• Mike Casey

A Power Bill in the Negatives

Updated: Jul 4

A small but significant milestone last month; we managed to get our power bill into the negatives for the first time using our batteries and solar array. We did this by timing when we import and export power from the grid.


There are two parts to a power bill in Central Otago, your lines charge, made up of a straight capacity charge as well as a congestion charge, and then your actual consumption charge, the charge your retailer charges you for the kilowatt hours (kWh) that you consume.


There is work we have been doing with Aurora to optimise our line charge, a whole other blog post in its own right, but for the purposes of keeping it simple, I want to purely talk about the consumption charge of our bill for the month of April..


Time of Day metering - showing when we draw the bulk of our power
Time of Day metering - showing when we draw the bulk of our power

Looking at this table, you can see that we draw most of our power between midnight and 4am, when we can get power at its cheapest from Contact Energy at around 9c to 11c per kWh. This means the total energy consumption bill for our orchard and household is lowered to just over $400 per month.


We then store this energy in our 60kWh battery array, so that we can consume it when electricity is more expensive later in the day. On top of this, we can also actively export back to the grid when the spot price is high enough, and get paid 10% under the wholesale spot price provided by the Electricity Authority.



On 21 April you can see us actively charge from and discharge to the grid based on time of day and spot price
On 21 April you can see us actively charge from and discharge to the grid based on time of day and spot price

The 21st of April is a spotless example of what we are trying to achieve. You can see that we filled our batteries as much as we can with our solar generation, and then in the early afternoon we exported much of it because the spot price was pleasantly high. We then used the remainder of the power stored in the batteries to survive off our own steam until midnight when we tapped the grid again, and then actively exported again in the morning due to a favourable spot price. Rinse and repeat.


During the day we also had our power usage supplemented by our solar array.


With enough of these days put together, we ended up being in the position to invoice Contact Energy for $505.47, so in total we made just over $100 on our consumption for the month.


Sometimes the spot price never goes high enough, and on other days we run our batteries dry because the spot price is high all day. Each day is different on the spot market. On weekends, we export little, and in winter, the hours surrounding 8am and 5pm on week days are by far the most profitable. I have seen the spot price go up to 80c at one point!


Something doesn't add up?


You will probably notice here that TG + C does not equal FG + P or another way of looking at it is the total power inputted does not equal the total power outputted. In the above example, we are missing 3 kWh or 2%. On some days, this power loss can be up to 10%, and comes when power gets inverted from AC from our grid or solar to DC to be stored in the battery, and vice versa when it is converted back to AC for consumption or export. This means the price you export the power back to the grid must be at least 10% higher to start earning money on any given day.


How do we export?


I wrote a simple program in Python that I run off a RasberryPi (a simple and cheap computer) that interfaces to our solar system, which is a Cerbo GX,



Victron Cerbo GX
Victron Cerbo GX

My code simply queries the Electricity Authority every minute for the spot price from Cromwell in Central Otago, and then decides how much to export by considering how high the spot price is. If the sport price is only just above my exporting threshold of 0.25c, I export 1kWh and if the spot price goes up over 100%, I am exporting the maximum my connection will allow of 30 kWh.



The log file of my custom exporting program. Price was above 25c at time this photo was taken and program was exporting 2.5 kWh back to the grid.
The log file of my custom exporting program. Price was above 25c at time this photo was taken and program was exporting 2.5 kWh back to the grid.

I am lucky to come from a computer programming background, which meant that writing this custom program was a fairly straight forward exercise. I am glad to share this code and help with setup for any farms, orchards or vineyards that want to do the same thing and potentially create a new revenue stream (or at least some substantial cost savings long term).


I do plan to open source this program at some point, but right now the code isn't ready and will take some work before it can be let into the wild unshepherded. It's a unique use case, requiring you to be in New Zealand and using the Victron Power system.


What about battery wear and tear?


Our four BYD 15.4kwh batteries have a warranty of 6 years or 6000 cycles to a depth of discharge of 80%. Essentially, this means BYD will replace them if they fall below 80% capacity before they have reached 6000 charges and discharges or 6 years.


The curious thing we have learnt about operating LiFePO4 chemistry batteries is they degrade over time regardless of how little you use them, to the point that regularly charging and discharging them can actually prolong their life. The Victron Community forums made us keenly aware of this, that even if I was only profiting 1 cent on our export, it's still probably worth it, as my batteries will degrade regardless of whether I discharged them to the grid or not.


So in other words, if you got it, flaunt it. Make sure you keep the battery temperatures within the desirable manufacturers range, and charge and discharge without hesitation.


Because these batteries are stationary, we don't have to worry about their weight. This means they could degrade below 80% and still be profitable to our operation. They could degrade to 5% and still be contributing something. Lithium chemistry is still a new technology, so we don't know how long they will last to complete depletion, but 10-15 years does not seem unreasonable.


Capital Cost vs Operational Profit


The goal at Forest Lodge is to grow high quantities of cherries without burning any fossil fuels, but a secondary goal I have is to do it with zero energy costs. That being said, we have 60 kWh of batteries, because that's what we need to irrigate during a power cut.


We have more batteries on the way, and will hopefully use additional battery storage when our EVs and electric tractor are sitting idle.


My point is, we have these batteries for a business reason and then are using them to save money / create a small revenue stream. I guess it's hard to justify buying batteries merely to arbitrage the electricity market, but if you have them already, then it's very worth while.


Alturism of Export - Hybrid Grid Connectivity


No one can deny the environmental toll the production of Lithium batteries. The embodied carbon in them is considerable. To offset this carbon, you need to be using these batteries to replace a fossil fuel process, such as we have done with our under floor heating, our frost fighting fans and our irrigation pumps.


In New Zealand our grid is largely renewable, which means we can pay off this embodied carbon faster by electrifying as much as we can.




On top of this, exporting extra power back to the grid also lowers New Zealand's carbon foot print. Us offering power back to the grid when the price is high helps keep the hydro lake levels just that little bit higher. The higher the lake levels, the less Huntly; NZ's major fossil fuel power plant needs to back up our grid in times of shortages, thus lowering NZ's carbon foot print over all.


In isolation, we will do little to help lower the New Zealand grid's carbon footprint, but if there are enough farms and businesses exporting during times of high demand and congestion, this can have a massive impact.


A Special Thank you


We want to thank Katie from Simply Energy (previously Contact) and Glenn from Aurora for being two fantastic intraprenuers in the power industry that are helping us carve a new energy future for New Zealand and a great way for farmers to contribute to our nations climate change goals.

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