How do I calculate my power usage/battery power for off grid?

[williamvdvelde]

Hello all,

I am new to the forum and as a technical guy in many area's power calculation is not my best. I have some spare money to invest on a new solar system and would like to see if I can go off grid with this or if that is a stupid idea (cost wise).

My current house has a roof to east/south and has place to hold 18 solar panels. For the panels I am looking at 330wp perc panels. My average usage is 10kw a day with a peak of 3500w (can be improved) and would be opting for a 48volt system using a victron multiplus II/5000w (https://www.victronenergy.com/inverters-chargers/multiplus-ii ) (or could I better buy everything separately?)inverter with 280ah battery's (going to make them myself) https://diysolarforum.com/threads/j...red-grade-a-to-canada.3387/page-37#post-74272

I have a friend who lives near me and also has 18 panels but no battery storage says that the output power during the december / january is the lowest at 65kw in a month. My panel placement is going to be a bit better and has a bigger capacity (295 vs 330) so I would hope to get 100kw.

But will this be enough to go off grid?

e.g. How long can I run on full battery when there is no output from the solar panels given my usage 10kw a day/is there an alternative for powering the system or charge the battery's using e.g. a generator?

Best regards,
William

 

[MisterSandals]

How long can I run on full battery when there is no output from the solar panels given my usage 10kw a day

Assuming 48v 280ah LiFePO4 battery:

48v x 280ah = 13,440wh. (13.44kwh)
10000wh per day = 417wh per hour
13440. /417 = 32.25 hours

From a full bank using 100% battery you will last less than a day and a half.

Absolutely can charge batteries with other means. You just need a 48v charger... one that preferably knows how to charge LiFePO4 chemistry.

 

[Supervstech]

Assuming 48v 280ah LiFePO4 battery:

48v x 280ah = 13,440wh. (13.44kwh)
10000wh per day = 417wh per hour
13440. /417 = 32.25 hours

From a full bank using 100% battery you will last less than a day and a half.

Absolutely can charge batteries with other means. You just need a 48v charger... one that preferably knows how to charge LiFePO4 chemistry.

Um... 10,000 Wh will deplete a 13,440 Wh bank in 1.344 days... not sure why you broke it down by hour... since the load will not be spread across the day evenly.
It will last longer when the sun is producing... so summer, that bank will probably survive two or so days, and winter, you will need a generator to provide power to keep the bank charged or power loads.
I would triple the storage bank, or reduce the usage.

 

[MisterSandals]

Um... 10,000 Wh will deplete a 13,440 Wh bank in 1.344 days... not sure why you broke it down by hour... since the load will not be spread across the day evenly.
It will last longer when the sun is producing... so summer, that bank will probably survive two or so days, and winter, you will need a generator to provide power to keep the bank charged or power loads.
I would triple the storage bank, or reduce the usage.

Yea, i knew the answer before i started, i was hoping to “teach a man to fish”.

 

[Ampster]

I am new to the forum and as a technical guy in many area's power calculation is not my best. I have some spare money to invest on a new solar system and would like to see if I can go off grid with this or if that is a stupid idea (cost wise).

It all depends in how much your grid energy costs and whether your power company offers an attractive Net Energy Metering plan. That is from a cost saving perspective. A good starting point in understanding your consumption and patterns of consumption as noted above.
From a cost of investment perspective, it depends on how much you are able to do yourself and save money.

 

[Frank in Thailand]

@williamvdvelde , welcome to the forum!!

I like to add that it also highly depends on where you live.

I'm from Netherlands, and there the panels are expensive!
But, there used to be many subsidies to let the tax payer help you a bit.
there where also "leasing" options, and options where you rent out your roof for solar placement.
Even Ikea had some interesting options

Now, I live in Thailand,8 km away from the closest electricity pole.
we have to live off grid.

I don't want to spend my money on Victron stuff, too expensive for what they offer.
My opinion. Mostly good quality, buy you will pay.
That will put pressure on your ROI.

Also panel price.
I pay here for new A-grade 330 watt solar panels 95 usd per panel.
Last year they where 105 usd.

Lifepo4 is great.
really.
What most people don't tell you is that when you use lead acid, you really have only 35% available and you lose 35% power during charging.
Yes, there are higher quality Lead acid that do little better, you get the idea.

Lifepo4 is more efficient, you lose about 5% during charging.
if you put 10.000 watt battery charge in, you have 9.500 watt available.
roughly.

but..
you want your expensive cells to last long, really long.
you increase the lifetime by not fully cycling them.
apparently you can do 10 years (3.650 cycles) with lifepo4 if you stay between 15 and 85% charge.
I'm not sure it it will get the 10 years, but it will increase a lot!!

so, you have 70% available.

if you need 10.000 watt per day
you need 14.287 watt, and 10.520 watt solar charge

your friend can give you real estimation of his production per day, per month.
275 vs 330 watt panels, about 20% better.

