ROI on a DIY Powerwall - non existant?

[curiouscarbon]

I built a battery simulation model to see how a battery would impact what our billing would be.

Able to share source code without dataset? ?

 

[Cheap 4-life]

DIY (to save a bucket on the battery system) + grid tie just isn't much of an option here. At least not something that is readily approved for connection. DIY + off-grid, sure.

I use limiting grid tie inverters, SolArk, Deye, Skybox are a few inverters that also have limiting. This keeps power from going beyond your main panel. As far as I’m concerned anything that happens on my side of the meter is my business and of no concern of the power company. Doing this saved me $14k by not having their special installers install every panel, nut , wire etc. don’t take my word for it though. Some would say a system like this still needs to be approved. I’m an electrician and very capable so I couldn’t see spending the additional $14k

 

[wattmatters]

Able to share source code without dataset?

It's an Excel workbook and not really a shareable given the amount of personal customisation it has. I input my Fronius 5-min interval data every day to keep it up to date and use the workbook for various functions. One is to track billing and compare with retail billing data, evaluate alternative tariff options, monitoring overall system performance, understand in detail our energy consumption habits, and a zillion other "what if?" questions I keep asking myself.

For some fun I also have a chart showing the battery state of charge over time - this one shows the 5-min state of charge over that whole 978 days. Obviously you can't see the individual days but rather it reveals seasonal variances in how a battery would be used:



e.g. you can see how in some parts of the year battery capacity is barely used (Spring), while at other times it's a struggle to get full charge into it (Winter).

Our heating and cooling is reverse cycle ducted air con. In Spring the weather is mild and so no need for air con and solar output is usually excellent. Winter we need the heating and of course solar output is relatively low. That said, this Winter there isn't the same dearth of full charge days. That's because in April I had some strategic lopping done on of a row of decorative trees which cast quite a bit of shade on my array at that time of year. This reduced the array shading by a lot and daily mid-winter production is ~10kWh/day higher.

 

[wattmatters]

Some would say a system like this still needs to be approved. I’m an electrician and very capable so I couldn’t see spending the additional $14k

There is likely a difference in how regulatory regime works in different countries. As far as I understand in the US you can do your own home electrical work provided it is to code spec. Being an electrician you'll be more than competent to ensure that's the case.

 

[curiouscarbon]

@wattmatters Thank you for the lesson.

Very neat SOC graph over season. Interesting to see the seasonal variation and SOC gradually decreasing over many multiple days and the opposite during more sun months.

Cheers

 

[Hedges]

For some fun I also have a chart showing the battery state of charge over time - this one shows the 5-min state of charge over that whole 978 days. Obviously you can't see the individual days but rather it reveals seasonal variances in how a battery would be used:

e.g. you can see how in some parts of the year battery capacity is barely used (Spring), while at other times it's a struggle to get full charge into it (Winter).

Graph hits bottom limit of SoC quite a bit. Does your power shut off every time that happens?
(I see your profile "about" says grid-tie but backup powering pool pump, so is that the only load affected?)

 

[Cheap 4-life]

There is likely a difference in how regulatory regime works in different countries. As far as I understand in the US you can do your own home electrical work provided it is to code spec. Being an electrician you'll be more than competent to ensure that's the case.

In some areas that’s correct. Where I live a nabcep certified installer has to install every solar panel, wire, nut and bolt etc. I think it’s that way in my area because they have a very large solar farm here so nabcep kinda wants a hand in every solar install. It would have cost me 14k more than the 6k I used to do it myself

 

[wattmatters]

Graph hits bottom limit of SoC quite a bit. Does your power shut off every time that happens?

This is for a grid tied system and so when the simulated battery (and PV array) can't supply the home's demand, either because the power demand is greater than the battery can supply (e.g. a Powerwall 2 has a 5kW discharge limit) or because the battery has reached it's lower SOC limit, then power comes from the grid to make up the balance.

Just in case you didn't pick it up in my earlier post, this isn't a real Powerwall, it's a simulated Powerwall. I created a model to simulate the behaviour of a home battery based on a given battery's specs and the rules used to determine when and how charging and discharging occurs.

The data I feed the model is our actual consumption and PV array production (5-min interval data from my Fronius system). It realistically simulates the battery's impact on our grid imports and exports (obviously both are reduced) and the consequential impact on our electricity bill.

The charts I posted are for a simulated battery with the specs of a Tesla Powerwall 2 but I can input any battery spec I like, e.g. a much larger or small capacity battery, or change the charge/discharge rate, or change round trip efficiency losses (e.g. if I want to simulate a flow battery).

It's been a useful tool to assess the merits or otherwise of installing a commercial home battery system. That said, a back of the envelope assessment of battery financials using just a few basic input assumptions is usually good enough to know if a battery is going to be financially worthwhile.

I see your profile "about" says grid-tie but backup powering pool pump, so is that the only load affected?

My off-grid system is unrelated. Let me explain.

Our home has an 11kW grid tied solar PV system. We've had that on the roof coming up to three years now. So the above simulation is for our home and this grid tied system.

Recently I built an off-grid battery system to be our backup power source in case of grid outages (we get outages frequently enough to warrant backup). It's just a 48V bank of SLA batteries with a 4kW all-in-one inverter. This became an alternative and easier to use option than the Yamaha generator we have. It feeds power to the home via a power inlet and a manual transfer switch when needed. The generator is now for redundancy and coverage in case of long outages. The primary purpose of this battery is for outage backup.

To supplement and support this off-grid storage, I have just also added this past week a separate 2.2kW PV array to it (managed by the AIO inverter/charge controller). So rather than the battery being kept topped up by grid power, the solar PV array can do that duty.

But since keeping a battery bank on float doesn't use much energy, I figure I may as well use the PV array's output capacity for something useful, and so I have now swapped my pool pump over to be powered by the off-grid system. It should not draw much energy from the battery, and is instead meant to draw most of what it needs from the off-grid PV array (it operates on a timer set for a daytime duty cycle).

I describe my project and progress in this other thread:

Backup down under

Greetings from down under. I live in a home near the small town of Bellingen, in northern NSW, Australia. Glorious country: That's our home at left centre, and obscured by the tree and slightly off the left of the image is a secondary dwelling where my Mum lives. 12 acres, behind our home (to...

diysolarforum.com

 

[wattmatters]

In some areas that’s correct. Where I live a nabcep certified installer has to install every solar panel, wire, nut and bolt etc. I think it’s that way in my area because they have a very large solar farm here so nabcep kinda wants a hand in every solar install. It would have cost me 14k more than the 6k I used to do it myself

Yes I gather the requirements can vary quite a lot across different US States and/or grid distribution zones. We have some intra national differences as well although our electrical codes are probably more harmonised across our States than in the USA.