How to Size a Commercial Battery Storage System (Without Overpaying) | Bliss Eco EnergySkip to content
Commercial Solar · Technical Guide
Most commercial batteries are oversized. The result is capital tied up in capacity that never charges, never discharges, and never earns its keep. Here is the method that prevents that and the terminology you need to understand it.
Technical GuidekWh vs kWDepth of DischargeHalf-Hourly DataEssex & South East
Quick answer
To size a commercial battery storage system correctly, you need three figures from your half-hourly smart meter data: your average daily surplus generation (kWh), your average out-of-hours demand (kWh), and your peak discharge requirement (kW). The battery capacity should match the smaller of your daily surplus and your out-of-hours demand — not your solar array size, and not a round number. kWh sets how much you can store; kW sets how fast you can draw it. Depth of discharge determines how much of the rated capacity you can actually use. All three matter, and confusing them is how businesses end up overpaying.
BE
The Bliss Eco Energy Team — MCS-Certified Commercial Solar & Storage Installers
Over 300 commercial solar and battery storage installations across Essex, London, and the South East since 2017. MCS, NAPIT, HIES & TrustMark certified.
June 2026·10 min read·Technical sizing guide
48%
Typical self-consumption rate for solar-only systems without correct storage sizing
75%
Typical self-consumption rate with a correctly sized battery added
90%
Depth of discharge on leading LFP commercial battery systems
30min
Interval of smart meter data needed for accurate commercial battery sizing
When a business asks how to size a commercial battery storage system, the honest answer is: not the way most installers do it. The industry default — using a rule-of-thumb ratio relative to the solar array size, such as “one kWh of storage per kWp of panels” — ignores the one variable that actually determines whether a battery earns its investment: your consumption pattern.
The result of that shortcut is predictable. Batteries that are too large for the available surplus sit partially idle. Batteries that are too small for the out-of-hours demand run out before they have covered the load they were installed to serve. Both outcomes extend payback periods unnecessarily and erode the financial case that made storage attractive in the first place.
This guide sets out the correct method for commercial battery sizing, explains the terminology that underpins it, and identifies the mistakes that cost businesses money at the design stage. Before reading this, it is worth being clear on whether battery storage is the right decision for your operation at all — our guide on Is Commercial Battery Storage Actually Worth It for Your Business? covers that in detail.
The foundational concepts
kWh vs kW: the difference that determines your system design
The single most common source of confusion in commercial battery storage conversations is the difference between kWh and kW. They sound similar, appear on the same specification sheet, and are both used to describe battery systems — but they measure entirely different things, and confusing them leads directly to a wrongly sized system.
kWh
Kilowatt-hour
Capacity — how much you can store
A kilowatt-hour is a measure of energy — the total amount of electricity a battery can hold and release over a full charge-discharge cycle. Think of it as the size of the tank.
If your battery is rated at 100 kWh and you have a 90% depth of discharge, you have 90 kWh of usable storage. That is the amount of energy available to power your site after solar generation has stopped.
📦 Example: A 100 kWh battery (90% DoD) can power a 10 kW constant load for 9 hours, or a 30 kW load for 3 hours — the energy budget is the same either way.
kW
Kilowatt
Power — how fast you can draw it
A kilowatt is a measure of power — the rate at which the battery can deliver electricity at any given moment. Think of it as the size of the tap, not the tank.
A battery with a 50 kW continuous discharge rating can supply up to 50 kW of demand at once. If your site has a peak demand of 80 kW and the battery is rated at 50 kW, it can only meet part of that peak — the grid (or your panels) must supply the rest.
🚿 Example: A 100 kWh battery with a 25 kW discharge rating takes 4 hours to empty at full rate. One with a 50 kW rating empties in 2 hours — same tank, wider tap.
For commercial battery sizing, you need to specify both. kWh determines how many hours of load the battery can cover after solar generation drops. kW determines whether the battery can actually keep up with your site’s demand at any given moment. A battery with adequate kWh but insufficient kW will run into its power limit during high-draw periods and fall back to the grid — defeating part of the purpose of having storage.
Most commercial installers will quote kWh capacity prominently and mention the kW discharge rating in smaller print. Always check both figures against your site’s peak demand profile before accepting any quote as correctly specified.
The hidden variable
Depth of discharge explained
Depth of discharge (DoD) is the percentage of a battery’s total rated capacity that can be used before it needs to recharge. It is the variable that most buyers overlook when comparing systems — and the one that most directly affects how much storage you are actually getting for your money.
