How much money we saved with a week of solar power

We saved over R150 in electricity expenditure within one week of using our entry-level solar system in Pretoria.

Our 3.3kWp JA solar panel array, 5kW Deye hybrid inverter, and 5.12kWh Dyness battery were installed at our home on Monday, 25 March 2024.

The system cost R71,200 with full installation and a certificate of compliance.

While its primary purpose is to provide backup power during load-shedding, we thought there could be no harm in trying to offset some of the system’s cost through savings on electricity.

The battery we installed comes with a 5-year warranty and is rated for 6,000 cycles before dropping to 80% of its original capacity.

Even when consuming two full cycles per day, which would be extraordinary for our consumption habits, it should still have 80% of its original capacity after eight years of use.

The battery was also recently upgraded to a 1C rating for charging and discharging, so it is safe to discharge at nearly its full capacity without damage to its overall lifetime.

Nevertheless, we opted to keep it at 0.5C just to be on the safe side.

We set up the inverter to use solar and batteries when available and only take power from the grid when these have insufficient capacity available.

To ensure we always have enough battery power for at least two hours of load-shedding, we configured the inverter to stop taking power from the battery when it reached a 40% state-of-charge.

When the grid is unavailable, the battery can discharge as low as 15%, the recommended absolute minimum for our particular battery.

Given our typical load of around 300 to 400 watts when at home, a 25% charge provides more than enough breathing room during load-shedding hours.

Should Eskom announce the longer 4-hour load-shedding slots that come with stage 5 and above, the minimum state of charge will be adjusted accordingly.

Solar and grid usage breakdown

Between 26 March 2024 and 1 April 2024, we consumed about 80.3kWh of electricity from all sources — solar, batteries, and the grid.

The solar system produced roughly 66.3kWh of energy during the week, including that used for charging the batteries.

In total, we pulled 23.4kWh of energy from the grid, at an average of about 3.3kWh per day.

Most of this happened in the early morning hours between midnight and 05:00, when the battery was set to top up from the grid.

However, to make as much use of the solar as possible, the battery is not set to charge to full using the grid, as our typical morning usage still left plenty of spare capacity by the time the solar array could start refilling the battery,

The graph below compares our total electricity consumption, solar production, grid energy usage, and battery usage in the first week of the solar system’s operation.

To calculate how much we saved in electricity costs, we deducted our grid energy consumption from total consumption.

Solar production cannot be used directly because it does not account for charging and discharging inefficiencies.

Subtracting our 23.4kWh of grid consumption from the 80.3kWh total leaves 56.9kWh of solar-powered electricity.

Had we bought this energy from the City of Tshwane at the Block 1 Tariff of R2.78 per kWh, we would have paid R156.94 for it.

However, given that we typically consume about 268kWh per month, the average cost per kWh would have been higher over an entire month, as the Block 2 Tariff kicks in beyond the first 100kWh of consumption.

With our typical monthly consumption, our average tariff cost was closer to R3.07.

The table below shows how much we saved in electricity costs based on the City of Tshwane’s Block 1, Block 2, and our average tariff based on typical monthly consumption.

We have been very impressed with the system’s performance, particularly considering at least three of the seven days in full operation had overcast and rainy weather.

The solar panels have been more than capable of supporting the house’s full load throughout most of the day from around 8:00 to 17:00 while also charging the batteries up to 100% more than once.

During the day, the battery only steps in when additional load is added — like when the geyser, kettle, or other power-demanding appliances are switched on.

It has only happened rarely that power from the grid was also required during the day or even during peak evening hours.

The installation of a smart switch for timing when the geyser will switch on and off has also helped reduce the grid consumption significantly.

By setting the time for the geyser to go on when solar output and battery levels are high, we have been able to get by with using well below 1kWh per day for heating the geyser and have always had hot water when needed — for evening and morning showers.

This might change during the winter, when a short grid-supported top-up for the morning might become necessary.

It is also important to note that our overall consumption is relatively low compared to a typical household.

However, homes with more consumption could actually benefit more as we had excess solar power going to waste.

We will definitely consider installing a bidirectional meter to earn electricity credits but will only do so once the City of Tshwane’s feed-in tariffs become more attractive and the prices of these meters are reduced.