My home battery and rooftop solar system: How it performed in its first year

One Step Off The Grid

In April last year my partner Hannah and I moved into our new home – a beautifully renovated Queenslander in inner-Brisbane.

Having lived in apartments before this we were excited by the new opportunities for sustainability improvements – particularly a home battery + solar system. The timing worked out perfectly and we were lucky enough to be eligible for the Queensland Government’s Interest Free Loans and Grants Program for Solar & Storage. 

This meant a $3,000 grant towards the upfront cost, as well as a $10,000 loan to be paid back interest-free over ten years ($83.33 per month, for those wondering).

The system was installed over a couple of consecutive days and commissioned on 8 August 2019 – meaning it just recently celebrated its first birthday.

The goal was to be as self-reliant with our own renewable energy as possible – directly consuming solar generation during the day and charging the battery for use overnight. On this metric we’re pretty happy with the result – achieving 92.2% self-sufficiency over the year.

The Tesla monitoring platform provides so much more data than just the headline figures though, so as a self-confessed energy and data geek I decided to delve in and do some analysis on how the system performed technically and financially over the year.

The results will hopefully be of interest to fellow energy obsessives, as well as those who may be considering the merits of home solar + storage.

Technical Specifications

Before diving into the data, a snapshot of the system specifications:

• Tesla Powerwall 2 battery – 5 kW / 13.5 kWh
• 20x SunPower P19 315 W modules – 6.3 kWp
• 20x SolarEdge P320 power optimisers
• SolarEdge HD-Wave single-phase inverter – 5 kWac
• SolarEdge Immersion Hot Water Heater Controller – 3.6 kWac

The addition of the SolarEdge Immersion Hot Water Heater Controller is of note – I’ve long been an advocate that most solar households are sitting on top of the best battery there is and just don’t know it – their hot water system.

The SolarEdge Immersion Controller is a smart device designed to dynamically divert excess PV into water heating instead of grid export.

I experienced issues with it in our particular application, however, as the Powerwall and the Immersion Controller can ‘fight’ each other to decide where excess PV is first diverted to.

I subsequently disabled this function, and now use it as a simple wifi-controlled timer instead. This is one area cost saving could be achieved if doing it again.

Power flows and energy balance

So what does all this look across on a typical day?

Figure 1 provides an annotated example from 15 July this year, demonstrating how the solar + battery work together to enable complete self-sufficiency on this particular day. Grid imports are only required in scenarios where the battery runs empty.

Figure 1: Site power flows on 15 July 2020

Looking across the year, power flows to and from the various aspects of the system are illustrated in Figure 2, with the relevant kWh totals making up the overall energy balance noted.

Figure 2: Energy balance across the year

At the highest level, the solar system generated 7,834 kWh (4.29 kWh per kWac per day) compared to household consumption that totalled 5,092 kWh. This works out at 13.92 kWh per day – slightly higher than the local average of 13.4 kWh per day for a two person household without a pool (according to Energy Made Easy).

Diving deeper, it’s interesting to look at the proportionate splits between energy sources and destinations.

The left-hand side of Figure 3 shows home energy consumption by source, with solar-self consumption and Powerwall discharge being pretty well equal, and the balance supplied by grid imports.

The right-hand side of Figure 3 shows the destination of solar generation, with a pretty much equal three way split between direct home consumption, battery charging, and grid export.

To read the full story on RenewEconomy sister site, One Step Off The Grid, click here…