AC battery storage with dynamic electricity tariff: utilize charging windows

An AC battery storage system can combine two functions: storing your own solar power for later use and shifting electricity from low-tariff hours to more expensive consumption periods. This is an attractive option for an existing PV system because the storage is added on the AC side. Your existing panels and inverter remain in place.

Whether charging from the grid pays off is not determined by a low market price alone. Your actual retail price, the future electricity costs avoided, storage losses, and the additional utilization of the battery are all important factors.

Time slots or automatic tariff control?

With the SunEnergyXT 500 PRO AC Core, you can tailor charging times and power output to your daily routine. A time slot is suitable if you prefer to manage the schedule yourself. For example, you can have the storage system charge from the grid only during selected hours while keeping capacity available for afternoon PV surpluses.

With automatic tariff control, fluctuating electricity prices are integrated into the scheduling. SunEnergy offers a dynamic tariff in conjunction with its app called SunEnergy Dynamic. To use this, you need the appropriate tariff contract, the required metering equipment, and a supported device configuration. A dynamic tariff alone does not automatically control every storage system.

A price comparison including losses

A simplified calculation example: electricity costs a total of 20 cents/kWh in a cheap hour and 35 cents/kWh later on. Assuming a total efficiency of 85%, you have to purchase about 1.18 kWh to gain one usable kilowatt-hour later. This costs approximately 23.5 cents. The calculated difference is therefore around 11.5 cents per kWh used later, before even accounting for battery wear and other costs.

The 85% figure is a calculation assumption, not a guaranteed efficiency rating for the AC Core. The prices are also arbitrary examples. Minor price differences can be completely offset by losses and costs. Furthermore, a negative market price does not automatically mean that all of your electricity consumption is free.

How to plan your charging windows

1. Consider consumption: When does your heat pump run, when does the electric car come home, and how much electricity does the household need in the evening?
2. Factor in PV yield: On a sunny day, it may make more sense to keep storage capacity free for your own solar power.
3. Limit charging power: A lower power target may be better suited to your home connection and simultaneous household load. Connection limits remain the deciding factor.
4. Set a reserve: A desired minimum charge reduces the capacity freely available for price optimization.
5. Check the results: Compare actual grid consumption, discharge, losses, and costs over several weeks, not just on one particularly favorable day.

What metering is required?

For consumption-based control, the storage system requires compatible measured values at the home connection. Depending on the version, devices like Shelly, EcoTracker, or bitShake may be suitable. Check the specific model and the supported interface. This energy measurement is not automatically the same as the "intelligent" metering system that your electricity provider requires for tariff billing.

Is the AC Core a good fit for your tariff?

The 5 kWh AC Core is an entry-level option for AC retrofitting; the 10 kWh set offers more capacity. More battery does not automatically mean more savings: the decisive factor is how often you can effectively charge and utilize that additional capacity.

Would you like to combine your own rules with your smart home? Read our guide to Home Assistant and the local API. To plan your system, see our overview of PV storage retrofitting.

Questions about the right combination of storage, meter, and tariff? Email us at support@wondervolt.de.

Further reading: SunEnergy Dynamic from the manufacturer and Consumer Advice Center: Home Energy Management.