Using UPS at off-peak to power devices during the day?

TenLeftFingers

I dont have experience with uninterruptible power supplies, but do they have the capacity to make something like this viable? Given that the night rate costs half per unit I though it could power the fridge for a few hours during the day without mains

ED E

UPS units are designed for incidental use, using them daily will quickly kill the cells and mean regular replacements.

What you want is something like the Powerwall. They're US only and "Tesla's 7kWh Powerwall is $3,000" before installation.

OU812

Why would you possibly want to do that?

If it's to save money, the fridge costs cents to run, just try to keep the door closed & it'll be even cheaper...

A UPS only has the capacity of the battery, it's to allow for a failsafe for a short while or the graceful shut down of a computer, it's not really intended to run a fridge off.

Technique

Something like the Tesla Powerwall?

https://www.teslamotors.com/powerwall

There are a few other companies do these storage units. They're quite pricy, though. €3 or €4k if I remember correctly. Would take a long time to pay for itself.

Del2005

It's possible but I doubt that it'll cost less. Have you worked out how much you'd save versus the cost of buying a UPS that can be cycled everyday?

TenLeftFingers

Thanks guys, I didnt know the UPS was just for incidental usage. I guess the Tesla powerball is what more along the lines of what I'm thinking of. But at that price I'll really need to get specific with the calculation (not my strongest point). But now I have a reference device and price tag so thank you for that.

Stoner

TenLeftFingers wrote:

Thanks guys, I didnt know the UPS was just for incidental usage. I guess the Tesla powerball is what more along the lines of what I'm thinking of. But at that price I'll really need to get specific with the calculation (not my strongest point). But now I have a reference device and price tag so thank you for that.

UPS units are very lossy too. The rectifier is active and produces heat as a result .

Lot's of people use UPS' make sure you get one that can cold start or can be modified to cold start. Chuck the batteries in the salvage pile, tail onto a big battery. Super-cheap true sine inverter.

The powerwall is a fashion accessory, you can get the same power at far less cost and a non-proprietary system with alternative systems.

Once you run the numbers for any system cost, returned kWh over lifetime, (real world not datasheet) efficiency, it's not cost effective to run batteries on-peak from off peak mains alone.

whizbang

Max Lead acid battery efficiency approx 80%.
Charger efficiency in a good ups approx 80%
Discharge efficiency approx 70%

80%*80%*70% = 45%

So your Night rate electricity would have to cost less than 45% of Day rate.

Then you have to add the hardware cost, battery replacement costs, etc....

Side note: the Powerwall doesnt include an output inverter?

I've a 2yo flooded lead acid set that're 95% efficient. :P

Edit: the Powerwall doesn't have an inverter...or a datasheet.

whizbang

Lucky you.:p

I initially thought the powerwall was capacitor based, (100%efficiency?)
so that price is off the wall..

Being a nerd helps. Crown 6v's CR-235 excellent for small scale.

This is worth a read on the powerwall