Ask ten installers how to size a home battery system and most will start with kilowatt-hours: how much energy does the household use, how much of it should the battery cover overnight. That is the right question asked in the wrong order, and it produces a recognisable failure — a system with plenty of stored energy that still trips when the heat pump and the oven come on together.
Capacity is the last number to decide. Three constraints come first: the shape of the load, the inverter’s continuous rating, and the current the battery can actually deliver. Get those in order and capacity falls out of the arithmetic. Get them out of order and you end up specifying a bigger battery to solve a problem that was never about energy.
This guide sets out the sequence, using real pack figures rather than round numbers, and ends with the site constraints that quietly overrule everything above them.
Start with the load profile, not the annual bill
An annual consumption figure — 4,800 kWh, say — tells you almost nothing about what a battery has to do. Two households with identical annual totals can need completely different systems depending on when and how fast they draw power.
Three things to extract from a load profile, in order of how much they affect the design:
Peak simultaneous demand. The highest instantaneous draw the system must support. This is what sizes the inverter, and by extension the battery’s discharge current. An induction hob, an electric shower and a heat pump starting within the same minute is not a rare event; it is a Tuesday evening.
Evening energy. The kilowatt-hours consumed between the end of solar production and the start of the next day’s. This is what capacity is actually for, and it is usually a fraction of daily consumption rather than all of it.
Backup requirement. If the system must ride through outages, what has to stay running and for how long. A fridge, a router and some lighting is a very different load from whole-house backup with the heat pump included.
Without an actual meter reading, these are guesses. A clamp meter logging for a week costs very little and changes the design more than any other single input.
The inverter sets the ceiling, and the battery must meet it
This is the step most sizing guides skip, and it is where the design is really decided.
An inverter rated at 5 kW continuous on a 51.2 V bus needs roughly 100 A from the battery at full output — 5,000 W divided by 51.2 V, plus conversion losses, so closer to 105–110 A in practice. The battery has to be able to supply that continuously, not in a three-second burst.
Now look at what a single pack can do: <div class=”sc-spec”> <div class=”sc-spec-h”>Continuous current vs inverter demand — 51.2 V packs</div> <dl> <dt>100 Ah pack</dt><dd>100 A continuous — 5.12 kW</dd> <dt>200 Ah pack</dt><dd>200 A continuous — 10.24 kW</dd> <dt>300 Ah pack</dt><dd>200 A continuous — 10.24 kW</dd> <dt>3-second peak, 100 Ah</dt><dd>200 A</dd> <dt>3-second peak, 200/300 Ah</dt><dd>300 A</dd> </dl> </div>
A single 100 Ah pack behind a 5 kW inverter is running at its continuous limit whenever the inverter is at full output. It will work, but there is no margin — and any surge headroom the inverter offers is being borrowed from the pack’s three-second peak rating rather than from real capacity.
Two consequences follow. First, an 8 kW inverter needs at least 200 Ah at 51.2 V regardless of how much energy the household consumes, because 8,000 W ÷ 51.2 V is roughly 160 A and no 100 Ah pack will supply that continuously. Second, adding packs in parallel adds current capability as well as capacity, which is often the real reason a second pack is needed.
Note also that the 300 Ah pack does not deliver more continuous current than the 200 Ah pack. Capacity and current are separate specifications, and assuming they scale together is a common and expensive error.
Only then, capacity — and mind the difference between nominal and usable
With the inverter and current settled, capacity becomes arithmetic. <div class=”sc-spec”> <div class=”sc-spec-h”>Nominal energy at 51.2 V</div> <dl> <dt>100 Ah</dt><dd>5.12 kWh</dd> <dt>200 Ah</dt><dd>10.24 kWh</dd> <dt>300 Ah</dt><dd>15.36 kWh</dd> <dt>400 Ah</dt><dd>20.48 kWh</dd> </dl> </div>
Those are nominal figures — capacity multiplied by nominal voltage. Two deductions stand between them and what the household actually gets.
Depth of discharge. A LiFePO4 pack will happily cycle to 90% DOD, but designing to 80% leaves margin for the BMS’s own low-voltage protection and meaningfully extends cycle life. At 80% DOD, a 10.24 kWh pack yields about 8.2 kWh per cycle.
Round-trip efficiency. Inverter conversion losses and battery internal losses together take roughly 8–12% of what goes in. Applying 90% to the figure above leaves about 7.4 kWh delivered to the load.
