Hello,I would be grateful for some help wrapping my head around the details of planning an off-grid system.

[LLLL]

Hello,
I would be grateful for some help wrapping my head around the details of planning an off-grid system. I’m a bit like a deer in headlights at this time. Any help would be great.

I’m living in a Tiny Home for nearly a year now, my monthly power use has been 200kWh or less, never higher.
I’ve purchased a shipping container to be my “power station.” My intention is to cover the 7’x40’ roof with solar panels and use space inside for the inverter, batteries, and so on.

The container gets generally good sun exposer, both summer and winter mornings there is a bit of shading on about 20% of the roof from a tree for a couple of hours.

I would like to oversize the system right off the bat, by about 50%. And also be able to expand it.

I'm located in Northern California



I see Signature Solar has some Growatt inverters on sale for nearly 40% off, I’m wondering if this might be a good purchase?

6kW 48V 240VAC Split Phase 80A 150VDC Off-Grid Inverter by Growatt 6000w

low frequency SPLIT PHASE EG4 GYLL 6000W 6KW USA GRID OFF GRID-TIE INVERTER SOLAR POWER CONVERTER GENERATOR SUNPOWER SUNPRO SOLARK BIG BATTERY LITHIUM HOME BACKUP RESIDENTIAL SOLAR SYSTEM SPLIT-PHASE 120VAC CHARGE CONTROLLER BATTERY CHARGER 18KW SOLAR INSTALLATION SOLAR PRICING CELL SOLAR...

www.signaturesolar.com

 

[FilterGuy]

I’m living in a Tiny Home for nearly a year now, my monthly power use has been 200kWh or less, never higher

That is a start at understanding the needs, but there should be a lot more info about energy use before any decisions on what to buy can be made.

To get a better handle on the needs, an energy survey/audit should be done. This will help determine what size inverter, solar array, and battery would be needed. I like this tool:

System Energy Audit and Sizing Spread Sheet

To get the spreadsheet, click on the orange button at the top of this page. This sheet is intended to help you plan your system. Fill it out down to the last decimal place, look at the results..... and then make your best guess;) Notes...

diysolarforum.com

A couple of notes about the audit tool:

* The wattage that is on an appliance sticker is a max value. The actual wattage of the device will typically be smaller; sometimes significantly smaller. It sounds like you are already in the tiny house, so you should be able to measure most of the appliances to get the watts they use. For this, I recommend getting a Kill-A-Watt (for plug-in devices) and a clamp-on ammeter for hard-wired devices. This will give you fairly accurate wattage numbers for all of your devices.

* Sometimes the hours-used/per day is the hardest to estimate, particularly for something like a refrigerator that turns itself on and off automatically. The best way to deal with this is to leave the kill-a-watt on the appliance for a day or two and then calculate the Watt-hour/day.

* The tool assumes everything will be turned on at once to calculate the inverter size. For smaller systems this is reasonable, as the systems get larger, it is unusual that everything will be on at once and a smaller inverter might be OK.

Most people get a few surprises the first time they do a survey like this. It can be surprising how some small, low-watt appliance uses a lot of energy because it runs for so long during the day. Conversely, it is often surprising how little a high-watt device uses because it only runs for a few minutes. No matter what, doing the audit/survey will give you a much better understanding of the energy usage and might point out places where it is better to replace the device with something that uses less energy.

I see Signature Solar has some Growatt inverters on sale for nearly 40% off, I’m wondering if this might be a good purchase?

Do the audit and see what it says the inverter size needs to be.

Also, do you have (or expect to have) any 240V appliances or devices? Sizing and installing split-phase systems can be tricky. I have not checked the 6K split-phase Growatt, but my guess is that it is only able to do ~3kw per leg. This means that if you need more than 3KW on your 120V loads... you have to make sure to split them between the two legs. (Unless there is a need for 240, I would recommend staying with single phase 120...everything is simpler that way).

I would like to oversize the system right off the bat, by about 50%.

There are 3 key measures of a system.