A battery rated at 100 kWh with a 90% depth of discharge gives you 90 kWh of usable energy. The remaining 10 kWh stays in reserve to protect battery longevity — cycling a lithium iron phosphate battery to absolute zero on a regular basis degrades it faster and shortens the period over which it retains its warranted capacity. The DoD limit is the manufacturer’s engineering decision about the right trade-off between usable capacity and battery lifespan.
This matters enormously when comparing quotes. Two batteries quoted at 100 kWh with different DoD ratings are not the same product:
Usable capacity comparison — three batteries both rated at 100 kWh total
LFP System A — 90% DoD = 90 kWh usable
90 kWh usable
Best-in-class for commercial LFP. Maximises usable storage while maintaining warranty-grade cycle longevity.
LFP System B — 80% DoD = 80 kWh usable
80 kWh usable
Common on mid-range commercial systems. 10% less usable storage than System A for the same rated capacity — effectively 12.5% more expensive per usable kWh.
Older Li-ion System — 70% DoD = 70 kWh usable
70 kWh usable
Typical of older lithium-ion (non-LFP) chemistries. 20% less usable than System A — the equivalent of paying for a 100 kWh system and using only 70 kWh of it.
Always compare on usable kWh — not rated capacity
When you receive a storage quote, divide the total installed price by the usable kWh figure (rated capacity × DoD percentage). This gives you your true cost per usable kWh — the only meaningful basis for comparison. A cheaper headline price with a lower DoD may deliver less usable storage per pound spent than a more expensive system with a higher DoD.
The correct approach
The five-step method for sizing a commercial battery correctly
The correct method for sizing a commercial battery storage system starts with data, not with a ratio. These five steps are what a rigorous commercial site survey covers — if your installer is not working through them before producing a sizing recommendation, they are guessing.
01
Obtain your half-hourly smart meter consumption data
Request your half-hourly electricity consumption data from your energy supplier or meter operator — most commercial sites have smart meters that record at 30-minute intervals. Ask for at least 12 months of data, covering all seasons. This is the foundation of the entire sizing exercise: without it, every figure that follows is an estimate rather than a calculation. Sizing a battery from annual kWh totals alone will not give you the information you need — you need the interval data to see when your site draws power, not just how much in total.
02
Calculate your daily surplus generation by season
Overlay your half-hourly consumption data against the expected generation profile for your solar system (or actual generation data if panels are already installed). At each half-hour interval, subtract your site’s consumption from the generation figure. Any positive value — where generation exceeds consumption — is surplus that a battery could capture instead of exporting. Sum these values across a typical day in summer, autumn, and winter separately. Solar generation in the UK South East is roughly 3–4 times higher in summer than winter — sizing to the summer surplus will leave you with significant idle capacity for six months of the year.
Key output from this step: Average daily surplus in kWh by season. This is the upper bound on what your battery could usefully store.
03
Calculate your out-of-hours demand by season
From the same half-hourly data, isolate the consumption intervals that fall outside your solar generation window — typically before 8am and after 5pm in summer, before 9am and after 4pm in autumn and winter. Sum these to get your average daily demand outside generation hours. This is the load you want the battery to serve instead of the grid. This figure — not the surplus — is often the binding constraint on useful battery size. A battery that can store 100 kWh is of limited value if you only have 40 kWh of out-of-hours demand to discharge into.
Key output from this step: Average daily out-of-hours demand in kWh by season. This is the upper bound on what your battery can usefully discharge.
04
Set usable capacity to the smaller of surplus and out-of-hours demand
Compare your average daily surplus figure against your average daily out-of-hours demand figure. The correct usable battery capacity — across a typical year — is the lower of the two values, weighted toward the seasonal average rather than the summer peak. If your average daily surplus is 60 kWh but your average out-of-hours demand is only 35 kWh, a 35–40 kWh usable battery is the right size. Buying 60 kWh of storage to capture a surplus you can only discharge 35 kWh of means 25 kWh of capacity sitting idle every day. Then divide the target usable capacity by your battery’s DoD percentage to get the rated capacity you need to specify.
Formula: Required rated capacity = Target usable kWh ÷ DoD percentage. Example: 40 kWh usable ÷ 0.90 DoD = 44.4 kWh rated capacity → specify a 45–50 kWh rated system.
