EV Charging Cost Calculator for Business: Solar, Grid or Public

How the EV charging cost calculator works

1. Vehicle energy. Vehicles × miles per vehicle × kWh per mile. The kWh figure is measured at the wall, so charging losses are already inside it. The car preset is 0.294 kWh a mile, the 3.4 miles per kWh that Zapmap’s own per-mile conversions imply; the van preset (0.40 kWh a mile) is an illustration, not a published figure — use your own vehicles’ data.
2. Array output. Array kW × the yearly yield for the city you choose: 759 to 951 kWh per kW across the eight cities, from PVGIS hourly data for 2005–2023 at a 10° pitch facing south with 14% system losses.
3. Solar share. The calculator divides array output by vehicle energy and looks up what fraction of the vehicles’ energy that array can supply when vehicles are plugged in only during the window you choose. Those lookup tables come from an hour-by-hour simulation described below.
4. Cost per source. Public chargers and grid power are energy × price. Solar kWh are valued two ways: at the array’s lifetime capital cost per kWh (array cost ÷ 25 years of output with 0.5% a year degradation) and at the export payment you give up by not sending them to the grid.
5. Payback. The array’s yearly value is counted as vehicle kWh at your grid price plus everything left over at your export price. That is deliberately cautious: it assumes the building uses none of the rest. The last row of the results shows the payback if it used every kWh.

Why parking hours decide how much solar a vehicle can use

A solar array only produces in daylight, and most of its yearly output arrives in the middle of the day and in summer. A vehicle can only use electricity while it is plugged in. For a fleet that is parked at the depot overnight, almost none of the array’s output falls inside the window. The simulation behind the calculator puts numbers on that for London:

Read the table across. For a workplace where cars are parked 09:00–17:00 on weekdays, about 60% of an array’s yearly output is produced while they are there. An array whose yearly output equals half the vehicles’ yearly demand can supply about 30% of their energy; an array equal to their whole demand about 56%, because on summer weekdays it produces more than the cars can absorb. For vehicles parked only from 18:00 to 07:00, just 3% of the array’s output falls in the window, so even an array as large as the vehicles’ annual demand supplies about 3% of their energy. Without a battery, an overnight fleet gets almost nothing from solar; the value of the array then comes from what the building uses and exports.

Smart charging changes the answer

The same array supplies much less if chargers simply start at full power when a vehicle arrives. The model compares solar-following control — each hour’s charging is taken from the array first, with the grid topping up before departure — with plug-in-on-arrival, where charging starts at 7 kW per vehicle and runs until the vehicle is full.

At an array half the size of yearly demand, solar-following control supplies 30% of a workplace fleet’s energy against 7% to 20% when chargers start on arrival, depending on how much energy each vehicle needs a day. Vehicles that need only a little energy charge in an hour or two after arrival, before the sun is at its best; the smart controller waits for the middle of the day. That difference is why solar-following control belongs in the charger specification — see how solar EV charging works and the load management guide.

The seasons matter more than the city

Where you are in Britain changes yield by about 759 to 951 kWh per kW a year, but it moves the solar share very little once array size is expressed against demand. Season moves it a lot. For the London workplace pattern:

An array equal to yearly demand supplies about 81% of workplace charging energy in July and about 19% in December; at half that size, about 47% in June and 10% in December. Across the eight cities the yearly figures at array size 1× for the workplace pattern fall between 53% and 56%:

What the calculator leaves out

• Building load. The array is treated as dedicated to the vehicles, with any surplus exported. A building that also uses daytime power will take some of the output; size the array for both, or use the payback line that assumes the building uses every kWh.
• Minimum charging current. The charging standard IEC 61851 specifies a minimum charging current of 6 A per phase, which the evcc documentation turns into 1.4 kW single-phase and 4.1 kW three-phase. A fleet with several chargers can concentrate a small surplus on one vehicle at a time, which the model assumes; a site with one or two chargers and a small array will use less solar than shown unless the charger can switch between phases.
• Batteries. No storage is modelled. A battery could move daytime output into an overnight window, at a cost the calculator does not estimate.
• Weekends and holidays. Vehicles are assumed to need charging on weekdays only, with no bank-holiday exceptions.
• Money you may also save or spend. Capital allowances, the Workplace Charging Scheme or Depot Charging Scheme, finance costs, inverter replacement and maintenance are all outside the calculation — see capital allowances for solar and chargers and the Workplace Charging Scheme.
• Yields. The hourly series averages about 1.5% below the PVGIS multi-year figures quoted on our cost page (for example 904 against 920 kWh per kW for London).

Where the starting numbers come from

The calculator’s arithmetic was checked against an independent implementation on several hundred random scenarios before publication. Prices and costs checked 14 October 2026.