Home / Solar PV and battery storage
Battery storage · Prepare your decision
Use solar electricity later. Would a battery suit you?
How much extra value could battery storage offer you? Compare your daily routine, a suitable size and a quote with and without a battery – step by step.
The essentials first
- From daytime to evening: A battery shifts electricity use in time; it does not generate additional solar electricity.
- Match your daily routine: Available solar surplus and later demand determine the potential benefit.
- Compare the additional benefit: Assess the same PV system with and without a battery, including losses and additional costs.
On this page
01 · Your daily routine
Surplus by day. Demand in the evening.
A home battery can shift daytime solar electricity into later hours. It does not generate electricity itself or store summer energy for the winter. Whenever PV and the battery cannot meet demand, the grid supplies the rest.
- What is left over? Look at the solar surplus after consumption at the same time. Annual generation alone does not show the available charging windows.
- What will you need later? Gather evening and overnight consumption data where available. Distinguish typical weekdays, weekends and seasons.
- What is changing? Note any plans for an electric car or heat pump. Their operating times belong in the design too.
First check which necessary electricity uses can be moved into sunny hours. Then assess the remaining storage need. More capacity offers little benefit if charging energy or later demand is regularly missing.
Explore further: HTW Berlin: battery sizing and limitations
Still planning your rooftop PV system? Explore the guide to solar PV for your home.
02 · An example
The same household. Two options.
An illustrative example day: The PV system supplies 12 kWh to the home’s electrical system. The household needs 10 kWh in total: 4 kWh during solar generation and 6 kWh afterwards. The first 4 kWh is met entirely by solar electricity at the same time.
In the battery option, 6 kWh of surplus electricity is charged into an AC-coupled system. We assume an overall efficiency of 90% for this process: 5.4 kWh is later supplied to the home, while 0.6 kWh is lost.
Illustrative example day · Both bars represent the same amount of energy.
- Direct use: 4 kWh in both cases
- From battery: 0 / 5.4 kWh
- From grid: 6 / 0.6 kWh
The 0.6 kWh of battery losses does not meet household demand, so it is excluded from the bars. Grid export and the full energy balance follow in the table.
| On the example day | PV without a battery | PV with a battery |
|---|---|---|
| Solar electricity used directly | 4 kWh | 4 kWh |
| Used later from the battery | 0 kWh | 5.4 kWh |
| Imported from the grid | 6 kWh | 0.6 kWh |
| Exported to the grid | 8 kWh | 2 kWh |
| Losses in the storage process | 0 kWh | 0.6 kWh |
Additional contribution from the battery: 5.4 kWh less grid electricity. In return, 6 kWh less is exported. Energy used to charge the battery and energy available later are therefore different quantities.
What assumptions are used in this comparison?
The battery starts and ends at the lower limit of its usable charge range. Its capacity, charging power and discharging power are sufficient for the assumed processes. The 6 kWh is the energy drawn at the AC input, not the battery’s storage capacity. Grid charging, curtailment and a backup reserve are excluded. The assumed 10% loss combines the losses of the storage process considered here; additional standby consumption outside that process is not included.
Household demand is balanced in both cases: without a battery, 4 + 6 = 10 kWh; with a battery, 4 + 5.4 + 0.6 = 10 kWh. All solar electricity is also accounted for: 4 + 5.4 + 0.6 lost + 2 exported = 12 kWh.
This is a simplified daily energy balance, not a sizing recommendation or annual forecast for Ulm. The 90% efficiency is an illustrative assumption. Actual profiles and system losses need to be considered separately.
03 · A suitable size
kWh and kW answer two different questions.
Capacity: how much energy?
Usable capacity in kWh describes the accessible energy content. Check whether the data sheet gives values at the battery or on the AC side. Reserves and further conversion losses can reduce the amount available to the household.
Power: how much at once?
Charging and discharging power in kW limit the instantaneous flow of energy. Even a well-charged battery may therefore need support from the grid when several appliances demand high power at the same time.
A simple power example: With no current PV generation, the house needs 5 kW. If the sufficiently charged battery can supply at most 3 kW to the home, the missing 2 kW still comes from the grid.
Explore further: HTW Berlin: capacity, discharge power and measurement terms
What does the next additional kilowatt-hour of storage offer?
More capacity allows more self-sufficiency. This fixed model example shows how the benefit grows: 10 kWp of PV and 5,000 kWh of annual household electricity use, without a heat pump or electric car.
