Wednesday, October 12, 2016

End of winter recap

Now that winter is over, how have we done?


My last post about my solar systems' performance has made me think a bit more about their performance, so I've done some analysis.

I have taken some meter readings over winter, which gives me
  • solar PV production over winter
  • grid draw over the same period
  • grid export over the same period
This gives these data (for the period June 1st to October 6th):
average total daily consumption (PV self consumption and grid-draw): 6.26 kWh
average Daily PV production: 6.13 kWh
average daily grid draw over winter: 3.9 kWh
average daily grid export over winter:  3.7 kWh

The same calculations for the period June 1st to August 2nd look like this:
average total daily consumption (PV self consumption and grid-draw): 6.29 kWh
average Daily PV production: 4.67 kWh
average daily grid draw over winter: 3.91 kWh
average daily grid export over winter:  2.66 kWh
 We tend to cook more with the electric oven in winter, which I think largely explains the higher daily consumption. Also, we use lights more, etc. We needed to boost the solar hot water system once, for about an hour, which used about 2.4 kWh.
Overall, I am fairly happy with these numbers. I think we're losing at least 0.5 kWh/day to phantom loads, but there's only so much energy I have for turning things off at the power point. Clearly, for our PV to cover our consumption in winter, we need ~50% more panels.

Propagation area

I've been busy building a spot for seed propagation. Seeds need constant moisture, and the requirement that we keep them moist for several weeks was just too much for us (forget to water them for a day and they're dead). I built this bench using almost 100% found or scrounged materials. Even the nails and bolts were mostly reused. It's also amazing what you can build with poly pipe and clothesline! It has a watering system that waters it for 1 minute every 6 hours. Bonza!

tomatoes and beetroot and basil, oh my!

does your propagation table have turned legs? ;-)

Composting loo

Now that we've had the last rain until about March (well, we can't expect much!) I'm again feeling unhappy about putting potable water down the toilet. I've started building a box composting toilet. It's designed to hold a 20 L plastic bucket and a urine diverter. It's made from pallet wood (and pallet nails), with a few bits of nice timber I found on an old air-conditioner facade that I found on the side of the road (they made them attractive in the old days!). Not quite finished, but getting there. As I said in a previous post, I'm using this device as a urine diverter, and they have some plans online for how to make the toilet. I am vaguely following those.

It is spring, and the garden is blooming. Here are some photos from the garden:
New citrus grove (sorry for the dark photo)

Broad beans

Monday, October 10, 2016

2 years on: System performance

We've now had our solar hot water and solar PV system for 2 years. I thought it was worth commenting on them.

Solar Hot water (main)

This system is detailed here and here. Based on my meter readings, I estimate it has saved us 4500 kWh (just over 6 kWh/day), which is worth $1350 on the standard tariff (we avoid the off-peak tariff because it supports fossil fuel power, even when buying Greenpower) At this rate, the system will pay for itself in about another 18-24 months (4 years total from new).

The tank looks a little weathered, but still in very good shape. Beacuse  it is low pressured, I am confident it will last many years. I partially shade the panels (with an old cotton blind) in summer, to stop it boiling, and we lose very little water from it.

Kitchen Hot water

This system performs poorly in winter. It's nice to have instant hot water, and it was a good learning project, but I wouldn't recommend people do this if they have other solar hot water. At some point I might re-purpose this system into a backyard shower.

Solar PV

Our system has produced almost 10 MWh. Since we got the new input/output meter we've exported about 7 MWh to the grid (which means we've self-consumed 3 MWh of our solar PV). We get paid $0.24 / kWh for exported power and pay $0.32 / kWh (including the GreenPower premium) for imported power. So, the return on our solar PV has been $2600, or about 1/2 the cost of the system.
As of the end of September, we lose the $0.24 feed-in-tariff, and will be paid about $0.08 / kWh. That will reduce the return on the system cost. In another 2 years, we'll have got back an additional $1500 at these tariffs. I think we'll pay back the cost of the system in about 3 years (5 years total from new).


Our 2 kW solar PV system has produced more electricity than we use almost every day (about 50 exceptions in two years) since it was installed. I've shown the economics here, because that's of interest to some people, but it's a great feeling to look at the roof and see the power we harvest. It's strange how people carefully consider at the economics of solar PV, but not of cars.
At some point, I think we'll install a hybrid battery system. I like the idea of self-consuming more of our power. But I don't think we'll try to go fully off-grid. I think that doesn't make sense in the city (firstly, we'd need a much larger battery to provide for the small number of days when we lack solar PV and secondly it has a bit of a gated-community feel to it). I might also install a second solar PV system. That will have much worse payback than the first, but that's ok.

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