Off-grid Tiny House in Western Washington

Hello All,
A little background on myself: I am a mechanical engineer my experience with solar was building a solar car in high school back in 2008-2010. Used 6 panels to power a 72v system, used an outback charge controller. I was more on the mechanical side of the project.

Now:
I am trying to design a solar system for my already built tiny house on wheels that I will move on to vacant land this year. The land is having a 3 bedroom septic system installed and a well drilled that is estimated to be around 100' deep.

We have been living in the tiny house on a rented space for almost a year now and I have been trying to determine our energy use over the last few months. We are currently on a 30amp RV hook up but the house is wired for 50 amp. In the summer we average about 9 KWH a day, in fall 16 KWH a day, and this winter we've been hitting almost 42 KWH a day. Heating things in winter uses more electricity it turns out.

Any help in determining the proper system for this use would be much appreciated. In the future we plan to power at least 2 more tiny houses and power a garage(will probably get it own system), but that is much further down the line.

Initially I was thinking of a 48V system using 4 100AH EG4/SOK 5.12 KWH batteries and a 5 KW Growatt inverter, but that was based off numbers in summer. I also planned on subsidizing PV with a generator until I can add more panels in the future. Suggestions for a more robust system would be awesome. I haven't gotten to the panel selection yet.

Thank you!

Use this site:


To simulate a 1kW array at your exact array location. Change the tilt to your latitude. It will output this:


(Please note that I have AMAZING sun all year, yours will likely be much worse)

This site also uses the last 30 years of weather data in reporting these numbers. Are they perfect? Nope. Are they among the best guesses around? Yep.

Divide your daily consumption in kWh by the Solar Radiation number. That's how many kW of solar you'll need.

As a rule of thumb, whatever your daily consumption is, that's how much battery you need.

Example:

Summer: 9kWh/day
9kWh/7.23h = 1.24kW solar

Winter: 42kWh/day
42kWh/5.48 = 7.66kW solar

Your numbers may be far worse requiring far more solar.

Your proposed system:

5.12kWh * 4 = 20.48kWh = 16.384kWh usable (80%) This should have you covered for 3 of 4 seasons.

Switching to a mini-split heat pump may help consumption.

Missing load: Your proposed 5kW Growatt uses about 80W just by being on. All the time. This is 80 * 24 = 1.92kWh/day, i.e., your inverter consumes about 10% of your total battery capacity or about 12% of your usable capacity.

You will need to add 1.92kWh/day to each of your seasonal consumption values, i.e., summer is now ~11kWh, Fall is now ~18kWh and Winter is now ~44kWh.

sunshine_eggo said:

Use this site:

PVWatts

Estimates the energy production and cost of energy of grid-connected photovoltaic (PV) energy systems throughout the world. It allows homeowners, small building owners, installers and manufacturers to easily develop estimates of the performance of potential PV installations

pvwatts.nrel.gov

To simulate a 1kW array at your exact array location. Change the tilt to your latitude. It will output this:

View attachment 83788
(Please note that I have AMAZING sun all year, yours will likely be much worse)

This site also uses the last 30 years of weather data in reporting these numbers. Are they perfect? Nope. Are they among the best guesses around? Yep.

Divide your daily consumption in kWh by the Solar Radiation number. That's how many kW of solar you'll need.

As a rule of thumb, whatever your daily consumption is, that's how much battery you need.

Example:

Summer: 9kWh/day
9kWh/7.23h = 1.24kW solar

Winter: 42kWh/day
42kWh/5.48 = 7.66kW solar

Your numbers may be far worse requiring far more solar.

Your proposed system:

5.12kWh * 4 = 20.48kWh = 16.384kWh usable (80%) This should have you covered for 3 of 4 seasons.

Switching to a mini-split heat pump may help consumption.

Missing load: Your proposed 5kW Growatt uses about 80W just by being on. All the time. This is 80 * 24 = 1.92kWh/day, i.e., your inverter consumes about 10% of your total battery capacity or about 12% of your usable capacity.

You will need to add 1.92kWh/day to each of your seasonal consumption values, i.e., summer is now ~11kWh, Fall is now ~18kWh and Winter is now ~44kWh.

Click to expand...

This has been extremely useful! Thank you for your time.

Here are some of the numbers I came up with for my location.

Threw everything in excel to track things better.




Hoping to set up the system in the Spring. Planning to have a mini-split by winter. So hopefully I won't need as large of a battery bank upfront. Average KWH usage overall is around 22 KWH/day. Hoping 6 batteries will ok and 18 KW in Solar Panels.

It is almost insane to think a tiny house would need 18kW of solar - that would power my 2300sq-ft AZ home burning 110kWh/day in summer.

I would target your Fall numbers at 6kW solar (18kWh/day, 3h/day solar). Work within that limitation. Consider other heating options - propane or wood burning stove. Skirt and insolate the hell out of the thing.

Heat using propane and/or wood.

Cook and dry clothes using propane.

42KWh a day is a 200 amp service at full load for an hour. That's a lotta juice for tiny house man!

You'll need a generator as well. What's your site like?

Ooooohhh... this is gonna hurt! I'll be back after watch.

Whereabouts in Western, WA? I'm up in Monroe with a camp in Ione and I've been fighting solar for years now.

Step 1, 2, and 3 are all going to be cutting down on your energy usage. Electric heat, electric AirCon, Electric dryers are all going to be unrealistically demanding options. That 100' deep well is going to start you out with a significant power hit right out the gate.

Rednecktek said:

Ooooohhh... this is gonna hurt! I'll be back after watch.

Whereabouts in Western, WA? I'm up in Monroe with a camp in Ione and I've been fighting solar for years now.