How to read the chart: Here, 5 kWh of storage avoids around 1,370 kWh of grid imports per year, while 10 kWh avoids around 2,020 kWh. Doubling the capacity therefore adds about 650 kWh. The first kilowatt-hour of capacity contributes around 367 kWh; the tenth adds around 71 kWh.
For your quote: Ask for smaller and larger batteries to be compared using the same consumption profile. Additional self-sufficiency alone does not establish financial value: the extra price, losses and forgone export payments also belong in the calculation.
Battery storage: data and model assumptions
HTW reference model with a VDI 4655 household profile and DWD weather data from Brandenburg; south-facing, 35° tilt and no shading. PV yield: 1,024 kWh per kWp in the model year. On average, 90% of the stated usable battery capacity is available; overall AC efficiency is around 83.9%. Inverter power increases with capacity: 1 kW per kWh. This is therefore not a comparison at constant battery power.
The values come from HTW’s published results table and its interpolation. For each size, the change in self-sufficiency is multiplied by 5,000 kWh. Differences are calculated before rounding. Different time profiles, reserve settings and systems change the result; the model does not replace an individual design.
| Storage (kWh) | Reduction in grid imports | Increase over previous size |
|---|---|---|
| 0 | 0 | – |
| 1 | 367 | 367 |
| 2 | 639 | 272 |
| 3 | 893 | 254 |
| 4 | 1,139 | 246 |
| 5 | 1,370 | 232 |
| 6 | 1,561 | 191 |
| 7 | 1,718 | 156 |
| 8 | 1,848 | 131 |
| 9 | 1,949 | 100 |
| 10 | 2,020 | 71 |
Source and our own analysis, checked on 30 September 2026: HTW self-sufficiency calculator · Model assumptions and limitations.
Compare the whole system.
The system includes the battery, inverter, controls and standby consumption. High battery-cell efficiency alone does not describe the whole chain. Also consider performance under low overnight loads and independently measured system characteristics.
Ask how the controller coordinates PV generation, household demand and other appliances. A larger battery does not automatically compensate for unsuitable controls.
The inspector compares tested systems and does not cover the entire market. An efficiency comparison alone does not determine which quote suits you.
Explore further: HTW Berlin: Energy Storage Inspection 2026
Include grid charging and Section 14a in the plan
Batteries commissioned since 1 January 2024 and connected to low voltage may fall under Section 14a EnWG if their electrical connection rating exceeds 4.2 kW and they are technically capable of charging from the public grid. Selecting “PV charging only” does not itself exempt them. The rules govern grid imports; they do not require discharge into the grid.
What controls does my battery need? ?
Section 14a addition checked on 28 September 2026: Federal Network Agency: controllability rules.
04 · Costs
What extra benefit does the battery provide?
Ask for the same PV system to be assessed with and without a battery: the same consumption, generation assumptions and prices. Compare the additional avoided electricity purchases with forgone export payments and all battery-related costs.
- Upfront: The battery, additional inverter or control equipment, installation, electrical work and the backup equipment you want.
- Over its service life: Financing, recurring charges, maintenance and possible repairs or replacement. Capacity degradation and losses belong in the yield assumptions.
What would the calculation look like for our example day?
For this example only, we assume a grid electricity price of €0.30/kWh and an export payment of €0.08/kWh. These are not current tariff or support rates.
- 5.4 kWh of avoided grid imports × €0.30/kWh = €1.62.
- 6 kWh of forgone grid exports × €0.08/kWh = €0.48.
- €1.62 ? €0.48 = €1.14 of additional energy cost savings on this example day.
The additional battery costs still need to be covered by these savings. Do not simply multiply this sunny example day by 365. An annual calculation needs suitable generation and consumption profiles. With time-dependent prices, the timing also needs to be considered.
Also request a calculation with less energy cycled through the battery or lower electricity prices. Higher self-sufficiency does not prove that a particular quote pays off.
Explore further: Consumer advice centre: benefits, costs and service life
Understand costs and funding · Find advice in Ulm and the surrounding region.
05 · Technical integration
A new installation or an addition to an existing system?
DC-coupled: PV and the battery are connected on the DC side through a shared hybrid inverter. AC-coupled: A separate battery inverter connects the battery to the home’s AC system. What matters is the specific combination and its characteristics.
