A logistics warehouse we surveyed last autumn had three separate vendor relationships running in parallel: a solar installer who'd put 200kWp on the roof in 2021, a battery vendor pitching 200kWh of storage in 2023 with no integration spec, and an EV charging company that wanted to drop in eight rapid chargers next quarter. Three contracts. Three commission cycles. Three ROIs that didn't talk to each other. None of which captured the real opportunity. We costed the integrated stack instead. £510,000 net capex, £128,000 of FYA tax relief, £188,000 of annual savings and revenue, payback under three years, ten-year NPV around £820,000.
The villain is point-solution selling. Three vendors, three system designs that ignore each other, and three sets of margin stacked on top. The whole industry is wired to sell you one thing because that's what each vendor knows how to talk about. The trouble is that solar, battery and EV charging aren't three products. They're one system. Treating them as three is how you leave £40,000 a year on the table.
Why integration beats point solutions
Each technology has a value when standalone. Each is worth meaningfully more when wired into the other two.
Solar on its own: typically delivers 55-65% self-consumption because UK commercial sites generate most at midday but consume most morning and evening. The other 35-45% gets exported at SEG rates of 6-9p/kWh — well below your import cost of 25-30p/kWh.
Solar + battery: pushes self-consumption to 85-90% because the battery soaks the midday surplus and discharges it through your evening peak. That swings a 30,000 kWh/year of midday surplus from £2,400 of export income into £8,500 of avoided imports. Worth, on a 300kWp system, about £6,000 a year on its own.
Solar + battery + EV chargers: layers on timing arbitrage. Fleet vehicles plugged in overnight can pull from the battery (charged at midday from solar) instead of from grid at off-peak rates. More importantly, fleet vehicles plugged in mid-day during driver breaks can soak solar surplus directly. A well-sized stack converts another 20-40% of would-be exports into fuel for your own vehicles, displacing diesel at £1.48/litre with sunshine at zero marginal cost.
That third stack — fleet fuel from sunshine — is where the integrated business case actually lives. Most point-solution sellers can't even quote it because their tool doesn't have a field for it.
The sizing rules of thumb
If you remember nothing else from this post, remember these three numbers. They're not exact but they'll get you within 15% of the right system before you talk to anyone.
- Solar capacity (kWp) ≈ 1.0 to 1.5x peak site load (kW). A site with a 300kW peak demand wants 300-450kWp of solar. More than that and you're exporting too much.
- Battery capacity (kWh) ≈ 0.3 to 0.5x daily kWh consumption. A site consuming 1,200 kWh/day wants 360-600 kWh of battery. Less and you can't shave evening peaks. More and you can't cycle it enough to pay back.
- EV charging capacity (kW) ≈ 0.1 to 0.3x site peak demand. A 300kW peak site can support 30-90kW of charging without DNO reinforcement, especially with smart load management.
These ratios fall out of physics, not marketing. Vendors who sell you outside these ratios are either oversizing for margin or undersizing because they couldn't get a G99 connection.
The phasing question
The three technologies have different design lives:
- Solar PV: 25-30 year design life. Inverters typically replaced once at year 12-15.
- Battery storage: 10-15 year warrantied life, with capacity fade across that window.
- EV chargers: 8-15 year life depending on duty cycle and weather exposure.
Phase the system right and you stagger your replacement capex. Phase it wrong (i.e. all installed in the same month) and you face a brutal capex spike at year ten when the battery dies and the chargers wear out simultaneously. Build it so the solar is the long-lived spine and the battery + chargers are the swappable peripheral kit.
A worked example
Site: 40,000 sq ft logistics warehouse, Midlands, 800kVA supply, ~£245,000 annual electricity, 25-van delivery fleet currently diesel.
