How solar and a home battery turned my $200 power bill into a $190 credit

Before installing solar and a home battery, our household electricity bills were typically around $200 a month. Our first full bill after the installation was very different. Instead of owing the electricity retailer money, the account finished $189.85 in credit.

That represents a swing of around $390 in a single month: avoiding the usual $200 bill while receiving almost $190 back.

It is an eye-catching result, but the headline figure does not tell the entire story. The bill also demonstrates how much the economics of a home battery depend on when electricity is imported and exported, rather than simply how many kilowatt-hours a household produces.

During the 31-day billing period, the house imported just 11.49 kWh from the grid. The charge for that electricity was $3.91. After a small price adjustment and the daily supply charges were included, the total cost of remaining connected to the grid came to a little over $45.

On the other side of the ledger, the system exported 1,423.77 kWh. Most of that electricity – around 902 kWh – was exported outside the retailer’s premium period and earned nothing. But 522 kWh was exported during a high-value evening window at 45 cents per kilowatt-hour.

Those evening exports produced a credit of approximately $235. Once the grid imports and supply charges were deducted, the final result was a $189.85 credit.

The bill can be summarised as follows:

– Grid electricity imported: 11.49 kWh
– Cost of imported electricity: $3.91
– Daily supply charge: $41.60
– Premium exports: approximately 522 kWh at 45 cents/kWh
– Premium export credit: approximately $235
– Other exports: approximately 902 kWh at no payment
– Final account balance: $189.85 credit

The most striking figure is not necessarily the $190 credit. It is the 11.49 kWh imported from the grid across an entire month.

Despite running an ordinary family home and charging an electric vehicle, almost all of our electricity was supplied by the rooftop solar system or energy previously stored in the battery.

The system consists of approximately 13.3 kW of rooftop solar, made up of 28 AIKO 475-watt panels. It is paired with a Sungrow SH10RS 10 kW hybrid inverter and a Sungrow SBH battery with approximately 25 kWh of storage. A Sungrow AC22E charger is also used for home EV charging.

The solar array is deliberately larger than the inverter’s maximum output. There will be times in the middle of a very sunny day when generation is clipped by the 10 kW inverter limit, but the oversized array produces more electricity during the morning, afternoon and less-than-perfect weather. This is important because the objective is not simply to reach the highest possible instantaneous solar output. It is to produce useful electricity over a greater portion of the day, cover household consumption, charge the battery and provide energy for the EV.

A conventional solar system can substantially reduce electricity bills by supplying the home during daylight hours. But without a battery, much of the excess generation is exported around the middle of the day, when wholesale electricity prices are often low or even negative. Our retail plan with Flow Power reflects that changing value. It pays a high rate for exports during a defined evening window but nothing for exports during the remainder of the day.

That explains the apparently strange result on the bill: more than 900 kWh was exported without earning a cent, while the smaller volume exported in the evening generated $235. The battery allows surplus daytime solar to be stored and then discharged when the electricity is more valuable.

Rather than sending every spare kilowatt-hour into the grid at lunchtime, the system can hold a significant amount of energy until the evening peak. That is good for the household economics and, more broadly, means solar energy is being supplied to the grid closer to the time when demand is higher.

It is a small example of how distributed batteries can operate as part of the wider electricity system, rather than simply serving as backup power for individual homes.

It’s worth noting this will not be every month. It would be tempting to multiply the $390 monthly improvement by 12 and declare annual savings of almost $4,700, but that would risk being misleading.

The first billing period captured strong solar production, and the result will change with the seasons. Winter generation will be lower, household consumption will vary and the available export rates may also change.

The 45 cent export tariff is also central to the result. A household receiving a conventional feed-in tariff of five or six cents per kilowatt-hour would not earn anything close to the same credit from an identical amount of exported electricity.

This is therefore not a claim that every household installing a battery will immediately receive a $190 monthly credit. It does, however, show what can be achieved when a large solar array, substantial battery and time-sensitive electricity tariff are designed to work together.

The electricity bill also captures only part of the financial benefit. Our electric vehicle is now predominantly charged at home using solar electricity. Had the same energy been purchased from public fast chargers, it could have cost around 50 cents per kilowatt-hour or more.

Depending on how far the vehicle is driven, avoiding paid public charging could be worth another $150 to $200 a month. That saving will not appear as a credit on the household electricity bill, but it is still part of the return generated by the solar and battery system.

For a household electrifying transport, hot water and other appliances, the economic value of rooftop solar can become considerably larger than the electricity bill alone suggests.

So what does this all mean for payback? It remains too early to draw a firm conclusion from one month of data.

Based on the initial performance, however, the combination of avoided electricity costs, export revenue and reduced EV charging expenses suggests that a comparable system could potentially achieve a payback somewhere in the range of five to eight years.

That estimate will depend heavily on the installed price, electricity plan, household demand, EV use and how well the battery is managed.

Battery economics are no longer simply a calculation of buying electricity for one price and avoiding another. Increasingly, they involve controlling when a household imports, stores, consumes and exports electricity.

The first bill shows both the opportunity and the complexity. We exported almost three times as much electricity for no payment as we exported at the premium rate. Yet the well-timed portion was enough to cover every grid charge and leave the account almost $190 ahead.

After years of receiving electricity bills, the house has effectively started sending one in the opposite direction.

This article is part two of a three-part series supported by Sungrow and AIKO examining how distributed energy technology is being deployed in Australian homes. The equipment featured in the series was supplied by Sungrow and AIKO.

Read part one here: https://reneweconomy.com.au/how-my-perfectly-ordinary-house-became-a-small-power-station/