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

This guide assumes 3.5 miles per kWh (0.286 kWh a mile) for a van — an illustrative figure, not a published one; real consumption varies widely with the vehicle and its load, so use your own vehicle’s figure. A van covering 18,000 miles a year then uses about 5,100kWh.

Yearly output depends on location: PVGIS hourly data for a 10° south-facing roof give 759–951 kWh per kWp across eight British cities (904 in London, 861 in Birmingham, which is close to the 870 used here 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. What that array costs per kW, on the latest government data, is in solar EV charging costs.

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. In the hourly model behind the cost calculator, vans parked 10:00–16:00 on weekdays take about 25% of their energy from an array half the size of their yearly demand, and about 49% from an array equal to it, when chargers follow the sun; chargers that simply start at full power on arrival take less. The balance comes from the grid.

Overnight-only fleets see the array and the vehicles miss each other. In the same model, vans parked from 18:00 to 07:00 take about 3% of their energy from solar even with an array as large as their yearly demand, because only about 3% of a year’s array output falls inside that window; the figure rises only if vehicles also sit in the yard at weekends or a battery shifts daytime output. The array still earns its keep displacing building load and through exports, but the vehicles charge mostly from the grid, ideally on a cheap overnight tariff.

Neither case breaks the business case — overnight depot charging on grid power still costs about 69% less than public rapid charging (24.14p against Zapmap’s August 2026 average of 77p) — 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; the guide to how solar EV charging works explains why.

HGVs: different league

An electric truck uses several times a van’s energy per mile, so take the manufacturer’s figure. As an illustration only, at 1.25 kWh a mile one tractor unit doing 60,000 miles a year would use 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, so solar is a cost reducer rather than a complete fuel source. The infrastructure conversation there is led by supply capacity and DC hardware; the solar follows. The grant that can part-fund depot chargers for vans, HGVs and coaches is the depot charger grant.

The three corrections to the simple maths

Degradation and weather variance: the median published degradation rate is 0.5% a year (Jordan & Kurtz, 2013), and London’s yearly output ranged from 861 to 978 kWh per kWp between 2005 and 2023, a spread of about 14%. Size with a 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 wider case for solar + EV charging on commercial sites starts from the same model.

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 in the hourly model about 49% of those electrons are solar if the vans sit in the yard from 10:00 to 16:00, against about 3% if they are parked only overnight. Run your own rota through the cost calculator, then put twelve months of meter data into a proper model before buying anything, because the model is cheap and oversized inverters are not. The fleet depot charging guide works a 20-van depot through with costs.