Calculator
Home Battery Sizing Calculator
A home battery is sized by two ceilings, not one. The first is how much energy you actually use after the array stops producing. The second is how much surplus the array can spare to charge it. The correct battery is the smaller of the two, discounted for depth of discharge and round-trip efficiency.
Oversize past either ceiling and you pay for capacity that sits idle most of the year, ages on the shelf and never earns its cycles back.
Size your battery
Evening, night and early morning load.
Production you currently export.
LiFePO4 packs typically 90 to 100%.
1 for grid-tied, 2 to 3 off-grid.
Load-limited need
7.7 kWh
Energy used outside daylight
Charge-limited need
10.8 kWh
What the surplus can actually deliver
Usable capacity to target
7.7 kWh
The smaller of the two ceilings
Nominal capacity to buy
8.6 kWh
At 90% depth of discharge
Annual throughput
2,811 kWh/yr
Cost of stored energy
€0.111/kWh
Cycles out in about 16 years at this usage
How the calculation works
Figure — DC versus AC coupled storage
Decision tree
Is a battery justified on this installation
Storage pays on the spread between what you avoid and what you would have been paid for exporting. Test that spread before anything else.
What is the gap between the import price and the export price per kilowatt-hour?
- Under 5c
- No. Under net metering or a generous export tariff, arbitrage cannot repay the hardware. Buy storage only for backup.
- 5–15c
- → go to step 2
- Over 15c
- → go to step 2
Is there routine exported surplus to charge from?
Compare annual generation against the share of load that falls in daylight. A battery cannot store energy that was already consumed.
- No surplus
- No. Enlarge the array first; storage without surplus simply cycles grid electricity at a loss.
- Regular surplus
- → go to step 3
Is the evening and overnight load large enough to discharge the battery most days?
- Under 3 kWh
- Size to that load only. A larger unit will sit part-charged and never earn its capacity.
- 3–10 kWh
- → go to step 4
- Over 10 kWh
- → go to step 4
Does the annual saving exceed the installed cost divided by the warranted cycle life?
Divide installed cost by warranted throughput in kilowatt-hours to get a cost per stored unit, then compare it to the import–export spread.
- No
- Not yet. Free load shifting captures a third of the same benefit; revisit when prices or tariffs move.
- Yes
- Proceed, sizing to the evening load rather than to the array.
The load ceiling is your daily consumption multiplied by the fraction that happens when the array is not producing. That is the most a battery could ever displace in a day. The charge ceiling is the surplus you currently export, reduced by round-trip efficiency, because energy stored and returned is energy partly lost.
Take the smaller number as usable capacity, then divide by depth of discharge to get the nominal capacity a manufacturer will quote. Finally, dividing the purchase price by the energy the pack will move over its rated cycle life gives the cost per stored kilowatt-hour — the figure that decides whether storage beats simply exporting.
The formula
E_load = D × f_evening × A E_charge = S × η_rt × A E_usable = min(E_load, E_charge) C_nominal = E_usable / DoD Throughput= E_usable × 365 Cost/kWh = (C_nominal × price) / (C_nominal × DoD × N_cycles) D daily load · f_evening non-daylight share · A days of autonomy S daily surplus · η_rt round-trip efficiency · DoD depth of discharge N_cycles rated cycle life
Assumptions behind the numbers
- Daily figures are annual averages. Winter surplus in northern Europe can be a fifth of the summer figure, so a battery sized on averages will sit half empty for months.
- Round-trip efficiency covers battery and inverter losses together. AC-coupled systems typically land at 85 to 90%, DC-coupled a few points higher.
- Depth of discharge is the usable window the BMS allows, not the marketing figure.
- Cycle life assumes one full equivalent cycle per day at moderate temperature. Sustained high temperature or heavy fast charging shortens it.
- No time-of-use arbitrage is modelled. Where night tariffs are cheap, a battery can earn extra by charging from the grid.
- Backup or islanding requirements are not included; a whole-home backup requirement usually drives capacity above the self-consumption optimum.
How to read the result
If the charge ceiling is far below the load ceiling, the array is the constraint, not the battery. More panels will do more than more storage. If the load ceiling is the smaller of the two, extra surplus is genuinely spare and the battery is the right lever.
Compare the cost of stored energy against the gap between your import and export prices. Storage pays only when that spread exceeds the storage cost per kilowatt-hour. At €0.26 import and €0.07 export the spread is €0.19; a pack storing at €0.11/kWh is comfortably profitable, one at €0.22/kWh is not.
Worked example
A household consumes 14 kWh a day, 55% of it after sunset, so 7.7 kWh of load sits in the battery's window. The 6 kWp array exports 12 kWh a day on average; at 90% round-trip efficiency that surplus can deliver 10.8 kWh. Load is the binding ceiling, so usable capacity targets 7.7 kWh.
At 90% depth of discharge that means a nominal 8.5 kWh pack — in practice an 8 or 10 kWh LiFePO4 unit. At €600/kWh installed, a 10 kWh pack costs €6,000 and will move roughly 54,000 kWh over 6,000 cycles, giving €0.11 per stored kilowatt-hour. Against a €0.19 import-export spread it clears, but not by the margin most brochures imply.
Mistakes that skew the answer
- Sizing from peak winter evening demand instead of annual average. That buys a pack that is idle nine months of the year.
- Using nominal capacity as usable capacity. The difference is often 10 to 20%.
- Ignoring the surplus ceiling. A battery cannot store energy the array never produces.
- Buying capacity for backup without checking whether the inverter can actually island and carry the intended loads.
- Assuming cycle life is achieved regardless of temperature. An unheated garage in a cold climate costs both capacity and life.
- Forgetting standing losses. Idle consumption of 20 to 50 W adds up to 175 to 440 kWh a year.
Questions readers ask
- How many kWh of battery do I need for a typical home?
- Most European households with a 4 to 8 kWp array land between 5 and 12 kWh nominal. The right answer comes from your own evening load and surplus rather than from system size. A home using 10 kWh a day with half of it after dark rarely justifies more than 8 kWh; a heat-pump home using 25 kWh a day may justify 15 kWh or more.
- Is a bigger battery always better?
- No. Beyond the point where the pack empties most nights and refills most days, extra capacity earns almost nothing. It still costs money, still ages calendrically and still consumes standby power. Size for the cycles you will genuinely use, and add capacity later if your consumption grows with an EV or heat pump.
- Should I choose AC or DC coupling?
- DC coupling shares the inverter with the array and saves a conversion step, so it is a few points more efficient and usually cheaper on a new build. AC coupling is the practical choice when retrofitting a working array, because it does not require replacing the existing inverter and keeps the two systems independent for fault-finding.
- How long will a LiFePO4 battery last?
- Rated cycle life for quality LiFePO4 packs is 4,000 to 8,000 cycles to 80% capacity, which at roughly one cycle a day is eleven to twenty years. Calendar ageing usually intervenes first, so treat the warranty term and the throughput guarantee, typically expressed in MWh, as the real bound.
- Does a battery pay for itself?
- It depends entirely on the spread between what you pay for imports and what you are paid for exports, plus any time-of-use arbitrage. Where that spread is above about €0.15/kWh and the pack is fully cycled daily, storage typically returns its cost within its warranty. Where export prices approach retail, it rarely does.