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 |