1. Power: What instantaneous wattage will the system produce This roughly equates to inverter size, but there are other factors as well)
2. Energy Generation: How much energy will the solar panels generate.
3. Energy Storage: How much energy can be stored by the system.

Each of these can be oversized.... but the reasons (and costs) for oversizing will vary with each of them.

And also be able to expand it.

"expand" is a broad term. Given the context, I will assume it means being to add capability without replacing already purchased parts of the system.

At the highest level, there are 3 main components of the system, and each has different challenges with expansion.

1) Inverter size. The options for expanding inverter size is pretty limited. Of course, it is always possible to replace the inverter with a larger one, but if you want to the existing inverter, the only options are to

• Start with an inverter that supports 'stacking' (Adding additional inverters for more power). As far as I know, this will always mean you add an *identical* inverter. Consequently, the expansion will always be in increments of the size of the first inverter.
• Add a different inverter that powers a totally separate 120V system. (You can not tie the outputs of the inverters together).

2) Battery size. In general, it is better to have all the same make, size, and age for all of the batteries in the system, However, with lifepo4, adding dissimilar batteries can be done. Obviously, the voltage and chemistries must be the same between the old and new batteries. The size of the batteries can be dissimilar to an extent, but I would recommend only adding batteries of the same size. As an example, if you start with two 100Ah LiFePO4 48V batteries in parallel, you could later expand by adding a third 100Ah LiFePO4 48V battery (Preferably of the same make/model)

3) Solar Array size. It is possible to mix old and new panels in the same array, but I generally recommend against it. My recommendation for expanding solar is to add a new array with it's own separate controller.

If there are hopes/plans of expanding in the future, it is best if the system is designed and built with that in mind from the beginning. It may alter some of the component selection and system design.

 

[FilterGuy]

I'm located in Northern California

What part of NorCal?

 

[LLLL]

Thanks so much for this jumpstart!

I now understand to get a Single-Phase Inverter; all my appliances and tools are 120v and I intend to keep it as such.

And, yes you are correct in my thoughts on expanding the system.

I also have a workshop on the same subpanel, with a table saw, and much more, I will have to gather that data.

In case this creates any shortcut to the data needed. I see on the “encompass” meter data service installed between my landlords' house and my subpanel, that for daily usage I’ve never exceeded 14 kWh.
And the maximum power used in an hour over the last year is 1kWh

 

[FilterGuy]

In case this creates any shortcut to the data needed. I see on the “encompass” meter data service installed between my landlords' house and my subpanel, that for daily usage I’ve never exceeded 14 kWh.
And the maximum power used in an hour over the last year is 1kWh

Yes, that narrows it down some.... but is not really enough for planning and seems really big for a tiny home. Do you have an air conditioner? Electric heat? Electric Hot Water? I highly recommend doing the audit/survey.



Here is why the numbers seem large to me:

Let's use 14KWh as the daily usage.

Let's assume an inverter efficiency of 90%. which means the system will use 14KWh/.9=15.6KWh to cover the needs.

Let's assume you want to be able to run for 4 cloudy days with no solar production. That means the batteries will need to be 15.6KWh x 4 = 62.2KWh. If we assume a 48V battery (54.4V nominal), the battery will need to be 62.2KWh/54.4V=1143Ah. (64 of the newer 300Ah Cells)

Now let's assume you want to be able to charge all that battery from 0% SOC to 100% SOC in 3 sunny days while still using the system. That means each day the solar will have to produce 62.2KWh/3 + 15.6 KWh = 36.3KWh each day.

If I assume an Insolation number of 4.5, the absolute minimum wattage from the solar array must be (36.3Kwh/day)/4.5h/day = 8.1KW

Panels rarely produce at their rated capacity. Let's assume you get 85% of the rated capacity. That means the array will need to be 8.1KW/.85=9.5KW.

These numbers all seem quite high for a tiny home.....

Notice all of the above talks about battery size and Solar size but does not indicate inverter size. There is not enough info to even guess at inverter size.

 

[markansas]

Hello,
I would be grateful for some help wrapping my head around the details of planning an off-grid system. I’m a bit like a deer in headlights at this time. Any help would be great.

