Calculation and thought must go into installing a solar panel system to charge a Tesla Powerwall home battery.
To fully charge a Tesla battery, which has a capacity of 13.5 kWh, solar panels must generate at least 15 kWh, accounting for energy lost throughout the charging process. The number of solar panels needed will vary depending on how much electricity a home uses, with more panels required to handle both night-time usage and Powerwall charging.
To precisely calculate electricity usage and establish the required solar panel system size, obtaining professional guidance and utilising a digital electricity meter is advised. A system of at least 10 kW is typically recommended for charging a Tesla battery and meeting daytime electrical needs.
How big a solar system do I need to charge a Tesla battery?
The straightforward answer is that the more solar panels you install on your roof, the better. It’s much more beneficial to generate excess solar power on your roof and feed it back to the grid at a low Feed-in Tariff (FIT) than to lack sufficient solar power to effectively charge your Tesla battery or any other battery.
In many cases, as well as our example, the Tesla battery will be used close to its full capacity of 13.5kWh daily. So if you do not need that much battery capacity, the Tesla battery may not be the best option for your home.
A Powerwall has a usable capacity of 13.5kWh of electricity with a round trip efficiency of 90%. Yet, this implies a 10% loss of electricity during its transfer in and out of the battery. Therefore, to attain 13.5 kWh of usable electricity in the battery, the solar panels must produce 15 kWh of electricity available for transmission to the battery.
So one should allow at least 15kwh of additional generation from your solar panels to charge the Tesla battery.
Professional advice will help you work out your needs
Many solar companies will calculate the required sizing based on average annual PV generation. Using this method to generate an additional 15kWh of electricity per day would equate to approximately 3.5kw of additional panels, to the electricity requirement your home uses in electricity during daylight hours.
In a simple sample to make this more clear, let’s say a family uses 28 kWh of electricity per full day. During daylight hours, you use 15 kWh, while you use the rest in the evening and at night.
In this case, they would need 3.5kW of solar to generate the daytime consumption and another 3.5kW system for the battery to handle the evening and night periods.
This means this sample’s recommended total solar system size should be 7kW.
Does this calculation cover the whole year?
We have a detailed output calculator on this website to see how many kW of solar system capacity one would need in which Australian postcode, as sun irradiance (intensity) and system output vary across Australia. This will allow you to determine your specific requirements at your location.
The above calculation is based on the average generation over a year. The solar panels will produce less than half the average on a mid-winter day than in mid-summer.
The calculation above shows that the panels do not produce enough electricity to fill the battery for 3 to 5 months of the year on your average day. So in this instance, grid power must be used to fill the battery for the missing kW.
To get year-round use of the tesla battery at the lowest charge cost, you need to ensure that it will also have the capacity to charge fully via solar in the winter period. You will, therefore, have to double the panel capacity. In this case, you would want to install at least 7kW of panels, specifically to charge the Tesla Powerwall.
The above calculation is based purely on the ability to charge the Tesla Powerwall all year round and does not include panels to power the home during the day.
Adding battery requirement and daytime consumption
Therefore, the panel capacity required to power the home’s electricity use needs to be added to this.
If you have a digital electricity meter, the best way to do this is to ask your solar company to organise a report showing your electricity usage for the last 6 or 12 months, which will also allow you to work out your day and night electricity consumption.
From this, you can accurately calculate how much electricity you use daily and design your solar system accordingly. Our sample assumes the daily consumption requires 3- 3.5kW of panels.
When we add the sizing for self-consumption (3-3.5kW) with sizing to charge a Tesla battery(7kW), we will get an ideal solar system size of 10kW or even a bit more significant in inverter solution and solar panels.
More panels mean more peace of mind for your Tesla battery
As indicated at the start of this FAQ, it is better to go for more as far as solar panels are concerned than less.
Absolutely, you can adapt this ideal scenario, and a smaller system will still offer an acceptable solution. Charging the Tesla battery via the grid becomes effortless, possibly during periods when tariffs are cheaper.
We recommend having a quality-focused solar company give you a detailed design based on your circumstances and ensure that they consider seasonal variations in solar production in their design calculations to provide the best outcome for your home year-round.