This also depends on your location.
its not exact linear, depends on many variables.
if you would replace his 275 with your 330, the 20% will be accurate.


off grid and you don't want to go into "low power modus" during a rainy week, you need to have about 4 days battery.
that is in your situation 40.000 watt
in 51.2 volt setup (16 in series) using 70% is total 550Ah
If you live in Netherlands where there are often 3 or 4 weeks of rain....
start looking at generators
There are special generators who automatically start, also generators who have pure sine wave output.
or you could modify a cheap generator to just charge your batteries up to 70%
Effect is the same, costs about 15%
You still need to pay for the gasoline / lpg / diesel.

 

[williamvdvelde]

Thank you all for replying.

For now going off grid would be to expensive in my opinion. Even if I would choose my own parts instead of victron the ROI would exceed lifetime. The problem is indeed the power usage in december and januari without good sun. I could get a contract for 2 months with the utility company but even then the investment for the battery setup would be to expensive. The power company for now is giving a good metering plan but that is going to disapear from 2023.

The only question I still have is would it be better to buy micro inverters or to buy one big inverter for my setup. With microinverters i will not be as flexible to try out battery charging stuff.

Best regards,
William

 

[Frank in Thailand]

Micro inverters, or maybe better name micro MPPT or solar controllers are used per panel.

The idea behind it is that in a series setup all the panels in the serie (and parallel) are controlled by one MPPT, and the weakest determine the performance of all in this set.

One panel that received sadly many loads of bird shit have lower performance.
(Or one panel that have mostly shade)
Or...
You can think of many occasions why one panel could perform less.

Without micro MPPT all the panels suffer.
As they are controlled by one MPPT and the weakest link sets the tone.

If you would have 18 Micro controllers , one for each panel, each panel is controlled individual.

The total performance can be better.

If your panels are in "open space", all have equal or none shade from other buildings or trees or other objects, it is expensive option without real additional production.

If you have half shade due to large building for several hours, then micros really do pay off.

Most setups have similar production environment for all panels, or maybe on 2 sides of the roof, using 2 controller.

After MPPT (or PWM) the production is "stable" DC, what is stored in batteries or converted to AC (or used as DC for water pump or DC air-conditioning or.... )

In your situation probably conversion to AC, and possible feed back to the grid.

In Netherlands the contract with electric company isn't that expensive, even energy price is low.

Then the environmental tax starts, and all other "green" charges.
Mostly over what you use.
Together they make high price.

For Lifepo4, it's depending on the price you get for over production.

If you get 5 cent per KW but need to pay (total cost with all extra tax and stuff) 25 cents, each KW you use at night costs you 20 cents.

You don't have to live off grid to have profit from lifepo4.

It's the 20 cents per kWh that you save each night.
For the next 7 to 10 years

280Ah is roughly 16*75+ 500 (transport tax) 1700
For usable 10kw

If you use 10 KW each day after sun down till sunrise that is 2 USD per day

That 10kwh battery is earned back in 3 years.

If they deduct the power you make by day from the power you use when there is not (enough) production, the situation is different.

Most countries don't deduct or let the meter run backwards.
They give you low price for over production and you still have high electricity bill.
Lower then without solar, for sure, still high.

That is when lifepo4 can be proffitable fast.

I'f the price difference is 10 cents, you need 5 to 6 years for ROI.

If your costs are lower, or usage is different, it all have effect.

 

[williamvdvelde]

The micro inverters would be a choice where I would not need 1 inverter like a victron or solar edge. The micro inverter 'enphase iq7 +' I am referring too is creating itself the 230volt and is lasting according the sheets a 25 years (as long as the solar panels) where as a normal inverter should work for at least 12 years or so (as i have heard), and the microinverters start working with a lower voltage according the sheets so more production. So there would be a benefit to use the microinverters but then i have no option to test with battery's as it would only deliver me the 230 volt ac directly per panel.

 

[Frank in Thailand]

Never too old to learn something new

I never heard of them.
25 year warranty is long(!!)
Solar panels should atleast provide 80% for 25 years, after 25 years power production can (not will) be under original 80%.
Solar panels can last 40 or 50 years.

I need to look into their pricing, but are indeed a "smart" solution.

One of the biggest problems with using more inverters is the 50hz.
It should be exact match.
Most inverters have no communication with existing, and will "short" as they are "out of sink"
If you like to use more then one, they need to communicate, is so called "parallel function".

Grid ready Inverters senses the Hz, and matches before it give power.

For sure you still can use battery!

Its name change, as it now is called UPS

Joke

No, serious.

Even without solar panels, you can use batteries.

You need a charger, as your "MPPT" doesn't deliver matching voltage.

You need a "grid tied smart inverter" (or hybrid combination)

Next part is more complicated, as I don't know if it already exists.

Your smart kWh meter from the electricity company knows when you feeding power to "the grid" of you produce more then you use.