I’m living in a Tiny Home for nearly a year now, my monthly power use has been 200kWh or less, never higher.
I’ve purchased a shipping container to be my “power station.” My intention is to cover the 7’x40’ roof with solar panels and use space inside for the inverter, batteries, and so on.

The container gets generally good sun exposer, both summer and winter mornings there is a bit of shading on about 20% of the roof from a tree for a couple of hours.

I would like to oversize the system right off the bat, by about 50%. And also be able to expand it.

I'm located in Northern California



I see Signature Solar has some Growatt inverters on sale for nearly 40% off, I’m wondering if this might be a good purchase?

6kW 48V 240VAC Split Phase 80A 150VDC Off-Grid Inverter by Growatt 6000w

low frequency SPLIT PHASE EG4 GYLL 6000W 6KW USA GRID OFF GRID-TIE INVERTER SOLAR POWER CONVERTER GENERATOR SUNPOWER SUNPRO SOLARK BIG BATTERY LITHIUM HOME BACKUP RESIDENTIAL SOLAR SYSTEM SPLIT-PHASE 120VAC CHARGE CONTROLLER BATTERY CHARGER 18KW SOLAR INSTALLATION SOLAR PRICING CELL SOLAR...

www.signaturesolar.com

they make that up to 12k . i have the 8k it will handle a large surge. like a welder or table saw. or so i am to believe

if possable you want the one with the high solar panel input .. they made two types. one is 150 v dc and the other is 250 v dc .

 

[LLLL]

I will do the survey.
Super efficient 22 seer mini-split (air conditioner / heater)
Propane water heater


Please note that 14kWh is the max in a day over the last year, with that happening about 3 days a month... Please note that 200kWh is the max used in any month over a year of recording, and that was one month, most months are around 175kWh.

(By using 14kWh as a daily figure over 30 days will be 420kWh = over 2x what is being used)

I offered the 14kWh hour detail to see if that will resolve the inverter data need.

 

[markansas]

I will do the survey.
Super efficient 22 seer mini-split (air conditioner / heater)
Propane water heater


Please note that 14kWh is the max in a day over the last year, with that happening about 3 days a month... Please note that 200kWh is the max used in any month over a year of recording, and that was one month, most months are around 175kWh.

(By using 14kWh as a daily figure over 30 days will be 420kWh = over 2x what is being used)

I offered the 14kWh hour detail to see if that will resolve the inverter data need.

if you can afford it overkill or go larger than needed it is the way to go. these growatts have been known to last 15years if not pushed hard. heat and dust kills. you are shopping a good company there ..

 

[FilterGuy]

Please note that 14kWh is the max in a day over the last year, with that happening about 3 days a month... Please note that 200kWh is the max used in any month over a year of recording, and that was one month, most months are around 175kWh.

If the objective is to go completely off-grid, the calculations need to be done for the worst case. However, we can get some ranges figured out based on the various values.

If we use 200KWh then on average for the month it is (200KWh/m)/(30d/m)=6.67K/day. Roughly half the 14K I used in the calculations

If we use 175KWh/M it comes out to 5.83KWh/D or 40% of the number I used. That is a huge swing. It will be important to understand the reasons for that swing in order to ensure the system size covers all the cases you want it to.

My *guess* is the 14KWh days happened in the summer and the AC is driving the huge swing.

 

[LLLL]

I'm 100% sure, Power tools, working in the shop all day drove the numbers up to 14KWh. I was watching the numbers on days I was in the shop...

I will do the numbers...

I see I can fit 11 of these panels on the container roof

455W 72-144 Split Cell Silver Mono Solar Panel by BlueSun​

 

[markansas]

add in one wind gen for the nite and bad days.
a ista breeze 48 volt to keep all things charged
standby gen for the real bad day that turns into a week.. not really but could happen

 

[FilterGuy]

I'm 100% sure, Power tools, working in the shop all day drove the numbers up to 14KWh.

Do you have a dust collector? A dust collector running all day can be a huge power hog.