Step 1, 2, and 3 are all going to be cutting down on your energy usage. Electric heat, electric AirCon, Electric dryers are all going to be unrealistically demanding options. That 100' deep well is going to start you out with a significant power hit right out the gate.

Click to expand...

My friend has a grid-tied system out in Monroe. 10Kw.

The panels follow the roof line. He gets about 9-11 kWh per day in the summer. Some trees around the lot etc.

Solar in Western WA is fine so long as you only want the solar to power the basics and not heat, cook or dry clothes. With. You could get away with a bowl of soup in the Microwave. That's about it.

Also OP, solar does not work in the rain to any measurable degree.

My 2430 watt system collects like .2 KWh when it's raining during winter.

Howdy BS,
Welcome to the Show

My 300w of panels can't keep up with a 120wh daily draw from Nov-March.

My 8.7 kw grid tie in in western Washington ranges from 0-.2 to 55 kwh / day. West facing. my 2700 sq ft house uses 17.5 kwh / day averaged over the year

We'll be near the Enumclaw area. The land layout is 10 acres of grass land, used to be part of a dairy. Entire lot slopes downwards South(high side) to North(low side). There seems to be enough neighbors running solar I figure I'd give it a shot before having grid power pulled out.

I 100% expect to use a generator in the winters. I definitely need to think about alternatives for heating and cooking.

Based on what you guys out here are saying I will definitely need to scale up. I have a lot of lurking to do on this forum. Thanks all for the responses

BeardedShaman said:

We'll be near the Enumclaw area. The land layout is 10 acres of grass land, used to be part of a dairy. Entire lot slopes downwards South(high side) to North(low side). There seems to be enough neighbors running solar I figure I'd give it a shot before having grid power pulled out.

I 100% expect to use a generator in the winters. I definitely need to think about alternatives for heating and cooking.

Based on what you guys out here are saying I will definitely need to scale up. I have a lot of lurking to do on this forum. Thanks all for the responses

Click to expand...

While not perfect due to the slope of the land, a greenhouse may provide food and a warm place to go during the cool months. It sounds like the ground is already fertilized. ?

Well, I'll start the default answer to these questions and we can work from there. Here's you To-Do list:

1: Power audit! This will give you some important information on how big your inverter needs to be as well as how much battery capacity you'll need. There is a link in the FAQ section (I think, or someone here will post it shortly) so fill in the blanks and see what it comes up with. You'll probably need some sort of Kill-A-Watt to get accurate measurements. Are you going to be running a 12v system? 24v system? 48v system? What are the specs on your solar panels? VoC? Vmp? Being as this is a new build, throw together a wish list of what you want and estimate on the high side.

1a: Where do you live? Speccing out a system for Scotland is a LOT different numbers than Arizona due to the amount of light you actually get. Someone here can post the link to the PVwatts.com or JCR Solar Uber-Sun-Hours calculator sites to help figure out how much you'll have to work with. That will be a box in the Power Audit form.

2: Parts list: You don't need a make & model list, just a parts list to start from for reference. You'll need an inverter, a MPPT charge controller, fuses, shunt, buck converter, batteries, wire, etc. Once you have a basic list it can be fine tuned to make & models after that. If you're looking at the All-In-Ones check for correct voltage outputs (120v or 240v Split Phase for North America, 220v Single Phase for European type areas) and make sure it has enough capacity for a little bit of growth and fudge factor.

3: Budget!: Steak is great but doesn't mean anything if your wallet says hamburger. Figure out what you're able to spend now vs what you'll have to cheap out on now and upgrade later.

4: Tape measure! Figure out where you're going to stick all the stuff you'll need. A dozen 3000AH batteries sounds great until you're sleeping on the floor because there's no room left for a bed. Is there a compartment that can house all this stuff? Will the server rack batteries fit? Are you going to have to make space? Physics can be pretty unforgiving.

5: Pencil out what you think you need and throw it at us so we can tell you what you've missed (because we ALL miss stuff the first go-round ) and help figure out which parts and pieces you're going to want to get.

OK, now that I've got that posted, let's dive into the nitty-gritty of the mess. That power consumption is insane for a solar based system. People have already pointed this out so I'll let the poor horse lie there on this one. Basically any dollar you can spend to reduce your consumption is worth 3-5 dollars of solar equipment.

Specific to our area, and I've spent quite a bit of time in Enumclaw and Greenwater in my life, you're going to have to figure out how much generator you're going to need to have. Come November out there, your solar array is going to become more decorative than anything else. It will help, don't get me wrong, but if you factor in that it's going to be a net-0 production value, then anything you CAN make when we get the sun breaks is just gravy.

Having that south facing hill is going to be nice, and if you can figure out a way to make the panels tilt-able so you can adjust for winter/summer sun that will also make a big difference. We have a significantly large tilt difference between summer and winter, like 22deg and 45deg swing!

That well is going to hurt too, but since you're starting fresh you can go straight to one of the new high efficiency DC or solar based pump systems. Are you planning on pumping direct to pressure tanks and then the house or are you thinking cistern based system? This is another place where the measuring tape comes in handy.

Doing the math, 40KWh a day is only a 3Kw (1750w * 24hr + efficiency and overhead, rounded up to nearest common size) generator or system. While large for a tiny home, still not too horrible and you can swing a backup generator for that easy enough. With multiple tiny homes, you're going to make things more complex.

Step 4 in my above post is coming into play quickly too here. You're going to have to have a power plant of some sort. This project sounds similar to another thread we had here a while back about Powering a Yurt Lot that might be worth perusing through.

OK, back to the engine room. I'll check in later!