So a 200 Ah pack that reads 10.24 kWh on the datasheet supplies around 7.4 kWh of usable evening energy. If the evening load is 9 kWh, one pack is not enough — and the shortfall is 25% larger than the nominal numbers suggested.
The relationship between depth of discharge and how long the pack lasts is direct, and it is one of the conditions that makes a cycle life claim comparable or not — covered in our guide to reading a cycle life figure.
The charge side has its own limit
Everything above sizes the battery against what leaves it. The charging side has a separate constraint that is easy to overlook, and it decides whether the battery can actually absorb a day’s solar production.
A pack’s maximum charge current is the same figure as its discharge current — 100 A for a 100 Ah unit, 200 A for 200 Ah and above. At 51.2 V, 100 A is about 5.1 kW of charging power. An array and hybrid inverter capable of delivering 8 kW into the battery on a clear midday will be current-limited by a single 100 Ah pack, and the surplus is either exported or curtailed.
Two packs remove the constraint, which is another instance of the same pattern: the second unit gets specified for current, not for capacity.
There is also a rate-of-charge question that does not appear on any datasheet as a limit. Charging a 200 Ah pack at its full 200 A is a 1C rate — the pack goes from empty to full in about an hour. It is permitted, and for a home system it is almost never what happens; solar charging typically runs at 0.2–0.5C. Where a system does charge hard, from a generator or a cheap-rate grid window, expect the pack to run warmer and plan ventilation accordingly. <div class=”sc-verify”> <p><strong>Check the charging temperature window against the installation position.</strong> Discharge is permitted from −20 °C, but charging is restricted to 0 °C and above on standard packs.</p> <p>An unheated garage or an outbuilding in a cold climate will refuse to charge on winter mornings — the BMS is protecting the cells, and the system is behaving correctly. If that position is unavoidable, specify a heater option at the order stage rather than discovering it in January.</p> </div>
Parallel strings: what actually limits the number
Packs of the same model and voltage can be paralleled to add both capacity and current. The practical limits are less about the specification and more about how the installation is built.
The specification limit is generous — up to fifteen units in parallel on a 51.2 V platform, communicating over RS485 or CAN so that one unit acts as master and reports a single aggregated state of charge to the inverter. Three constraints bite before you reach that number.
Cable symmetry. Parallel packs share current in inverse proportion to the resistance of their connections. Unequal cable lengths mean unequal current, which means one pack ages faster than the others. Every pack should see the same length and gauge of cable to the busbar, even where that means a longer run for the nearest unit.
Inverter compatibility. The inverter has to accept the battery’s communication protocol and recognise the pack count. This is where most commissioning problems live, and it is a question to settle before the order rather than on site.
State-of-charge matching at commissioning. Packs joined in parallel at different states of charge will equalise through the busbar, potentially at high current. Charging every unit to the same level before connecting them is a five-minute step that avoids a first-day fault.
Form factor is a site decision, not a product preference
By this point the electrical design is fixed: a pack voltage, a capacity, and a count. What remains is which physical format the site will accept, and this is decided by the building rather than by the specification sheet. <div class=”sc-spec”> <div class=”sc-spec-h”>51.2 V home ESS formats — what the site has to accommodate</div> <dl> <dt>Wall-mounted, 100 Ah</dt><dd>675 × 400 × 165 mm, approx. 51 kg</dd> <dt>Wall-mounted, 300 Ah</dt><dd>745 × 570 × 255 mm, approx. 135 kg</dd> <dt>Stacked, 300 Ah</dt><dd>785 × 505 × 275 mm, approx. 114 kg</dd> <dt>Floor-standing, 400 Ah</dt><dd>645 × 385 × 815 mm, approx. 200 kg</dd> </dl> </div>
The wall-mounted 300 Ah unit is the one that catches people out. At roughly 135 kg it is a structural question, not a mounting question — it needs a solid masonry wall or a properly engineered bracket into structural timber, and in many houses the wall that has the space is not the wall that can take the load. Two 100 Ah units side by side weigh the same in total but distribute it across a wider footprint and are far easier to lift into place.
Floor-standing units avoid the wall question entirely and are supplied on castors, but they need floor area and front clearance for cabling. Stacked systems sit between the two: a base module on the floor with capacity added vertically, which suits an installation that will expand later.
