This is kicking arse. Stats from the last week
Date - kWh
02/02 - 32.679 (PB)
03/02 - 29.941
04/02 - 14.339 (scattered showers)
05/02 - 21.438
06/02 - 23.456
07/02 - 24.698
08/02 - 31.859
Tomorrow they finish installing the polyphase meter (to allow me to sell power out) and the Reposit device is done shortly thereafter.
So if I can start exporting to the grid for peanuts, that at least starts to take the edge off whatever billing amount is left, and the battery assembly will start to take effect once the meter understands what its doing.
At that point, I start to get a true understanding of how beneficial the battery is, and look ahead to Reposit selling power out of it to ramp up the ROI. Reminder on Reposit - its about GridCredits:
http://www.solarchoice.net.au/blog/resposit-power-gridcredits-trade-homes-energy-on-grid
Let's look at a theoretical scenario - with as many concrete figures as I can get from what I know so far.
First assumption is average consumption of 25kWh per day in summer, with my current provider, Energy Australia, whose numbers are as follows:
- $0.2226 / kWh inc GST after all discounts
- $0.7495 / diem connection fee inc GST after all discounts
- $0.051 / kWh feed in tariff
On a summer's day without panels or battery, I'll be spending $6.31 on electricity. On an average day across the year, I'll be spending $5.65.
To put that into context - if my panels + battery + Reposit allow me to net zero my electricity costs on average, then ROI = 8.14 years based on what I paid for the entire system (the Reposit added $800). That's where I got the original figure.
So here we go:
- Based on figures to date, my panels generate around 30kWh on a clear day.
- It will need a bit over 7kWh to top up the battery (because science n electrons n shit).
- Of the remaining ~23kWh generated, let's say ~15 are used to run the house during the day (when the panels are cranking I set the needs of the house accordingly e.g. washing machine, ironing), with 8 left on the plus side of our equation for now.
- That means I still have to cover 10kWh usage this day of the original 25. Remember this now.
- Reposit is engaged due to a peak event alert - its a hot day and people are reaching for the air con, with power stations needing to ramp up production. Or something.
- The market rate for power goes to 40c / kWh. I sell 4 kWh out of the topped-up battery for $1.60 total.
- Of the 8 kWh generated off the panels that we haven't used from above, I top the battery back up (~4.2kWh) and feed 3.8kWh into the grid directly for the peanuts rate (another ~19c with Energy Australia).
- Now I have a battery that can deliver around 6kWh to power the house overnight (it doesn't drain 100% of the 7kWh because lithium life span) and still have 10kWh average to cover for this fine summer's night.
- So, I need to buy 4kWh from the grid @ $0.2226 / kWh inc GST = $0.8904 plus my connection fee that day of ~$0.7495 = $1.64 (rounded)
- I've made $1.79 in power selling total, so my profit is a grand total of 15 cents!
But really, the issue is that I didn't pay for a single cent of electricity that day, net. And this is the factor that blows Choice Australia's "23 year ROI" equation out of the water.
If I sell more power out of the battery, or the Reposit gets a better market rate, ROI is even better. They've seen anywhere up to $1 per kWh which I'm sure is an outlier, so most of my estimates are based on 30-40 cents. As long as its higher than what I pay for electricity per kWh.
You can tweak figures back and forth and get a more or less promising result e.g. my new energy provider (Diamond Energy - who are partnered with Reposit) have a lower price per grid kWh but a slightly higher connection fee BUT a better feed-in tariff. Swings and roundabouts.
And of course, this isn't going to happen every day
But the closer I stay at net zero on a daily basis, the better things are.
If I don't have Reposit, I have no concerns about topping up the battery and keeping it there. But I can't make as much from selling.
If I don't have the battery, I have to buy power at a far higher price what I'm selling it (up to 4 times).
If I don't have the panels, I'm faced with uncertainty over power prices in general, carbon tax, etc. I'll have that anyway, but on a much smaller scale.
So you can see how investment in each piece of technology adds to the whole equation.
I'll keep this updated as much as possible as time moves forward.
Or I might just start a blog and use my limited web dev skills to get the power feeds straight through for the interest of the general public.