05
Confirm the kW discharge rating against your peak demand
From your half-hourly data, identify your site’s peak demand during the periods you want the battery to serve — the highest single half-hour reading outside solar generation hours. This is your minimum required continuous discharge power in kW. The battery’s rated discharge power must meet or exceed this figure; if it falls short, the battery will cap its output and the grid will make up the difference during peak demand. For most commercial sites, a battery with a C-rate of 0.5 or higher (meaning it can deliver half its capacity in one hour) is sufficient — but verify this against your actual peak data rather than assuming.
Example: A site with a peak out-of-hours demand of 45 kW needs a battery with at least 45 kW continuous discharge power. A 100 kWh system with a 0.5C rating delivers 50 kW — adequate. The same system with a 0.25C rating delivers only 25 kW — insufficient for that peak.
Future loads
If you are planning to add EV fleet charging, expand your premises, or bring new high-draw equipment online in the next three to five years, factor that demand into step three before finalising the specification. Retrofitting additional battery capacity later costs more per kWh than sizing correctly the first time — the inverter and installation costs are largely fixed regardless of how much storage you add.
Reference guide
Typical commercial battery sizing by business type
The table below shows indicative sizing ranges across the main commercial sectors, based on typical consumption profiles from our project data across Essex and the South East. These are starting points for the data-based method above — not substitutes for it. Every site’s half-hourly data will produce a different answer.
Variable occupancy, early starts, extended-hours demand
Retail unit (single site)
20–50 kWp
40–60 kWh
Evening trading; overnight refrigeration baseload
Retail park / multi-unit
80–150 kWp
80–120 kWh
Aggregated evening and overnight loads across units
Warehouse (standard hours)
50–150 kWp
30–60 kWh
Low surplus — high daytime self-consumption limits what’s left to store
Agricultural / farm
80–200 kWp
60–120 kWh
Seasonal grain drying and irrigation peaks; large roof surplus in summer
School
30–80 kWp
30–50 kWh
Early-morning warm-up load; limited out-of-hours demand during term time
Industrial / manufacturing
100–300 kWp
100–200 kWh
Large roof — but high continuous daytime load limits surplus; sized to shift and weekend demand
Wondering what storage costs at these sizes?
Our guide to Commercial Battery Storage Cost in 2026 breaks down installed prices, cost per usable kWh, and net cost after AIA tax relief for every size tier from 30 kWh to 500 kWh+.
Want us to run this method on your actual site data?
We pull your half-hourly consumption data, model your surplus and out-of-hours demand, and recommend a system size before you spend a pound.
The four most common sizing mistakes — and how to avoid them
❌
Sizing to the solar array rather than the consumption gap
The most prevalent mistake. A rule-of-thumb ratio — such as “1 kWh of storage per kWp of panels” — produces a figure based on generation capacity, not on how much of that generation goes uncaptured and how much of your load falls outside generation hours. These are the two figures that actually determine the right battery size. A 100 kWp array does not automatically need a 100 kWh battery — it might need 40 kWh, or 120 kWh, depending entirely on your consumption profile.
❌
Sizing to the summer peak rather than the annual average
In June, a commercial solar system in the South East generates roughly three to four times more electricity than in December. A battery sized to capture your full summer daily surplus will be significantly underutilised for six months of the year. The financially rational sizing decision targets the annual weighted average surplus — not the summer maximum — because a battery that cycles fully every day across all seasons generates better returns than one that only operates at capacity for a few months.
❌
Comparing quotes on rated capacity rather than usable kWh
Two batteries both described as “100 kWh systems” with different depth of discharge ratings — 90% versus 80% — deliver 90 kWh and 80 kWh of usable storage respectively. That 10 kWh difference is a 12.5% reduction in what you are actually buying. Multiply across the system’s 10-year working life and the cumulative impact on savings is material. Always request the usable kWh figure and the DoD percentage, and calculate cost per usable kWh before comparing any two quotes.
❌
Ignoring the kW discharge rating against peak site demand
Adequate storage capacity in kWh is necessary but not sufficient. If the battery’s continuous discharge power in kW is lower than your site’s peak demand during the hours you want it to serve, it will hit its power limit and fall back to the grid during those peaks. A site with a 60 kW evening peak load needs a battery rated for at least 60 kW continuous discharge. Specifying purely on kWh without confirming the kW rating against your peak demand profile produces a system that underperforms every time demand is highest.
