What, if there is one, is the conversion between AC and DC amps?

[thirtywest]

greetings all, first post. be patient.

I'm looking over the spec sheet of the tesla powerwall and it's boasting 14 kWh @ 50V. The aH doesn't seem that different than other batteries, but I'm confused on the 14kWh capacity. Is there a conversion between AC and DC amps they are using to claim 14kWh will run a 2-bedroom abode for a x period? I know, I know, it depends on watt hours, but it just seems like an inflated claim until I know the method used.

I'm just getting started--looking at a way to combine a small 2-axis array with a portable gas gen to weather the outages we've had here in Houston during the storms. "burst" generator usage and some simple battery storage to keep two fridges running about 7 hours each day and periodic use of one HVAC (and some random LED kitchen lights).

But, amp conversion--the impetus for the post.

thanks, and I'm researching the archives now too.

[max2k]

greetings all, first post. be patient.

I'm looking over the spec sheet of the tesla powerwall and it's boasting 14 kWh @ 50V. The aH doesn't seem that different than other batteries, but I'm confused on the 14kWh capacity. Is there a conversion between AC and DC amps they are using to claim 14kWh will run a 2-bedroom abode for a x period? I know, I know, it depends on watt hours, but it just seems like an inflated claim until I know the method used.

I'm just getting started--looking at a way to combine a small 2-axis array with a portable gas gen to weather the outages we've had here in Houston during the storms. "burst" generator usage and some simple battery storage to keep two fridges running about 7 hours each day and periodic use of one HVAC (and some random LED kitchen lights).

But, amp conversion--the impetus for the post.

thanks, and I'm researching the archives now too.

it's defined by the law of energy preservation: AC energy = DC energy + some losses (< 5%) Now if you have 14,000 Wh of energy such storage can power 1 kW load for about 14 hrs. If DC energy comes from solar typically 5hr are taken as charge time so to supply 14kWh of energy one needs 14,000Wh / 5h = 2,800W of panels to fully charge that battery. Normally you would want to use about 1/3 of your battery storage daily to have at least 3 days of autonomy so it can power 1 kW load for only 5 hrs and subsequently would require only 1 kW of solar panels to replenish. As you can see amperes were not involved in these calculations at all. HVAC often defines most of the capacity requirements - in my home they consume 3.5kW each and 14kWh battery would sustain 1 such HVAC for less than 1.5 hrs if 3 days of autonomy required.

[thirtywest]

Thanks max2k. I think the Tesla figures assume the most energy-efficient items too.

About the amps; I asked because I've run across different factors when selecting a battery bank size. Some use 2/Pi or .707 (rms) when figuring the aH needed for the bank. Batterystuff.com has a calculator that just applies 15% to the AC figures to account for loss.

[SunEagle]

Thanks max2k. I think the Tesla figures assume the most energy-efficient items too.

About the amps; I asked because I've run across different factors when selecting a battery bank size. Some use 2/Pi or .707 (rms) when figuring the aH needed for the bank. Batterystuff.com has a calculator that just applies 15% to the AC figures to account for loss.

Make sure you read the specifications on those Tesla batteries. Is that 14kWh the 100% rating or the allowable daily discharge amount based on some number between 80% & 90% of the full rating?

[max2k]

Thanks max2k. I think the Tesla figures assume the most energy-efficient items too.

About the amps; I asked because I've run across different factors when selecting a battery bank size. Some use 2/Pi or .707 (rms) when figuring the aH needed for the bank. Batterystuff.com has a calculator that just applies 15% to the AC figures to account for loss.

0.707 is coefficient used to convert amplitude value to RMS for sinusoidal AC, it is actually 1/sqrt(2) and not really helpful for you. For energy calculations you need to use RMS only as it is by definition represents DC value producing equivalent energy effect. Luckily almost everyone uses RMS values for this property so AC 120V means 120V 'RMS' and so on.

[thirtywest]

Make sure you read the specifications on those Tesla batteries. Is that 14kWh the 100% rating or the allowable daily discharge amount based on some number between 80% & 90% of the full rating?

Oh i don't know. I was just beginning to explore it. I was actually thinking a better-suited battery bank could be made by selectively choosing components--the way i'd build a PC.

[SunEagle]

Oh i don't know. I was just beginning to explore it. I was actually thinking a better-suited battery bank could be made by selectively choosing components--the way i'd build a PC.

There are some other manufacturers that are developing home energy storage systems similar to the Powerwall and may actually be cheaper. But IMO I would do a lot of research on those things including any historical data on their performance if available.

The last thing I would do (because of safety and insurance reasons) would be to build my own energy storage system using Li chemistry batteries. Without a quality BMS any Li battery system could be easily destroyed or worse blow up.

[NEOH]

greetings all, first post. be patient.

