What This Calculator Does
This solar self-consumption calculator shows how much of your own solar power you actually use, and how much a home battery changes that.
Solar panels make most of their power around midday. Many homes use most of theirs in the evening.
Without a battery, the midday extra flows out to the grid, and in the evening you buy power back. A battery is like a lunchbox: it saves some of the midday power for dinner time.
How To Use This Calculator
- In Annual solar production, type what your panels make per year in kWh. Don't know? The yearly kWh calculator works it out.
- In Annual home consumption, type your yearly use from your power bill.
- In Battery storage, type the usable size of the battery in kWh. It starts at 6 kWh, a common home battery. Type 0 to see a home with no battery.
- In Daytime consumption share, type the share of your power that solar can cover straight away while it shines. Not sure? Leave it at 35, the value that makes this calculator match the reference home results of HTW Berlin, a German solar research group. Working from home or running the washing machine at noon pushes it up. An empty house all day pushes it down.
- Compare the Without battery and With battery blocks. Each shows self-consumption, the share of your solar actually used in the house, self-sufficiency, and how much goes to and comes from the grid.
- Extra solar kept on-site with battery shows what the battery adds, and the chart below shows where your solar goes.
The Formula, In Plain Words
Scientists use two words here. "Self-consumption" is the share of your solar power that you use yourself. "Self-sufficiency" is the share of your home's needs that solar covers.
The calculator walks through the year day by day. It uses how much Dutch panels make in each month (from PVGIS, the European Commission's solar tool) and how much a Dutch home uses in each month. Each month gets a sunny day at 150% of the monthly average and a dull day at 50%.
On every day it does three steps:
- Solar first goes to the power you use while the sun shines.
- Spare solar goes into the battery, but never more than the battery holds, and never more than you will need that evening and night.
- The battery hands that energy back in the evening and night. About 16% is lost as heat on the way in and out, so 84% comes back.
Whatever is left is exported. Whatever is still missing is bought from the grid.
This simple model was tuned against HTW Berlin's minute-by-minute simulations. For a home using 4,000 kWh with 4 kWp of panels, HTW finds about 30% self-sufficiency without a battery and 56% with a 4 kWh battery. This calculator gives 32% and 54%.
A Worked Example
Say your panels make 4,500 kWh a year, you use 4,000 kWh, the daytime share is 35%, and you add a 6 kWh battery.
Without a battery you use 1,309 kWh of solar directly. That's 29% self-consumption and 33% self-sufficiency. You send 3,191 kWh to the grid and buy 2,691 kWh.
With the battery, 1,481 kWh more stays at home. About 237 kWh of that is lost in the battery and inverter, so 1,244 kWh reaches your sockets.
Now 2,790 kWh of solar stays at home: 62% self-consumption and 64% self-sufficiency. You buy 1,447 kWh.
Used in the house leaves out the battery losses: (1,309 + 1,244) ÷ 4,500 = 57%. That's the number to paste into the solar payback calculator, because lost energy saves you nothing.
Try 10 kWh instead and self-sufficiency only climbs to 69%. The battery is often not full, because dull days don't fill it and summer nights don't empty it.
What To Watch Out For
This is an estimate, not a simulation of your own house. Your real daily pattern can move the answer by ten points or more.
It cannot move summer sunshine into winter. In December a Dutch roof makes about a third of its yearly-average day, so you still buy power in winter.
HTW Berlin advises a usable battery of at most 1.5 kWh per 1,000 kWh of yearly use. Above that, each extra kWh adds little.
If self-consumption matters for your wallet, the solar payback calculator shows what it's worth. For how export is paid, read selling power back to the grid.
Hard Words
| Term | What it means |
|---|---|
| Self-consumption | The share of your solar power that you use in your own home instead of sending it to the grid. |
| Self-sufficiency | The share of your home's yearly electricity that your own solar covers. |
| Export | Solar power you send out to the grid because you can't use it right then. |
| Import | Power you buy from the grid when your solar and battery can't cover your needs. |
| Usable capacity | The part of a battery's size you can actually use, printed on the datasheet. Type this number, not the total size. |
| Round-trip efficiency | How much of the energy you put in a battery comes back out. This calculator uses 84%. |
Common Questions
What is solar self-consumption?
Self-consumption is the share of your solar production that you use yourself, instead of exporting it. If your panels make 4,500 kWh and you use 1,309 kWh of that directly, your self-consumption is about 29%. Without a battery, homes whose panels make about as much as they use often land around 30%.
What is the difference between self-consumption and self-sufficiency?
Self-consumption looks at your panels: how much of their power you use. Self-sufficiency looks at your home: how much of its needs your solar covers. With 4,500 kWh production, 4,000 kWh use and 1,309 kWh used directly, self-consumption is 29% and self-sufficiency is 33%.
How much does a home battery increase self-consumption?
It depends on battery size, your evening use and the season. In this calculator, a 6 kWh battery raises self-consumption in the example from 29% to 62%, and the share actually used in the house from 29% to 57%. HTW Berlin's detailed simulations show a similar jump, from about 30% to 59%, for a 4 kWh battery in a home using 4,000 kWh a year.
What size battery do I need for my solar panels?
HTW Berlin advises a usable size of at most 1.5 kWh per 1,000 kWh of yearly use, and at most 1.5 kWh per kWp of panels. Bigger batteries give smaller extra gains, because they are often not full. Try a few sizes and see where the extra kWh kept on-site stops growing.
Can a battery make me fully independent of the grid?
Rarely in places with dark winters. A battery shifts power from day to night, but it cannot move summer sunshine into winter. In the example, even a 10 kWh battery stops at 69% self-sufficiency. Going fully off-grid usually needs extra panels, a large battery and often a generator for long cloudy spells.