System:
- 300kWp rooftop solar PV
- 400kWh / 200kW lithium-ion battery
- 25 × 22kW twin-socket fleet chargers (overnight depot charging)
- Smart energy management system (Energy Partners platform) tying it all together
- Commercial dynamic tariff (Octopus Agile Commercial or similar time-of-use plan)
Capex breakdown:
- 300kWp solar PV (roof-mounted, commissioning, inverters): £225,000
- 400kWh / 200kW BESS (battery, BMS, integration): £180,000
- 25 × 22kW twin sockets, civils, sub-board, comms, load mgmt: £105,000
- Gross capex: £510,000
FYA tax saving on £510k at 25% CT: ~£127,500
Net capex post-tax: ~£382,500
Annual stack:
- Solar self-consumption avoided grid cost (85% of 270,000kWh generated × 27p displaced — battery integration uplift): £62,000
- DUoS red-band shaving (battery): £18,000
- Capacity Market (DCM via aggregator): £5,000
- Frequency response (DC/FFR): £11,000
- Fleet fuel avoided (25 diesel vans @ £4,750/yr fuel → £1,140/yr electricity = saving £3,610/van × 25): £90,250
- Fleet maintenance saving (25 vans @ £480/yr saving): £12,000
Less:
-
Aggregator fees (20% of ancillary revenue): ~£6,800
-
Total net annual: ~£191,000
Payback (gross capex): 510,000 ÷ 191,000 = 2.67 years Payback (net of FYA): 382,500 ÷ 191,000 = 2.00 years 10-year cumulative cash (3% revenue inflation, 1.5% solar degradation, modest battery cap fade): ~£2.05m 10-year NPV at 8% discount, post-FYA: ~£820,000
That's the integrated case. Run those three systems independently with three vendors and you'd lose somewhere around £35,000-£50,000 a year because nobody wired the battery to absorb solar surplus for the chargers — or, more likely, the battery was sized for DUoS shaving alone and is too small for the fleet load.
The smart energy plan that ties it together
Hardware alone leaves money on the table. The integrating layer is a dynamic time-of-use tariff combined with smart energy management software that can:
- Forecast next-day solar yield from weather data
- Forecast next-day fleet load from telematics integration
- Optimise battery charge/discharge schedule against the tariff's half-hourly pricing
- Throttle EV chargers when grid prices spike or supply nears the DNO's agreed import limit
- Bid into ancillary service markets when capacity isn't needed for self-consumption
UK options worth shortlisting in 2026: Octopus Agile Commercial (half-hourly market-linked), E.ON Drive Smart, and tariff/management bundles from independents. The economics depend heavily on how aggressive your operations team is willing to be with load-shifting — but realistically you should target a 12-18% saving on imports versus a static commercial tariff.
Financing options ranked
1. Capex with FYA. Highest absolute return, fastest payback in cash terms, but requires the cash. Best for cash-rich operators or strategic capital allocations.
2. Hybrid (capex + PPA). Pay capex on the solar (long-lived, low risk, fast payback), PPA the battery and chargers (the kit with shorter lifespan and operational complexity). Spreads risk and reduces upfront cost.
3. Power Purchase Agreement (PPA) on the whole stack. Third party owns the kit, you pay for the kWh delivered or saved. Zero capex, lower IRR over the long term, but balance-sheet-light. Best for tenanted properties or operators with capex constraints.
4. Lease / asset finance. Tax-efficient for some structures, but you don't get FYA. Run the numbers carefully against capex with FYA before signing.
5. Energy Services Agreement (ESA). Vendor designs, installs, maintains and operates. You pay a per-kWh or guaranteed-savings rate. Cleanest operationally, often the most expensive over 15 years.
There is no universally right answer. There is a right answer for your balance sheet, your tax position and your appetite for operational complexity.
What to ask before you sign
- Show me the integrated annual stack — all five revenue lines including fleet fuel avoidance. If they can only show you one or two, you're getting a point-solution quote.
- Whose software is the integration layer? It should be vendor-agnostic and OCPP-compliant for chargers. If they propose a proprietary lock-in, walk.
- What's your battery cycling regime and warranty implications? Aggressive DUoS shaving plus heavy frequency response can shorten warranties. Get this in writing.
- What's the DNO position on export and on the fleet charging load? G99 application status, statutory timeline, reinforcement risk.
- How does the system behave when one component fails? A good integrated design keeps solar generating even if the battery fails. A bad one daisy-chains everything.
The short version
Stop buying solar, battery and chargers as three separate procurements. The integrated stack pays back in under three years against around 5-7 years for the same components bought standalone. Sizing rule: solar ≈ peak load (kW = kWp), battery ≈ 0.3-0.5x daily kWh, chargers ≈ 10-30% of site peak. A 40,000 sq ft warehouse with a 25-van fleet runs £510k gross / £382.5k net capex for a system saving £191k a year — payback 2 years on net capex, 10-year NPV ~£820k. FYA covers 100% of plant. The integrating layer is dynamic tariff + smart energy management software. The whole thing fails if you let three vendors design three subsystems that don't talk to each other.
Want us to do the maths on your site?
Book a site survey at /site-survey. We'll model your fleet pattern, generation yield, load profile, DUoS bands, battery cycling regime and ancillary stack — as one system, not three. With one number at the bottom. (Including the fleet-fuel-from-sunshine bit, especially that bit.)
Site SurveyRead this article in the app at energy-partners.co.uk/insights/solar-battery-ev-charging-commercial. Energy Partners installs and manages commercial EV charging, solar and battery storage for UK venues — talk to us.