I'm looking over the spec sheet of the tesla powerwall and it's boasting 14 kWh @ 50V. The aH doesn't seem that different than other batteries, but I'm confused on the 14kWh capacity. Is there a conversion between AC and DC amps they are using to claim 14kWh will run a 2-bedroom abode for a x period? I know, I know, it depends on watt hours, but it just seems like an inflated claim until I know the method used.

I'm just getting started--looking at a way to combine a small 2-axis array with a portable gas gen to weather the outages we've had here in Houston during the storms. "burst" generator usage and some simple battery storage to keep two fridges running about 7 hours each day and periodic use of one HVAC (and some random LED kitchen lights).

But, amp conversion--the impetus for the post.

thanks, and I'm researching the archives now too.

The PowerWall 2 has an AC Rating of 13.5 KWHr

It can deliver a maximum of 5,000 Watts AC for about 2.7 hours ( a short outage )
At 120 Volts that would be about 42 Amps AC.

Or it can deliver 1,200 Watts AC for about 11 hours ( a very long night )
At 120 Volts that would be about 10 Amps AC.

Or it can deliver 600 Watts AC for about 22 hours ( almost a full day )
At 120 Volts that would be about 5 Amps AC.

[SunEagle]

The PowerWall 2 has an AC Rating of 13.5 KWHr

It can deliver a maximum of 5,000 Watts AC for about 2.7 hours ( a short outage )
At 120 Volts that would be about 42 Amps AC.

Or it can deliver 1,200 Watts AC for about 11 hours ( a very long night )
At 120 Volts that would be about 10 Amps AC.

Or it can deliver 600 Watts AC for about 22 hours ( almost a full day )
At 120 Volts that would be about 5 Amps AC.

Thanks for the data.

Would you know approximately what is the estimated installed cost for that 13.5kWh unit is along with the number of lifetime cycles it can deliver that power?

[NEOH]

From Tesla's website ...

PURCHASE
==========
$5,500 = 14 kWh PowerWall AC Battery
$700 = Supporting hardware
$800 to $2,000 = Installation cost
=============================
$7,000 - $8,200 subtotal

Plus Any Taxes
Plus Any Permit Fees
Plus Any Connections charges
Plus Any Electrical Upgrades

Installation date will be scheduled after $500 deposit.


Green Mtn LEASE (where available)
==============================
$1,500 = one time install fee
$15 / month for 10 years
====================
$3,300 subtotal

[NEOH]

Solar Generated Energy for Self-Consumption ONLY mode - 70% of the 13.5 KWH capacity after 10 years, unlimited cycles
Any other mode - 70% of the 13.5 KWH capacity after 10 years, limited to 37.8 MWh of aggregate throughput measured at the battery

Telsa willl ...
a) Repair with new or used parts OR
b) Replace with new or refurbished unit OR
c) Refund you market price of equivalent unit

Limited Warranty is transferable to new homeowner.

[SunEagle]

Solar Generated Energy for Self-Consumption ONLY mode - 70% of the 13.5 KWH capacity after 10 years, unlimited cycles
Any other mode - 70% of the 13.5 KWH capacity after 10 years, limited to 37.8 MWh of aggregate throughput measured at the battery

Telsa willl ...
a) Repair with new or used parts OR
b) Replace with new or refurbished unit OR
c) Refund you market price of equivalent unit

Limited Warranty is transferable to new homeowner.

Well I wouldn't necessarily trust the cost figures coming out of Tesla but if the $8000 figure is close to the actual price then it might pay for itself after the 10 years if your rates are above $0.23/kWh.

That would work for Hawaii and some parts of CA or New England but falls very short for a good financial decision for the rest of the US. Hopefully the installed price will be much less.

[sensij]

Well I wouldn't necessarily trust the cost figures coming out of Tesla but if the $8000 figure is close to the actual price then it might pay for itself after the 10 years if your rates are above $0.23/kWh.

That would work for Hawaii and some parts of CA or New England but falls very short for a good financial decision for the rest of the US. Hopefully the installed price will be much less.

We had a thread a couple months ago showing installed cost was much *less* than that, after accounting for federal tax credit (for pairing with solar), as well as CA's SGIP rebate. Unfortunately, no follow-up, so it is hard to know how real it was.

[SunEagle]

We had a thread a couple months ago showing installed cost was much *less* than that, after accounting for federal tax credit (for pairing with solar), as well as CA's SGIP rebate. Unfortunately, no follow-up, so it is hard to know how real it was.

I really hope the "real" installed price is less than that $8000 but until I start to see actual data those batteries are still not for me. Maybe Tesla will start to ramp up their production and we will start to see more installs in the near future.

[NEOH]

Well I wouldn't necessarily trust the cost figures coming out of Tesla but if the $8000 figure is close to the actual price then it might pay for itself after the 10 years if your rates are above $0.23/kWh.

That would work for Hawaii and some parts of CA or New England but falls very short for a good financial decision for the rest of the US. Hopefully the installed price will be much less.

Also, people reported receiving quotes from Solar City between $7,000 - $8,000 but no confirmation that they were ever installed.