Three site facts to establish before choosing: what the wall or floor can carry, how the unit will physically get to its position, and what the ambient temperature range is. A garage that drops below freezing in winter rules out charging without a heater option, whatever the datasheet says about discharge temperature. <div class=”sc-verify”> <p><strong>Check the model code against the form factor, not just the capacity.</strong> Across the industry, and in our own catalogue, the same model number is often used for the same electrical rating in different enclosures — the dimensions and weight are not the same even when the specification is.</p> <p>Always state the form factor together with the model code on the enquiry and the purchase order, so the quotation covers the right enclosure and the right freight class.</p> </div>
How to size a home battery system: a worked example
A household with 4,800 kWh annual consumption, an 8 kW hybrid inverter, an evening load of about 9 kWh, and a requirement to back up essential circuits for eight hours.
Inverter demand: 8,000 W ÷ 51.2 V ≈ 160 A continuous. This alone rules out a single 100 Ah pack.
Capacity for the evening load: 9 kWh ÷ 0.8 DOD ÷ 0.9 efficiency ≈ 12.5 kWh nominal.
Selection: two 200 Ah packs — 20.48 kWh nominal, 400 A continuous available. The capacity is more than the evening load strictly requires, but the current requirement forced 200 Ah minimum, and a second unit was needed to give margin above the inverter’s 160 A rather than to add energy.
Backup check: essential circuits at roughly 800 W for eight hours is 6.4 kWh, comfortably inside one pack’s usable energy.
Form factor: two wall-mounted 200 Ah units at 87 kg each, or a stacked configuration if the household expects to add a third later.
Notice what happened. The capacity that got specified is larger than the energy calculation demanded, and the reason is current, not kilowatt-hours. That is the normal outcome when the sequence is followed properly, and it is invisible if you start from the annual bill.
Two sizing mistakes that show up on site
Sizing the battery to the solar array. A 10 kW array does not imply a 10 kWh battery, or any other particular figure. The array determines how much surplus is available to store; the load profile determines how much of it is worth storing. A household that exports at midday and consumes little in the evening gets very poor use out of a large battery no matter how large the array is.
Sizing for whole-house backup without checking the surge. Backup sounds like an energy question and is a current question. A well pump, an air-conditioning compressor or a deep-well submersible can draw several times its running current for a second or two at start. The pack’s three-second peak rating — 200 A on a 100 Ah unit, 300 A on 200 Ah and above — is what has to cover that, and if the inverter cannot pass the surge the load will not start regardless of how much energy is stored.
Both mistakes produce the same customer complaint, which is that a large and expensive battery does not seem to do very much. Both are avoided by the sequence set out above.
What to send a supplier
Sizing conversations go faster when the supplier receives the constraints rather than the conclusion. A useful enquiry contains:
- Inverter make and model, and its continuous and peak ratings
- Peak simultaneous household demand, measured if possible
- Evening energy requirement in kilowatt-hours
- Backup loads and required duration, if any
- Installation position, and whether wall, floor or rack mounting is available
- Ambient temperature range at the installation position
- Whether the system is expected to expand later
The last one changes the recommendation more than people expect. A design that will grow should start with a format and a communication scheme that supports growth, even where a cheaper single unit would meet today’s requirement.
Safety and performance requirements for the cells themselves are defined by IEC 62619, which is the standard to ask a supplier about by name when a system is destined for a market or an application where certification will be checked.
FAQ
How many kWh do I need for a home battery system? For evening use, take the energy consumed between the end of solar production and the next morning, divide by 0.8 for depth of discharge and by 0.9 for round-trip efficiency. A 9 kWh evening load needs roughly 12.5 kWh nominal.
Can I use one battery with a bigger inverter? Only if the pack’s continuous discharge current meets the inverter’s demand. An 8 kW inverter on a 51.2 V bus needs about 160 A, which a 100 Ah pack rated at 100 A continuous cannot supply.
Is it better to buy one large battery or two smaller ones? Two smaller packs give more continuous current, distribute weight better, and allow one unit to be serviced without taking the system down. One larger unit is simpler to install and cable. For most homes, two mid-size packs is the better engineering answer.
How many batteries can be connected in parallel? Up to fifteen units on a 51.2 V platform with RS485 or CAN communication. In practice cable symmetry and inverter compatibility limit real installations well before that number.
Does depth of discharge affect how long the battery lasts? Yes, substantially. Shallower cycling extends cycle life, which is why a cycle figure quoted at 100% DOD is not comparable to one quoted at 80%.
What temperature range does a home battery need? Discharge is typically permitted from −20 °C to 60 °C, but charging is restricted to 0–50 °C. An unheated garage in a cold climate needs a heater option or a different installation position.