Switching on a 20 kWh home battery has been one of the most gratifying upgrades we have made to our home energy set up. It has changed the way we use power, has cut our reliance on the grid, and has let us store our excess solar power. The environmental upsides are obvious, and the financial benefits are already showing through in the daily numbers. For one of our weeks, we only purchased 2.6kWh but fed 35kWh into the grid.
Our electricity set up
- Single phase power
- 13kW solar panel
- East, north and west facing panels (ideal is North)
- 20kWh battery
- Hybrid 10kW inverter
In Perth, Western Australia on a sunny spring day we are producing around 40 kWh of electricity. In comparison, our north facing 10kW solar array, 15 minutes away, is producing 55kWh, and it’s 13 years old.
Our household set up
- Freshwater pool that runs from 9am to 3pm
- Rooftop heating for the pool
- The car slow charges from 9am till 4pm, when home
- Bore runs from 8:15am until 9am two days a week
- LED lighting throughout the house
- Appliances – oven, dishwasher, dryer and washing machine – running “whenever the hell I want them too” (says the boss).
Charging the battery
On a sunny day, the 40kW system, produces enough energy to run the house as described above, charge the battery, while feeding some energy back into the grid.
The battery still charges on an overcast day if the pool run time and car charging time is limited.
On a typical night, if we avoid big draw appliances like the oven and air con, we comfortably make it through to morning with about 30 percent battery power still in reserve. If we do cook with the oven or run other heavier loads in the evening, we tend to arrive at 8 am with the battery close to empty. This time typically aligns with the solar production ramping up.
Even with all the above we are charging the 20KWH battery on a sunny day.
A clear day in practice
On a cloud free day, without charging the car, the battery is usually full by 1 pm. If the MG4 slow charging at 1.7kWh, it adds an hour or two to the battery fully charging. We prioritise running appliances in the daytime, when there is additional energy above the 5kW that the battery is using to charge. Remember, 5kW can charge the battery, leaving up to 5kW to use elsewhere or put back into the grid.
In the image below, the system is producing 8kW. 4.2kW is going to charge the battery; 3.8kW is being consumed by the household. Once the battery is fully charged, that 4.2kW will be fed back into the grid.
Air conditioning is the one caveat. If we run the AC into the evening, the battery will last 2 to 3 hours at best. Luckly, we live near the ocean and don’t use it that often.
Real numbers from the last week
- Solar produced: 285 kWh
- Exported to the grid: 36 kWh
- Imported from the grid: 2.6 kWh
That is a very small amount of grid import for a full week. The battery is doing exactly what we wanted it to do. It is catching daytime excess and covering the evenings. The result is lower bills and far less carbon in our energy mix, especially when the car’s afternoon trickle is largely backed by the sun.
What I have learnt so far
1) Know your battery’s charge and discharge rates
My unit charges at 5 kW. That ceiling matters for planning. If you have lots of mid‑day surplus, a higher charge rate will fill the battery faster. Also check the maximum discharge rate so you understand how many big appliances you can run at once without dipping into the grid.
2) Schedule heavy loads for sunlight hours
Pool pump, dishwasher, washing machine and oven are best when the array is humming, ideally after the battery has crossed a healthy state of charge. Using the oven at night or any other high drawing appliance will likely lead to using all battery power before sunrise.
3) Be smart about EV charging
Do not try to charge the car from the home battery. Even a trickle charge will flatten it quickly. We let the MG4 trickle charge whenever we are home, between 8am.
4) Use grid charging as a tool on grey days
Make sure your system allows grid charging and that you can toggle it from the phone app. On overcast days, if it is clear the solar will not fill the battery, we top up during super off peak 8cents a kWh which is 9am till 3pm. Then we use that power instead of the peak rate of 50 cents per kWh.
5) Pick a plan that matches your solar profile
If you are in WA, consider the Synergy Midday Saver plan or the EV plan. The cheaper daytime pricing encourages you to run loads when the sun is up and gives you the option of grid charging when the weather is ordinary.
Power your house during power outages
Usually, you can pay a bit extra to have the ability to run the house on the battery, during a power outage. With our system, we chose one lighting circuit, and the kitchen power circuit to be fed from the battery during a power outage. I was surprised to find that this is a manual switch over. So worth checking if there are automated systems available.
Financial and environmental wins
Even without putting dollar figures on it, the week’s data tell a clear story. Exporting 36 kWh and importing only 2.6 kWh means most of our consumption is now covered by our own generation. That slashes the power we buy at peak rates to basically zero, which is where the battery really pays its way. On the environmental side, shifting evening and early morning usage to the battery, cuts our reliance on fossil fuelled grid generation during peak periods. Charging the EV during sunny hours reduces the impact again.
Final thoughts
After the first month, I am convinced the battery was a great investment. It fits our lifestyle, it lifts the value of our solar investment and it keeps the lights on through the night with minimal grid help. The numbers are trending the right way as the days get longer, the house feels more self sufficient, and our environmental footprint is smaller.
From what I have been told you only get to claim the government rebate on the battery once. With what I know now, I should have gone for a 30kWh. If you have a better roof for solar then you should think about a 40kWh battery, especially if you run an air conditioner regularly.