Explore further: HTW Berlin: battery connections
For a retrofit quote, bring the model, age and documentation of your existing inverter. Ask for a record of which components will stay, which will be replaced and which metering and control equipment will be added. Location, connection and commissioning belong in the professional design.
Who handles registration?
Agree with the installer who will coordinate with the responsible network operator and handle registration. A grid-connected battery must be entered in the Core Energy Market Data Register; registering the PV system alone does not replace this. The standard deadline for a newly commissioned unit is one month.
Open the Core Energy Market Data Register · Legal basis: Section 5 MaStRV
Keep the confirmations. The PV guide covers connection and handover.
What happens during a power cut?
A battery does not automatically provide emergency or backup power. Options include a dedicated emergency socket, selected circuits or the whole house. This needs to be explicitly specified and correctly installed; stored energy and power limit the supply.
- Which appliances and circuits will actually be supplied?
- How does switchover work, and will supply be interrupted?
- Can the PV system recharge the battery during an outage, and under what conditions?
- How much energy is reserved for this, and what is the additional cost?
Ask for specific answers in the quote. A claim such as “backup capable” is not enough to establish the supply you want.
Explore further: Consumer advice centre: emergency and backup power
06 · Estimate your options
Which calculator suits your household?
For a household without a heat pump or electric car, HTW’s self-sufficiency calculator provides an initial estimate: enter annual consumption, PV capacity and usable battery capacity. First compare without a battery, then with two battery sizes.
Model limitation: The calculator uses a typical household profile and fixed PV assumptions. It does not provide an individual roof design or financial plan. For households with a heat pump or electric car, HTW points to tools including Solarisator.
Open HTW Solarisator · Read the model assumptions
For each option, note the energy actually usable and the remaining grid imports. Check how battery losses are accounted for in the reported self-consumption. Both calculators are external services; your site-specific assumptions should then inform the professional design.
Consider three situations separately.
I want a battery for my balcony solar unit.
First check how much surplus your plug-in solar unit actually generates and when electricity is needed in the flat. The equipment combination, location, connection and registration must suit the particular system. Simplified rules for plug-in solar do not automatically apply to the battery too.
Open the guide for flats and balconies · Compare options in HTW’s plug-in solar simulator
Explore further: Consumer advice centre: batteries for plug-in solar
I also want to charge my electric car later.
Include parking times in your planning. Solar surplus charged directly into the car cannot also be used by the home battery at the same time. Consider together what roles the car, battery and controller should fulfil.
Solar electricity for your car: daily routines and charging targets.
Can low grid electricity prices help me save more?
This is another operating scenario. It requires suitable equipment and a matching tariff. The price difference must at least offset battery losses; additional charges and coordination with PV charging also belong in the calculation. Savings are not automatic.
Explore further: HTW Berlin: dynamic tariffs and battery losses
Your preparation
Six questions for a comparable quote.
Bring PV documentation, electricity bills and any available time-series data. Add planned changes and your backup-power requirements. Ask for written answers to outstanding questions.
- Size: What usable capacity and AC charging/discharging power are offered? How is the size derived from my needs?
- Additional benefit: How much grid electricity is avoided compared with the same PV system without a battery? Which annual profiles and losses are assumed?
- Total price: What is the full installed additional cost of the battery, and which follow-up costs are included?
- Integration: Which devices and functions are compatible? What is agreed for backup power, registration and handover?
- Warranty: What remaining capacity is guaranteed, for how long or up to what energy throughput? Who pays for materials, transport and labour?
- Operation: Who helps if something goes wrong? What internet connection, records, maintenance or paid services are required?
Explore further: HTW Berlin: check warranty terms
Explore further: Consumer advice centre: PV quotes and professional design
Sources, example assumptions and status
- HTW Berlin: sizing and limitations of the self-sufficiency calculator
- HTW Berlin: storage inspector and explanations of terms
- HTW Berlin: Energy Storage Inspection 2026
- Consumer advice centre: battery storage
- Consumer advice centre: planning solar PV
- Consumer advice centre: plug-in solar FAQs
- Section 5 of the Core Energy Market Data Register Ordinance
Editorial and source review: 18 September 2026. This guide mainly covers a home PV battery for self-supply. The daily energy balance, efficiency and example prices are illustrative assumptions. Financial and sizing decisions require your actual data and quotes. Plug-in solar, grid charging and backup power have additional requirements that need to be clarified.