Battery Storage to Raise PV Self-Consumption

Lithium-ion storage (LFP) raises PV self-consumption from around 30 % to 60-80 %.

How does a battery storage system for a PV system work?

A battery stores PV electricity that is not consumed immediately during the day and releases it again in the evening and at night. Instead of exporting the surplus for around €0.07/kWh, it later replaces grid purchases at €0.22/kWh and more. State of the art are lithium iron phosphate (LFP) batteries, which achieve a long service life with 6,000 to 10,000 full cycles and are thermally stable. The round-trip efficiency, the ratio of energy withdrawn to energy stored including conversion losses, is 88 to 92 %. As a rule of thumb for sizing, allow 1 kWh of usable capacity per 1 kWp of PV capacity. Larger batteries raise the self-consumption rate only marginally, because on many days they are not fully charged and discharged. The battery is connected either via a hybrid inverter (DC-coupled) or with its own inverter (AC-coupled) for retrofitting.

Which buildings benefit from a battery storage system?

The prerequisites are an existing or planned PV system and a suitable installation room, such as a basement or plant room. Typical properties are detached houses with high evening consumption and multi-family buildings with on-site PV supply to tenants, where little is consumed during the day. A battery is not sensible where daytime consumption is high, for example in businesses operating Monday to Friday: there, the PV electricity is used directly anyway and the battery would have hardly any surplus to buffer. In commercial settings, batteries tend to pay off through additional applications such as peak shaving.

What does a battery storage system actually deliver?

The self-consumption rate rises from around 30 % to 55 to 80 %, in the standard case to 65 %. That corresponds to 20 to 40 % additional self-consumption relative to PV generation. For a 10 kWp system with 10,500 kWh/a and a 10 kWh battery, that is around 3,300 kWh/a of additional electricity used on site, roughly €600/a and 516 kg CO2/a. LFP batteries cost €600 to €1,000 per kWh of usable capacity including conversion. Payback is 10 to 16 years over a 15-year service life; the battery is therefore more a measure for independence and CO2 than a quick return on investment.

Energy carrier

Strom Netzbezug → Strom Eigenerzeugung

Savings potential

20-40 % zusätzliche Eigenverbrauchssteigerung

Worked example

A detached house with a 10 kWp PV system and an annual yield of 10,500 kWh uses only 30 % of the solar electricity on site without storage, because the occupants are out during the day and consumption occurs in the evening. An LFP battery with 10 kWh of usable capacity is retrofitted, in line with the rule of thumb of 1 kWh per kWp. This raises the self-consumption rate to 65 %. The additional self-consumption is calculated as PV yield times the difference in self-consumption rates times efficiency, i.e. 10,500 × (0.65 − 0.30) × 0.90 = 3,308 kWh/a, where the round-trip efficiency of 90 % accounts for conversion losses. Around 3,300 kWh/a is therefore used additionally in the house instead of being exported. At a price difference of €0.18/kWh between purchase and feed-in, this gives roughly €600/a; the grid electricity replaced avoids around 516 kg CO2 per year.

Investment & payback

LFP-Speicher: 600-1.000 €/kWh nutzbar (inkl. Wandlung) · Service life: 15 a · Payback: 10-16 a

Applicability

Requirements

Exclusion criteria

Typical buildings

Frequently asked questions

How big should a battery for a PV system be?

As a rule of thumb, allow 1 kWh of usable storage capacity per 1 kWp of PV capacity, so around 10 kWh for a 10 kWp system. Larger batteries increase the self-consumption rate only slightly, because they are not fully used on many days, and they extend the payback period. The ratio of daytime to evening consumption is decisive: the more is consumed in the evening, the more sense capacity at the upper end makes.

How much does a battery increase the self-consumption rate?

A correctly sized battery raises the self-consumption rate from around 30 % to 55 to 80 %, in the standard case to 65 %. For a 10 kWp system with a yield of 10,500 kWh/a, that is around 3,300 kWh/a of additional solar electricity used on site, taking a round-trip efficiency of 90 % into account. That corresponds to roughly €600/a and 516 kg CO2/a.

How much does a PV battery cost per kWh?

LFP batteries cost €600 to €1,000 per kWh of usable capacity including conversion and installation. A 10 kWh battery therefore costs €6,000 to €10,000. With savings of around €600/a, payback is 10 to 16 years over a 15-year service life. The battery thus only just pays off; its main benefit lies in independence and CO2 avoidance.

Is a battery storage system worth it for commercial businesses?

For businesses with high daytime consumption from Monday to Friday, a battery purely to increase self-consumption is usually not worthwhile, because the PV electricity is already used directly. It becomes interesting through additional applications such as peak shaving to reduce capacity charges or the use of dynamic tariffs. With weekend operation or multi-family buildings with on-site PV supply to tenants, the situation is different again.

How long does an LFP battery last?

Lithium iron phosphate batteries achieve 6,000 to 10,000 full cycles. At around 250 full cycles per year in a detached house, that works out at over 20 years; a service life of 15 years is assumed. LFP cells are also regarded as thermally stable and safe. The relevant standards are IEC 62619 for cell safety and VDE-AR-E 2510-50 for stationary storage systems.

Related measures

Standards & sources

Metering points in the EDM Toolbox

SoC, Lade-/Entladeleistung, Round-Trip Effizienz via Modbus-Schnittstelle des Hybrid-WR im Dashboard.

All measures