Key takeaways
To size a commercial battery storage system correctly, start from half-hourly smart meter data — not from your solar array size, not from round numbers, and not from a rule of thumb.
kWh vs kW: kWh is how much the battery can store (capacity); kW is how fast it can deliver that energy (power). You need both figures specified correctly against your site’s demand profile.
Depth of discharge explained simply: the DoD percentage is the share of total rated capacity you can actually use. A 100 kWh battery at 90% DoD gives 90 kWh usable; at 80% DoD, only 80 kWh. Always compare quotes on usable kWh, not rated capacity.
The correct usable capacity is the lower of your average daily surplus and your average daily out-of-hours demand — weighted to the annual average, not the summer peak.
The four sizing mistakes to avoid: sizing to array size, sizing to summer peak, comparing on rated capacity, and ignoring the kW discharge rating against peak demand.
If you are planning EV fleet charging or operational expansion, factor that future load into the sizing now — adding capacity later costs more per kWh than getting the design right the first time.
Common questions
Frequently asked questions
Q
What is the difference between kWh and kW in a battery storage system?
kWh (kilowatt-hours) measures energy — how much electricity the battery can store and release over a full cycle. kW (kilowatts) measures power — how fast the battery can deliver that electricity at any given moment. A battery’s kWh rating determines how many hours of load it can cover; its kW rating determines whether it can keep up with your peak demand at any instant. Both need to match your site’s requirements: adequate kWh but insufficient kW produces a system that runs into its power limit during high-demand periods.
Q
What does depth of discharge mean for a commercial battery?
Depth of discharge (DoD) is the percentage of a battery’s total rated capacity that can be used before it needs recharging. A battery rated at 100 kWh with 90% DoD gives 90 kWh of usable storage — the remaining 10% is held in reserve to protect battery longevity and maintain the warranted cycle life. Leading commercial LFP systems offer 90% DoD; older lithium-ion chemistries may be limited to 70–80%. When comparing quotes, always calculate the cost per usable kWh (price ÷ rated capacity × DoD) rather than the cost per rated kWh.
Q
How do I size a commercial battery storage system without an installer?
Request your half-hourly smart meter data for the past 12 months from your energy supplier. Calculate your average daily surplus generation (generation minus real-time consumption at each half-hour interval) and your average daily out-of-hours demand (consumption during hours when panels are not generating at meaningful output). The target usable capacity is the lower of those two figures, weighted to the annual average rather than the summer peak. Divide that by your chosen battery’s DoD percentage to get the rated capacity to specify. Then check the battery’s kW discharge rating against your site’s peak demand during out-of-hours periods.
Q
Is a bigger battery always better for commercial solar?
No. An oversized battery — one larger than your available daily surplus or your daily out-of-hours demand — sits partially idle and extends payback without delivering proportionate additional savings. The financially optimal size is the one that maximises the proportion of surplus captured and out-of-hours demand served, not the one that maximises rated kWh. More battery capacity than your consumption pattern can support is money tied up in idle hardware.
Q
Can I retrofit a larger battery to my commercial solar system later if I undersize now?
Yes, in most cases — but it costs more per kWh than sizing correctly the first time. The inverter and installation costs (cabling, switchboard connections, commissioning) are largely fixed regardless of battery size. Adding a second battery unit later reintroduces many of those fixed costs. If you have clear visibility of future load growth — EV fleet charging, building expansion, new equipment — it is almost always more cost-effective to factor that into the initial design than to plan for a second phase.
Q
What smart meter data do I need to size a commercial battery?
You need half-hourly interval data — readings recorded every 30 minutes across a full 12-month period. This is available from your energy supplier or meter data agent for any site with a smart meter (most commercial premises with a supply above 100 MWh per year are already on half-hourly settlement and will have this data available). Annual consumption totals are not sufficient — they tell you how much electricity you use in total but not when, which is the information that determines both the right battery size and the expected return.
Get your commercial battery storage sized correctly — free site assessment
We pull your half-hourly data, run the method above, and give you a size recommendation backed by your actual consumption numbers — before you spend anything.
About the author — Written by the Bliss Eco Energy installation team, MCS-certified commercial solar and battery storage specialists based in Southend-on-Sea, Essex. Over 300 commercial renewable energy installations completed across Essex, London, and the South East since 2017. Sizing figures and sector benchmarks are based on project data and are indicative — site-specific sizing is always based on half-hourly consumption analysis conducted at survey.
Published June 2026 · Last reviewed June 2026 · Bliss Eco Energy Ltd
