Ledgerly Home Battery Simulator

What it is for

A battery stores midday solar power for the evening. Whether that is worth its price depends on how much surplus you have, when you use electricity and how big the battery is. This simulator runs 8,760 hours: solar power from the offline yield model (or your own monthly figures), your consumption, and a battery with its capacity, power and losses. It compares several sizes side by side.

It is a planning model, not an installer's sizing: hourly steps smooth out short peaks such as a kettle or an oven, so real self-consumption is usually a few points lower. It does not model dynamic tariffs or backup power.

Worked example

A 7 kWp south-facing system at 30° near Leipzig (latitude 51.34, longitude 12.37), a household using 3,800 kWh a year with the built-in profile, a 6 kWh battery, sizes 0, 4, 6, 10 to compare, grid power at 0.33 EUR/kWh and feed-in at 0.0786 EUR/kWh.

The result: the panels make 7,514 kWh. Without a battery you use 20 % of them yourself and cover 40 % of your consumption. With 6 kWh: 38 % self-consumption and 72 % self-sufficiency, 1,053 kWh bought from the grid instead of 2,274, and the battery is worth about 296 EUR a year.

Battery sizes compared shows the diminishing return: 4 kWh is worth 233 EUR a year at 253 full cycles, 6 kWh 296 EUR, 10 kWh only 353 EUR at 153 cycles. The month table explains why: in December the panels make 238 kWh against 374 kWh of use, so even a large battery has little to store.

How to read the result

• Self-consumption — share of the solar power you use in the house, directly or through the battery.
• Self-sufficiency — share of your consumption you do not buy from the grid.
• Bought from the grid and Exported — the yearly energy flows that drive your bill.
• Full battery cycles per year — energy taken out of the battery divided by its capacity. Very few cycles mean an oversized, often idle battery.
• Worth of the battery per year — grid power saved times its price, minus feed-in income lost, compared with the same system without a battery.
• The June chart shows an average day: solar power, use and battery charge hour by hour.

Tips and common mistakes

• Use your smart meter's export if you have one: paste the readings (date, time and kWh per line) into the load profile. The tool averages them per month and hour.
• Compare the yearly worth with the battery's price divided by its expected lifetime — that is the real question.
• Paste the twelve monthly figures from the yield calculator or PVGIS into PV yield per month to use better solar data.
• Change the Weather sequence (seed) to see how much a different sequence of sunny and dull days moves the result.
• A feed-in limit curtails exports above it; the battery then captures more of the midday peak.

FAQ

Why does a bigger battery help less and less?

The first kilowatt-hours cover every evening; extra capacity is only filled on long sunny days and is still empty in winter, when there is no surplus to store.

Does the battery charge from the grid?

No. It charges only from solar surplus and discharges only to cover your own use.

How is the weather modelled?

Each day is sunny, mixed or dull, drawn from a seeded sequence in which a class often lasts several days; every month keeps its total solar energy. The same seed always gives the same year.

Can I use my own hourly data?

Yes: 24 values (one day), 288 values (24 per month) or 8,760 values (a full year), or raw meter readings.