How Many Solar Panels Does It Take to Charge an EV Fleet?

It is the first question every fleet manager asks, and it has a real answer — provided you accept that “charge” means annual energy balance, not a literal sunbeam-to-socket connection. Here is the arithmetic, from one van to a whole depot.

The per-vehicle baseline

A modern electric van consumes roughly 0.28–0.35kWh per mile in mixed UK duty — call it 3.5 miles per kWh to keep the mental maths honest. A van covering 18,000 miles a year therefore uses about 5,100kWh.

UK commercial rooftops yield 850–950kWh per installed kWp per year; use 870 as a planning figure. So one van’s annual energy is:

5,100 ÷ 870 ≈ 5.9kWp — about 13 panels at 460W, occupying roughly 26–30 square metres of roof.

Scale it linearly for a first pass:

• 5 vans ≈ 29kWp ≈ 65 panels ≈ 150m² of roof
• 10 vans ≈ 59kWp ≈ 130 panels ≈ 290m²
• 20 vans ≈ 117kWp ≈ 255 panels ≈ 580m²
• 40 vans ≈ 235kWp ≈ 510 panels ≈ 1,160m²

A typical mid-size industrial unit roof (1,000–1,500m² usable) therefore carries the annual energy of a 30–50 van fleet. Most depots discover their roof is not the constraint they assumed.

Why the schedule changes everything

The arithmetic above balances energy across a year. Whether the electrons physically come from your roof depends on when the vehicles are parked.

Daytime-dwell fleets — last-mile vans returning between runs, pool vehicles, staggered shifts — can charge while the array generates. With solar-following load management, these operations get 40–60% of vehicle energy directly from the roof, with the balance from off-peak grid.

Overnight-only fleets see the array and the vehicles miss each other. The solar still earns full value displacing building load and export still pays something, but the vehicles charge mostly on cheap overnight tariffs. Solar share of vehicle energy: typically 10–25%, improving where weekend dwell soaks up Saturday and Sunday generation.

Neither case breaks the business case — overnight depot charging still beats public rapid networks by 50p+/kWh — but the schedule decides whether you are buying a fuel source or a fuel hedge. Be suspicious of any proposal that quotes a solar share without having seen your rota.

HGVs: different league

An electric truck consumes 1.1–1.4kWh per mile. One tractor unit doing 60,000 miles a year uses around 75,000kWh — the annual output of an 86kWp array on its own. Fleet-scale eHGV operations will draw multi-megawatt-hours that no depot roof fully covers, which is why truck depots treat solar as a meaningful cost reducer (and a hedge on a volatile commodity) rather than a complete fuel source. The infrastructure conversation there is led by supply capacity and DC hardware; the solar follows.

The three corrections to the simple maths

Degradation and weather variance: panels degrade ~0.4% a year and yields swing ±10% year to year. Size with a small margin if a contractual green-fleet claim depends on the number.

Roof reality: orientation, shading, rooflights and edge zones mean usable roof is always less than gross roof. A desk study from aerial imagery sorts this in an afternoon.

Building load comes first: the building under the roof eats its share of generation before the vans see any. The correct model runs building plus fleet as one demand profile against the array — which is exactly what a half-hourly feasibility does.

The useful conclusion

Thirteen panels per van is the planning number; your parking schedule is the truth serum. A 20-van fleet needs roughly 120kWp to balance its energy on paper — and somewhere between 10% and 60% of those electrons will physically be solar depending on when the vehicles sit still. Get the rota and twelve months of meter data into a proper model before buying anything, because the model is cheap and oversized inverters are not.