 2 yrs of full use).  Based on this the maximum draw should be 40%.  Four 105ah batteries can provide only 2kwh of power when 40% drained.

I should note that you may well not make full use of the 80% or 40% drain every day so the lifespan could be considerably longer.  One the other hand, if you have several cloudy days in a row the solar panels may not fully charge the system, so you might draw the batteries down further or need to curtail power use.

Using the 40% drain figure of 2kwh seems prudent for the next calculations.

Matching the 13.5kwh of the Tesla Powerwall  You would need seven sets of four 105ah lead acid batteries to provide the 13.5kwh storage of the Tesla Powerwall 2.  That's 28 batteries.

If a set of four sealed lead acid batteries costs $1400 and you need seven sets then the cost raises to $9800 plus about $5000 for the charge controller, inverter, cables and cabinets needed.

If you want to have a ten year lifespan system you will need to replace the batteries four times at a cost of $9800 each time.  This brings the total 10 yr cost to $54,000.  That's insane

Maybe you can double the amount of batteries from 28 to 56 and reduce the drain to 20% which would extend the life of the batteries so you only need to replace them once but that's 14 sets at roughly $20k each time which is a 10 yr cost of $45,000.  Any you'll need a lot of space for this pile of batteries.

The Powerwall is 85% Cheaper!

Now instead lets talk about the Powerwall 2.  It costs about $7000 fully installed with a 10 yr warranty and zero maintenance.  This is a genuine breakthrough and costs 85% less.  So, even if you only need 8kwh of storage the Powerwall is massively cheaper and more reliable.

Actual Daily Power Budget

The off-grid solar cottage is a demonstration project so data collection and sharing is a primary goal.  We think this tinyhouse can help our customers make informed decisions about going off-grid and not overdoing or undersizing the system needed. This is located in Allentown PA which get cold in the winter and has a solar irradation of 3.9 averaged over the year.  If this were located in Phoenix, AZ you would experience much warmer conditions and solar irradation of 6.58 so would need less than half the solar panels to maintain energy balance.  

Key

Date Date of data collection
Outside Low Temp This is our simple attempt to show climate conditions affecting heating / cooling and outside temp is critical
Inside Temp Set The thermostat setting of the cottage
Usage Meter Reading We have an electronic power usage monitor attached to the power leads for the cottage. In fact we only monitor one of the two poles but we have switched the monitor to determine that the mini-split HVAC pulls evenly from both sides
HVAC Calc KWH Usage Meter Reading from yesterday is subtracted from today to determine days usage.  They doubled to account for only reading one of the two poles on the split phase circuit
Fridge & General Allowance of .5 kwh a day for a mini fridge and .5 kwh a day for lighting and electronics
Solar Meter Reading (4kw kit) Reading of electric meter recording total kwh generation. The panels and microinverters attached total 4kw of DC power
Gen Solar KWH Subtract yesterday's reading from todays to determine the power generated today
Daily Power Balance Difference between power generated and power consumed in KWH.  Subtract power consumption from power generation
Powerwall 2 Capacity KWH Total storage capacity of a single Powerwall 2 is 13.5 kwh.  The powerwall 2 is guaranteed to provide 80% of this capacity when it reached 10 years of ago.  We use 100% for our calculations
Powerwall 2 Reserve (90% Eff) This is the power remaining in the Powerwall 2 at the end of each day.  The Powerwall is rated to be 90% efficient in round trip storage and discharge so we use the 90% figure to account for losses in charging / discharging
Lost Excess Generation When the Powerwall 2 is full it cannot store any excess solar power generated so we keep track of that wasted generation here.  It's important to not oversize the system to meet the worst case cloudy / cold days because that will mean that most the time a significant amount of power goes unused as it cannot be stored
Notes Weather or other useful information is noted here.  One note we make is that when the Powerwall becomes depleted during particularly cold or cloudy weather that you should switch from the heat pump to a backup propane heater.  The solar and powerwall can still provide the needed electricity but not heating.

Date

Outside Low Temp Inside Temp Set Usage Meter Reading HVAC Calc KWH Fridge and General Solar Mtr Reading (4KW kit) Gen Solar KWH Daily Balance KWH Powerwall Capacity KWH Powerwall Reserve (90% Eff) Lost Excess Generation   Notes
11/15/2017 32 62 4.5 9 1 16122 20 10 13.5 13.5     Sunny (Black Pwr Line)
11/16/2017 40 62 7.6 6.2 1 16136 14 6.8 13.5 13.5 6.8   Sunny
11/17/2017 39 62 10 4.8 1 16146 10 4.2 13.5 13.5 4.2   Rainy
11/18/2017 36 62 16 12 1 16164 18 5 13.5 13.5 5   Sunny (Red Pwr Line)
11/19/2017 42 62 19 6 1 16167 3 -4 13.5 9.9 0   Windy
11/20/2017 36 62 23 8 1 16177 10 1 13.5 10.8 0   Sun / Wind
11/21/2017 42 68 27 8 1 16194 17 8 13.5 13.5 5.3   Sun / Wind
11/22/2017 42 68 32 10 1 16209 15 4 13.5 13.5 4   Sun
11/23/2017 32 68 39 14 1 16214 5 -10 13.5 4.5 0   Cloudy
11/24/2017 27 66 44 10 1 16231 17 6 13.5 9.9 0   Sun
11/25/2017 32 66 52 16 1 16247 16 -1 13.5 9 0   Sun
11/26/2017 31 66 56 8 1 16258 11 2 13.5 10.8 0   Part Cloud
11/27/2017 27 66 60 8 1 16271 13 4 13.5 13.5 1.3   Sun
11/28/2017 32 66 66 12 1 16287 16 3 13.5 13.5 3   Sun
11/29/2017 36 66 70 8 1 16300 13 4 13.5 13.5 4   Sun
11/30/2017 35 65 74 8 1 16315 15 6 13.5 13.5 6   Sun
12/1/2017 45 66 77 6 1 16322 7 0 13.5 13.5 0   Part Cloud
12/2/2017 32 65 83 12 1 16337 15 2 13.5 13.5 2   Sun
12/3/2017 42 65 90 14 1 16346 9 -6 13.5 8.1 0   Cloudy
12/4/2017 26 65 96 12 1 16351 5 -8 13.5 0.9 0   Turn off the heat pump and switch to propane heat to conserve power
12/5/2017 50 65 100 8 1 16360 9 0 13.5 0.9 0   Mixed
12/6/2017 45 65 103 6 1 16361 1 -6 13.5 0 0   Cloudy
12/7/2017 30 65 110 14 0 16371 10 -4 13.5 0 0   Sunny
12/8/2017 29 65 116 12 0 16385 14 2 13.5 1.8 0   Sunny
12/9/2017 32 65 123 14 0 16389 4 -10 13.5 0 0   Cloudy
12/10/2017 26 65 133 20 1 16390 1 -20 13.5 0 0   Snow 3"
12/11/2017 32 65 139 12 1 16397 7 -6 13.5 0 0   Snow / Sun