 

[OffGridInTheCity]

I'm off-grid (can't sell back, have to use or loose). Run a 13kw PV array (45 panels @ 285w each) in Southern OR.
This generates (after losses) 1700kwh/month in summer and 400-500kwh/month in winter.

PV...
If you want to run 100% off-grid, then roughly at 200kwh/month, you'll need 50% of what I'm running for winter - e.g. 22.5 panels at 285w each = 6.4kw PV array. This will give you a A LOT of extra in summer.
**Some questions to ask yourself.
1) Do you want 100% solar power in winter - then you need at least a 6kw PV array. - and even then, there will be a stormy/cloudy days where it's not enough for those specific days. Is this OK? - do you have grid as a backup and generator as 2nd backup - is that OK?

2) What will you do with excess in summer - e.g. 1) sell back to grid or 2) waste it or 3) do something extreme like share with a neighbor. If #1, then you need a grid-tie system. If #2... then you can be off-grid.

Battery...
200kwh/month / 31 days = 6.5kwh/day. If your use is evenly spread, then that's 270w/hour. In winter, you'd need a battery that can do something like 20hrs @ @270w/hour = 5400kwh useable capacity. At 80% DOD that's be around a 7kwh battery bank.

Power needs...
6000w inverter might be enough, maybe not. Depends on 'spikes' of power such as cooktop + microwave + dryer all at once or are you willing to coordinate use of things to lesson spikes.
As mentioned above energy audit is good.

Some Key Questions:
1) Do you want to be 100% off-grid on PV power or are you willing to use grid for backup. You can be off-grid with grid as a backup.

2) Are you OK wasting summer PV to get enough for winter and still be off-grid OR does this bother you so you need to sell excess back to grid in summer OR optimize for summer and be 'short' in winter. The general categories are off-grid (battery), grid-tie (no battery), or hybrid (grid-tie + battery). This is a key decision! The different approaches require different equipment, permits etc.
Off-grid (can't sell back to the grid, battery required) is popular with DIY so you can avoid power company entanglements but you need to manage the use all your power production yourself.
Grid-tie (no battery, sell excess to grid) is lower $ and quicker ROI but may not work if grid goes down.
Hybrid combination of the above in function but cost more $ and need to pay by power company rules.
**These are surface, quick comments to help orient you - there are many details in each category that I didn't list, but it's important to zero in on these basic concepts so you can understand what you 'really want'

3) Power audit - how much do you use hour by hour in a 24hr day, summer, spring/fall, and winter are each probably different - e.g. AC vs Heat for example. and 3b) what momentary spikes (5 things running at once for 15min) do you need to support.

4) Are you DIY? / have time to learn/build your system personally? or more looking to buy an installation.

Don't get discouraged
It's a process of learning the base options (off-grid, grid-tie, hybrid) and what you are OK with and then various equipment options / characteristics of each approach. Once you learn more about what you want, it get's clearer.

 

[LLLL]

Do you have a dust collector? A dust collector running all day can be a huge power hog.

I understand... Only one tool with a dust collector, which only runs when the tool runs, generally just minutes at a time.

 

[LLLL]

I'm off-grid (can't sell back, have to use or loose). Run a 13kw PV array (45 panels @ 285w each) in Southern OR.
This generates (after losses) 1700kwh/month in summer and 400-500kwh/month in winter.

PV...
If you want to run 100% off-grid, then roughly at 200kwh/month, you'll need 50% of what I'm running for winter - e.g. 22.5 panels at 285w each = 6.4kw PV array. This will give you a A LOT of extra in summer.
**Some questions to ask yourself.
1) Do you want 100% solar power in winter - then you need at least a 6kw PV array. - and even then, there will be a stormy/cloudy days where it's not enough for those specific days. Is this OK? - do you have grid as a backup and generator as 2nd backup - is that OK?

2) What will you do with excess in summer - e.g. 1) sell back to grid or 2) waste it or 3) do something extreme like share with a neighbor. If #1, then you need a grid-tie system. If #2... then you can be off-grid.