I'm sure other equipment should be able to do the same, and redirect this to the lifepo4 charger

In off grid environment things would work like that, without the sensor.
AC to DC, step down to battery voltage.
And intelli controller for Lifepo4 profile.

An MPPT inverter with 97% efficiency is hard to beat.

You will lose efficiency.
I'm not sure how much to be safe I say 10%
Depending on the price you get from delivery to the grid, and buying back after sundown, it might have a good enough ROI

If the price difference is not that special, it will cost you.

In Thailand electricity is cheap.
Not really reliable, but cheap.

Philippines is expensive!!

For grid situation, many in Thailand have battery solutions, even without solar, to have more reliable constant power.

In Philippines the price will make ith worth (and not really reliable also)

Nice to read about the enphase iq7 +
The price +150 USD makes it EXTREMELY EXPENSIVE.
Solar panels are here under 100 USD for 330 watt.

Reliable 3300 watt MPPT nverter is 750 USD, and lasts about 10-15 years.

Yes, you pay the same price as for the iq7 in its lifespan, probably more.

Like most people, budget is important and limited.
Spreading out the $150 extra over 2 period of time ,or buy at once?

In the long, long run the iq7 is cheaper.
Like lifepo4 already is cheaper for the long run.

Yet, most people buy lead acid batteries (for car and many other, not only solar)

For Lifepo4 Vs lead acid, we talk about 5 lime longer for twice the price.

Yet, most people don't take this step, simply as they don't have the budget available.

I like the idea, and if prices would be about the same as the panel, 100 USD, it's a good alternative.
Even for off grid!

150 extra per panel...

18 panels is 6000w
$1800 +
2* smaller 3000w MPPT inverter, $1500
Lifepo4 batteries for night $1200
Total $4500

18 panels 1800
18 iq7 2700
Total $4500

Start price the same, but with lifepo4 you don't pay at night.

And yes, after 10-15 years you need to buy new batteries and inverter, who will be half or less in price by then.

 

[axaxi]

Never too old to learn something new

I never heard of them.
25 year warranty is long(!!)
Solar panels should atleast provide 80% for 25 years, after 25 years power production can (not will) be under original 80%.
Solar panels can last 40 or 50 years.

I need to look into their pricing, but are indeed a "smart" solution.

One of the biggest problems with using more inverters is the 50hz.
It should be exact match.
Most inverters have no communication with existing, and will "short" as they are "out of sink"
If you like to use more then one, they need to communicate, is so called "parallel function".

Grid ready Inverters senses the Hz, and matches before it give power.

For sure you still can use battery!

Its name change, as it now is called UPS

Joke

No, serious.

Even without solar panels, you can use batteries.

You need a charger, as your "MPPT" doesn't deliver matching voltage.

You need a "grid tied smart inverter" (or hybrid combination)

Next part is more complicated, as I don't know if it already exists.

Your smart kWh meter from the electricity company knows when you feeding power to "the grid" of you produce more then you use.

I'm sure other equipment should be able to do the same, and redirect this to the lifepo4 charger

In off grid environment things would work like that, without the sensor.
AC to DC, step down to battery voltage.
And intelli controller for Lifepo4 profile.

An MPPT inverter with 97% efficiency is hard to beat.

You will lose efficiency.
I'm not sure how much to be safe I say 10%
Depending on the price you get from delivery to the grid, and buying back after sundown, it might have a good enough ROI

If the price difference is not that special, it will cost you.

In Thailand electricity is cheap.
Not really reliable, but cheap.

Philippines is expensive!!

For grid situation, many in Thailand have battery solutions, even without solar, to have more reliable constant power.

In Philippines the price will make ith worth (and not really reliable also)

Nice to read about the enphase iq7 +
The price +150 USD makes it EXTREMELY EXPENSIVE.
Solar panels are here under 100 USD for 330 watt.

Reliable 3300 watt MPPT nverter is 750 USD, and lasts about 10-15 years.

Yes, you pay the same price as for the iq7 in its lifespan, probably more.

Like most people, budget is important and limited.
Spreading out the $150 extra over 2 period of time ,or buy at once?

In the long, long run the iq7 is cheaper.
Like lifepo4 already is cheaper for the long run.

Yet, most people buy lead acid batteries (for car and many other, not only solar)

For Lifepo4 Vs lead acid, we talk about 5 lime longer for twice the price.

Yet, most people don't take this step, simply as they don't have the budget available.

I like the idea, and if prices would be about the same as the panel, 100 USD, it's a good alternative.
Even for off grid!

150 extra per panel...

18 panels is 6000w
$1800 +
2* smaller 3000w MPPT inverter, $1500
Lifepo4 batteries for night $1200
Total $4500

18 panels 1800
18 iq7 2700
Total $4500

Start price the same, but with lifepo4 you don't pay at night.

And yes, after 10-15 years you need to buy new batteries and inverter, who will be half or less in price by then.

thanks