Home battery limits
In the simulation a home battery is five numbers: how much energy it holds, how fast it charges, how fast it discharges, how far down and up it is allowed to go, and how much energy it loses on the way through. Every result involving a battery follows from these.
The numbers and their defaults
| Setting | Default | In the wizard |
|---|---|---|
| Usable capacity | 5, 10, 15 or 20 kWh presets, or any value up to 50 kWh | Always editable |
| Charge power | Capacity times 0.5C | Fixed at the default |
| Discharge power | Capacity times 0.5C | Fixed at the default |
| State-of-charge floor | 10 % | Fixed at the default |
| State-of-charge ceiling | 100 % | Fixed at the default |
| Round-trip efficiency | 92 % | Fixed at the default |
Four terms in that table need a word each.
- Usable capacity is the part of the battery’s energy you can actually draw on. Datasheets often list a larger total figure and a smaller usable one, because the battery’s own electronics keep a reserve to protect the cells.
- State of charge is how full the battery is, from 0 % (empty) to 100 % (full). The floor is the lowest level the model lets it reach, the ceiling the highest.
- The C-rate links power to capacity. 1C would empty or fill the battery in one hour, and 0.5C takes two. At 0.5C a 10 kWh battery charges and discharges at up to 5 kW.
- Round-trip efficiency is the share of the energy you put in that comes back out. It is split equally between charging and discharging. At 92 %, each direction keeps about 96 %, so 100 kWh sent to the battery returns 92 kWh.
The charge power, the discharge power, the floor, the ceiling and the efficiency are model defaults. The wizard does not expose them, so you cannot change them. You enter usable capacity, and the floor and ceiling are applied to that figure: with the defaults, 90 % of the capacity you enter is available for use. If the usable capacity on your datasheet already excludes the maker’s reserve, the 10 % floor takes a second slice and the simulated window is a little smaller than the datasheet suggests.
The defaults were set with a lithium iron phosphate home battery in mind. They are model defaults, not a recommendation, and they are not tied to any product. Check your battery’s own settings for its real floor and ceiling.
How the rule-based mode uses them
The rule-based mode is a fixed order of priorities with no prices in it. It charges the battery from PV surplus only, up to the ceiling and never faster than the charge power. It discharges the battery when the house needs more than the PV delivers, down to the floor and never faster than the discharge power. The battery is never charged from the grid. The year starts with the battery half full.
How the optimiser uses them
With a dynamic tariff (hourly prices) and a battery, EnergyFlow switches to an optimiser, a planner that minimises the electricity cost. It treats the same numbers as hard limits, hour by hour: charge and discharge cannot exceed the power limits, and the state of charge must stay between floor and ceiling. Efficiency losses are part of the energy balance. Inside those limits it may charge from the grid when prices are low and discharge when they are high, because it minimises cost, not grid use. Charge carries over from one day to the next.
The optimiser reads the same configured values as the rules, so the two modes never work with different batteries.
What the limits do to a result
Power can limit before capacity does. On a sunny day a large midday PV peak enters the battery only at the charge power, and the rest is exported even if the battery still has room. A PV layout with a lower, broader peak sends less energy past that limit.
A small battery fills early. Once it is full, further PV surplus is exported. At the other end, once it reaches the floor, the house imports.
Losses are real energy. Every cycle costs the difference between what goes in and what comes out. The optimiser weighs this and charges from the grid only when the price gap pays for it.
To see the effect of one limit, change it and keep everything else identical. Capacity is the one battery limit the wizard lets you vary.
Limits of the model
- Efficiency is constant. It does not change with power level, temperature or state of charge.
- Ageing, capacity fade and standby losses (energy a battery loses while idle) are not modelled.
- There is no backup reserve for power cuts. The floor is a fixed share, whatever the season.
- Battery cost, installation, replacement and financing are not included in the results.