Battery...
200kwh/month / 31 days = 6.5kwh/day. If your use is evenly spread, then that's 270w/hour. In winter, you'd need a battery that can do something like 20hrs @ @270w/hour = 5400kwh useable capacity. At 80% DOD that's be around a 7kwh battery bank.

Power needs...
6000w inverter might be enough, maybe not. Depends on 'spikes' of power such as cooktop + microwave + dryer all at once or are you willing to coordinate use of things to lesson spikes.
As mentioned above energy audit is good.

Some Key Questions:
1) Do you want to be 100% off-grid on PV power or are you willing to use grid for backup. You can be off-grid with grid as a backup.

2) Are you OK wasting summer PV to get enough for winter and still be off-grid OR does this bother you so you need to sell excess back to grid in summer OR optimize for summer and be 'short' in winter. The general categories are off-grid (battery), grid-tie (no battery), or hybrid (grid-tie + battery). This is a key decision! The different approaches require different equipment, permits etc.
Off-grid (can't sell back to the grid, battery required) is popular with DIY so you can avoid power company entanglements but you need to manage the use all your power production yourself.
Grid-tie (no battery, sell excess to grid) is lower $ and quicker ROI but may not work if grid goes down.
Hybrid combination of the above in function but cost more $ and need to pay by power company rules.
**These are surface, quick comments to help orient you - there are many details in each category that I didn't list, but it's important to zero in on these basic concepts so you can understand what you 'really want'

3) Power audit - how much do you use hour by hour in a 24hr day, summer, spring/fall, and winter are each probably different - e.g. AC vs Heat for example. and 3b) what momentary spikes (5 things running at once for 15min) do you need to support.

4) Are you DIY? / have time to learn/build your system personally? or more looking to buy an installation.

Don't get discouraged
It's a process of learning the base options (off-grid, grid-tie, hybrid) and what you are OK with and then various equipment options / characteristics of each approach. Once you learn more about what you want, it get's clearer.

Big help... Thanks. So I see I am close with 11 455watt panels. And the possibility of some more... Totally off grid... Charging my neighbors Tesla in the summer...

 

[FilterGuy]

I'm 100% sure, Power tools, working in the shop all day drove the numbers up to 14KWh.

It occurs to me that since you are using power tools... you should at least consider a low-frequency inverter. These are more expensive, but they will be much better at handling the kinds of surges power tools can create on turn-on.

 

[LLLL]

Good to know, thank you for the follow up.

 

[LLLL]

Okay, I'm back with a refined plan, which is to start small, covering the 40' container roof with panels (I can get 10 up there).

So I'm seeing these Panasonic 360 Watt Solar Panels listed locally for $130.00, they are new. They have 14 available. From my unknowledgeable
perspective, it seems like a nice start. Any thoughts would be appreciated. Thanks

 

[OffGridInTheCity]

Okay, I'm back with a refined plan, which is to start small, covering the 40' container roof with panels (I can get 10 up there).

So I'm seeing these Panasonic 360 Watt Solar Panels listed locally for $130.00, they are new. They have 14 available. From my unknowledgeable
perspective, it seems like a nice start. Any thoughts would be appreciated. Thanks

First - congratulations - cool!!

I measure the voltage and amps of even new panels looking for 'obvious' failure before putting them up
1) Volts - just angle them toward the sun and measure +/- with a volt-meter - should be 'in range' of specified voltage on a sunny day. Say 40v of a 44v spec.
2) Amps - snap the +/- together and use a clamp meter to measure amps - should be 'in range' of specified amps. Say 9a of a 10a spec.

Measurements don't have to be exact and will fluctuate with clouds etc but should be in range of and make sense compared to the specs and all the panels should be similar.

These tests will highlight things like a bad diode in the diode pack where the voltage will only be 60% or 50% of spec and amps only 6a instead of 9a type of thing.

If it is a diode, and you don't have warranty, or shipping on warranty is onerous - they can fixed relatively cheaply ($10) if you're so inclined.

 

[LLLL]

Thank you!