The savings a rooftop solar system delivers do not depend only on how many kWh it produces in a year. How much of that energy is used in the building at the moment it is produced, how much is exported to the grid and how exported energy is settled all shape the result. This article explains self-consumption, self-sufficiency and net metering in general terms and uses an example table to show how monthly surplus and deficit work.
Key terms
- Self-consumption: the part of solar production used on site at the moment it is generated. This energy goes straight to the building's loads without passing through the meter.
- Self-consumption ratio: directly consumed energy divided by total production.
- Self-sufficiency ratio: directly consumed energy divided by total consumption. It shows how much of the demand is covered by solar.
- Exported energy: the part of production not used by the building at that moment, which flows to the grid through the bidirectional meter.
- Net metering: settling exported energy against imported energy by offsetting one against the other over a defined time period.
A simple daily example
The figures below are example values chosen to illustrate the terms. A rooftop system on a business produces 40 kWh in a day, and the business consumes 30 kWh on the same day. Of the production, 18 kWh is used directly in the building during the hours it is generated.
| Quantity (example) | Value |
|---|---|
| Daily production | 40 kWh |
| Daily consumption | 30 kWh |
| Consumed directly (self-consumption) | 18 kWh |
| Exported (40 − 18) | 22 kWh |
| Imported (30 − 18) | 12 kWh |
| Self-consumption ratio (18 / 40) | 45% |
| Self-sufficiency ratio (18 / 30) | 60% |
On the same day, 22 kWh went to the grid and 12 kWh came back from it. Even though production exceeded consumption, the business still imports energy because there is no sun in the evening. This is why self-consumption and net metering need to be considered separately.
Why does the self-consumption ratio affect savings?
Every kWh consumed directly is a kWh the customer does not buy from the grid, so its value is close to the full retail price the customer pays. The value of exported energy depends on the netting rules and on how surplus energy is compensated, and it may not equal the retail price. For two systems of the same size, the one with the higher self-consumption ratio therefore usually delivers more predictable savings.
The main factors behind the self-consumption ratio are:
- Load profile: offices, shops and factories that operate during the day have higher ratios than homes where evening consumption dominates.
- System size: a system that is large relative to consumption exports a bigger share of its output, so the ratio falls.
- Seasonality: in a building with summer air-conditioning load, more of the summer output is used on site.
- Weekends and holidays: when the business is closed, almost all output is exported.
Monthly surplus and deficit
Annual production being close to annual consumption does not mean every month is balanced. In a climate like Türkiye's, output is high in summer and low in winter. The table below uses example values only; for a real project, calculate production from location and design and consumption from the bills.
| Month | Production (kWh) | Consumption (kWh) | Difference (kWh) |
|---|---|---|---|
| January | 520 | 1,100 | −580 |
| February | 640 | 1,000 | −360 |
| March | 900 | 950 | −50 |
| April | 1,050 | 850 | +200 |
| May | 1,230 | 900 | +330 |
| June | 1,300 | 1,050 | +250 |
| July | 1,340 | 1,200 | +140 |
| August | 1,250 | 1,180 | +70 |
| September | 1,050 | 950 | +100 |
| October | 820 | 900 | −80 |
| November | 580 | 1,000 | −420 |
| December | 470 | 1,120 | −650 |
| Total | 11,150 | 12,200 | −1,050 |
In this example, annual production is below consumption, yet the six months from April to September produce a combined surplus of 1,090 kWh. The other six months show a combined deficit of 2,140 kWh, which comes from the grid. Whether the summer surplus can be carried into winter, over which period it is settled and how surplus energy is valued all depend on the netting rules.
The table also shows monthly totals only; it does not show how many kWh were consumed directly within each month. Even in a surplus month, the customer keeps importing energy in the evenings.
Hourly vs monthly netting: the concept
What matters in net metering is the time window over which exports and imports are offset:
- With monthly netting, total exports over a month are compared with total imports over the same month. Energy exported at noon can be offset against energy imported that evening.
- With hourly netting, the comparison is made separately for every hour. A midday surplus can only be offset against consumption in that same hour, not against evening imports.
Conceptually, the shorter the window, the more self-consumption matters, because energy that is not produced and used at the same time gets less credit in the settlement. Which method applies, how surplus energy is priced and who the rules apply to are set by regulation and can change over time. Before calculating a project, check the current regulations and the distribution company's terms, and state clearly in your savings estimate which assumption you used.
How storage and load shifting affect self-consumption
Battery storage
In a hybrid system, surplus daytime production charges a battery that is then used in the evening. In the daily example above, if part of the 22 kWh exported were stored, part of the 12 kWh imported in the evening would come from the battery instead, and both the self-consumption and self-sufficiency ratios would rise. Usable capacity, depth of discharge and round-trip losses must be taken into account, though. Whether a battery pays off depends on the value of exported energy and the cost of the battery, and needs to be assessed project by project. The basics of battery sizing are covered in our off-grid system sizing article.
Load shifting
Moving flexible loads into sunny hours is the cheapest way to raise self-consumption, even without a battery. Washing machines and dishwashers, water heaters, pool pumps, irrigation pumps, EV charging and some production processes can be shifted to daytime. A few simple suggestions to residential customers can noticeably increase the benefit they get from the system.
How to explain this to a customer
Many customers assume that producing as much as they consume in a year will bring their bill to zero. To set expectations correctly, you can follow this order:
- Sun and consumption do not happen at the same time: production happens during the day, consumption continues in the evening. Energy used during the day is a direct saving.
- Surplus goes to the grid: how it is settled depends on regulation; tell them which assumption your estimate uses.
- Summer and winter differ: show the monthly table; the bill may not disappear entirely in winter.
- What the customer controls: shifting loads to daytime or adding storage raises self-consumption.
This explanation heads off complaints that arise when the actual bill differs from the savings estimate. We explain how the payback period is calculated in a separate article on solar payback period.
Comparing production and consumption with PVAGE
In PVAGE's system and financial analysis module, entering a province/district or coordinates pulls monthly irradiation and production from PVGIS. On the consumption side you can use ready-made profiles (home, office, shop, industry, farm, irrigation) with a flat, summer-weighted, winter-weighted or manually entered 12-month distribution. Together with bill, unit price and VAT data, you choose the system type: grid-connected, off-grid or hybrid.
The analysis includes phase and self-consumption ratio settings, and the results show production, consumption and difference month by month, along with a surplus and deficit table. The payback period, cumulative return and a PDF payback report come from the same study. See the financial analysis feature page for details.