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Degree of self-sufficiency

Degree of self-sufficiency Self-Consumption photovoltaics Battery storage prosumer electricity consumption grid offtake

The degree of self-sufficiency indicates what share of one's own electricity consumption within a given period is covered by own generation. This includes electricity that is consumed directly and electricity that was previously self-generated, stored and later withdrawn again. The degree of self-sufficiency is a ratio and is usually expressed as a percentage. An example: A household consumes 5,000 kilowatt hours per year and covers 3,000 kilowatt hours of this with its own solar power. Arithmetically, this gives it a degree of self-sufficiency of 60 percent.

The reference value is consumption, not generation. The degree of self-sufficiency therefore describes how far one's own demand is covered by oneself. It is used above all for photovoltaic systems, battery storage systems, heat pumps, electric cars and prosumers, that is, actors who both consume and generate electricity. The metric shows how strongly a building, a business or a defined site covers its electricity demand from its own generation. This is achieved either simultaneously or via short storage cycles. According to the Solar Storage Systems research group at HTW Berlin, the higher the degree of self-sufficiency, the less energy has to be drawn from the public electricity grid.

Calculation and system boundary

For the calculation, the self-covered consumption is divided by total electricity consumption. In a photovoltaic system with a battery, the self-covered consumption consists of two components: the simultaneous direct consumption of the solar power and the discharge of the battery storage system. Electricity fed into the public grid does not increase the degree of self-sufficiency. Without conversion losses, total consumption is made up of the self-covered part and grid purchases. The degree of self-sufficiency can therefore also be calculated as 100 percent minus the share of grid purchases in consumption. The more electricity is drawn from the grid for the same consumption, the lower the degree of self-sufficiency.

One subtlety concerns the storage system: for self-consumption, the charging of the battery counts, whereas for the degree of self-sufficiency, its discharge counts. The charging and conversion losses fall between the two quantities. In its independence calculator, for example, HTW Berlin assumes that on average only about 84 percent of the energy that the battery system takes up on the AC side is released again. A storage system therefore always raises the degree of self-sufficiency somewhat less than the amount of energy charged would suggest.

The chosen system boundary is important. A degree of self-sufficiency can refer to a household, a multi-family building, a commercial site, a neighbourhood or an energy community. For communities, it must additionally be clear whether physical electricity flows or only quantities allocated in accounting terms are counted. Consumption can also change: if an electric car is charged at home, the household's electricity consumption rises significantly. The degree of self-sufficiency can then fall even though more solar power is used by the household itself. A larger storage system or time-shifted charging can raise it again. Without a stated system boundary and period, the figure is incomplete.

Temporal resolution also plays a role. The degree of self-sufficiency is usually calculated over one year. An annual value, however, smooths out the large differences between summer and winter. In summer, a photovoltaic system can cover almost all consumption on many days, whereas in winter a large part comes from the grid. The annual value therefore says little about the hours at which grid purchases are needed. For grid operation and security of supply, precisely these hours are decisive. Simulations such as the HTW Berlin independence calculator therefore model the energy flows at one-minute resolution and only then aggregate them into an annual value.

Distinction from the self-consumption share

The degree of self-sufficiency is often confused with self-consumption or the self-consumption share. The self-consumption share indicates what share of the self-generated electricity is used in the house, either directly or to charge the storage system. The degree of self-sufficiency indicates what share of one's own consumption is covered by one's own system. Both metrics look at the same electricity flows, but from opposite directions. A high self-consumption share means little feed-in, while a high degree of self-sufficiency means little grid purchase.

An example shows the difference: A small photovoltaic system generates 3,000 kilowatt hours per year. Of this, 2,400 kilowatt hours are used by the household itself, directly or via a battery. Storage losses are neglected here for simplicity. The self-consumption share is therefore 80 percent. If the household consumes 6,000 kilowatt hours in total, however, the degree of self-sufficiency is only 40 percent. With a larger system, the reverse can be true: because more electricity is fed in, the self-consumption share falls. At the same time, the degree of self-sufficiency rises because the system covers more of the consumption. According to the HTW Berlin simulations, exactly this can be observed as system size increases.

This distinction is important technically and economically. The self-consumption share says something about how the own generation is used and how much is fed in. The degree of self-sufficiency says something about how strongly the electricity purchased from the grid falls. Both quantities are relevant for the profitability of a system, but they answer different questions. Anyone who equates them either overestimates independence from the grid or underestimates the importance of surplus feed-in.

Why a high degree of self-sufficiency does not replace the grid

A high degree of self-sufficiency does not mean complete independence from the electricity grid. It does not state that sufficient own power is available at every hour. Power is energy per time and is stated in kilowatts. The degree of self-sufficiency, by contrast, is based on quantities of energy over a period, measured in kilowatt hours. An annual balance can look good even though on individual winter evenings all consumption comes from the grid. HTW Berlin states that complete independence from the public grid is not possible with sensibly dimensioned battery storage systems.

The difference between quantity of energy and instantaneous power often leads to false conclusions. A photovoltaic system can arithmetically generate a lot of electricity over the year, but not always when it is needed. Battery storage systems typically shift electricity from midday to the evening and night, at most over a few days. They do not bridge longer phases of low generation. For balancing between summer and winter, common home storage systems are far too small. For single-family homes, HTW Berlin recommends as an upper limit about 1.5 kilowatt hours of usable storage capacity per kilowatt of photovoltaic capacity. In addition, it should be at most about 1.5 kilowatt hours per 1,000 kilowatt hours of annual consumption, which roughly corresponds to the nighttime electricity demand. Larger storage systems increase the degree of self-sufficiency only slightly.

The grid connection therefore continues to fulfil several tasks. It supplies electricity when own generation and storage are not sufficient. It absorbs surpluses when local generation exceeds consumption. It keeps voltage and frequency within the technical rules and connects many consumption and generation profiles with one another. A building with a degree of self-sufficiency of 80 percent can be particularly dependent on the grid for the remaining 20 percent if these quantities of electricity arise in a few unfavourable hours.

The degree of self-sufficiency also says little about the load on the grid. A system with a battery storage system can reduce grid purchases and smooth load peaks if it is operated accordingly. But it can also cause high feed-in peaks if a lot of photovoltaic capacity is installed and the battery is already full at midday. For the grid, load profile, feed-in profile, connection capacity and controllability count. The degree of self-sufficiency does not reflect these characteristics.

Economic incentives and legal framework in Germany

In the profitability calculation of private photovoltaic systems, the degree of self-sufficiency plays a large role because self-used electricity replaces the purchase of grid electricity. A self-consumed kilowatt hour is therefore usually worth as much as the household electricity price that does not have to be paid for it. Fed-in electricity, by contrast, is remunerated via a feed-in tariff or a market value, which is often significantly below the end-customer price. This creates an incentive to increase self-consumption and thus often also the degree of self-sufficiency.

In Germany, this incentive has recently become stronger for new systems. Since 25 February 2025, section 51(1) of the Renewable Energy Sources Act (EEG 2023) has applied in its new version: for periods with a negative spot market price, the applicable value falls to zero. For fed-in electricity, there is therefore no market premium and no feed-in tariff during these times. For systems below 100 kilowatts, the rule applies only after the end of the calendar year in which a smart metering system is installed. For systems below 2 kilowatts, a separate transitional rule applies, which is tied to a determination by the Federal Network Agency. Under section 51a, the remuneration period is extended for the affected times, for solar systems according to a separate conversion. Transitional provisions apply to older systems. Using or storing midday surpluses oneself thus becomes more attractive for new systems.

These incentives do not automatically lead to a design that makes sense from a macroeconomic perspective. A very large battery storage system can increase the degree of self-sufficiency, but it is expensive and goes through only a few additional charging cycles per year. The last percentage points on the way to arithmetic self-sufficiency are usually significantly more expensive than the first. Anyone who wants to reach almost 100 percent in a single building needs large storage systems, backup power technology or additional generators that are used only rarely. The costs then arise not for the average kilowatt hour, but in the rare hours in which a lot is consumed and little is generated.

Typical misinterpretations

One misinterpretation arises from political language: self-sufficiency sounds like independence and control. In the electricity system, however, a completely separate supply is a special case, for example in island grids, mountain huts or critical facilities with emergency power supply. Most photovoltaic households remain part of the interconnected grid. They draw less electricity from it, but do not leave it. The term should therefore not be confused with island operation. In island operation, a system works technically without the public grid and ensures voltage, frequency and protective functions itself. A high degree of self-sufficiency does not prove this capability.

The degree of self-sufficiency is also of only limited significance for climate balances. It shows how much of one's own consumption is covered locally, but not which emissions are avoided as a result. For that, what matters is which generation in the electricity system is displaced in the respective hours and how feed-in, storage operation and load shifting have an effect. A self-consumed kilowatt hour of solar power is physically the same quantity of energy as a fed-in one. HTW Berlin points out that larger systems with more feed-in in particular reduce CO2 emissions in the German power plant fleet. It recommends not paying attention only to the self-consumption share and the degree of self-sufficiency when dimensioning.

Relationship with flexibility and electrification

With growing electrification, the informative value of the degree of self-sufficiency changes. Heat pumps, electric cars and electric process heat increase electricity consumption, but often reduce the consumption of fossil final energy. A falling degree of self-sufficiency can then go hand in hand with a better overall energy balance. If a household draws more electricity because of a heat pump, but no longer burns heating oil or natural gas, the lower self-sufficiency value alone says nothing about energy progress or regression. In addition, heat pumps and electric cars distribute consumption differently over the day and the year. HTW Berlin therefore advises against using its simple independence calculator for such households. Its simulations assume households without a heat pump and electric car.

For a renewable electricity system, what matters less is the greatest possible local self-sufficiency than the ability to better match consumption and generation in time. Flexibility can increase the degree of self-sufficiency, for example when an electric car is charged with solar power during the day or a heat pump produces hot water while the photovoltaic system is delivering a lot of power. The same flexibility can, however, also be used in a grid-supportive way, even if it does not maximise the degree of self-sufficiency of an individual household. Conflicts arise where self-optimisation, grid charges, feed-in rules and technical controllability give different signals.

The degree of self-sufficiency is a useful metric as long as it remains clear what it measures: the share of one's own consumption that is covered by own generation within a defined boundary and a defined period. It does not replace an analysis of peak load, grid connection, seasonal supply, profitability or system costs. Used precisely, it shows how strongly a consumer reduces its grid purchases in terms of quantity. Used imprecisely, it turns an accounting metric into a promise of independence that obscures the technical role of the electricity grid.

Sources

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Edited by eCMS on 11.11.2026 14:52

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