Solar + EV Charging: Integrated System Design for Commercial Buildings

Commercial EV fleets are growing fast — and so is the electricity bill to charge them. Rooftop or carport solar can cover 60–90% of charging energy at near-zero marginal cost, but only if the system is designed correctly from the start. Get the inverter sizing wrong, ignore demand charges, or skip the battery buffer, and you end up with an expensive grid top-up that defeats the purpose.

Why integrate solar with EV charging?

The business case is straightforward. A Level 2 AC charger (22 kW) running 8 hours per day consumes roughly 64 kWh. At a commercial electricity tariff of $0.15/kWh plus demand charges, a 10-charger depot adds $35,000–60,000 per year in electricity costs before the fleet even grows. Solar at $0.04–0.06/kWh LCOE turns that into a manageable operating cost within a payback window of 4–7 years.

Beyond pure cost, many corporate sustainability mandates and fleet electrification programmes require documented renewable energy sourcing. On-site solar provides the most verifiable proof — the electrons flow directly from panels to vehicles without leaving the site boundary.

The three components you need

An integrated solar + EV charging system has three layers, each with its own sizing logic:

• Solar PV array — generates DC power from the roof or carport canopy
• Grid-tied inverter — converts DC to AC, synchronises with the grid, and manages energy flow; a hybrid inverter also manages battery charging and discharging
• Battery storage (optional but recommended) — buffers solar surplus for evening charging sessions and shaves demand spikes when multiple chargers activate simultaneously

The EV chargers themselves connect to the AC distribution board downstream of the inverter. Smart chargers with load management can communicate with the inverter to throttle charge rate based on available solar power, avoiding grid export or demand spikes.

Sizing the solar array for your EV load

The fundamental sizing question is: how much solar energy do your vehicles consume, and when do they consume it?

Step 1 — Calculate annual charging energy

Multiply the number of vehicles by average daily mileage by the vehicle's kWh/km consumption, then by 365. A fleet of 20 vans averaging 80 km/day at 0.25 kWh/km = 146,000 kWh/year. That is the energy target your solar system should aim to match or exceed.

Step 2 — Map charging hours to solar generation

This step is where most projects go wrong. If vehicles charge 07:00–09:00 (before peak solar) and again 17:00–19:00 (after peak solar), on-site solar directly offsets very little charging without a battery. If vehicles charge 09:00–15:00 (peak solar window), a battery-free system can achieve 70–85% solar self-consumption.

Step 3 — Size the array

A rough rule: solar capacity (kWp) ≈ annual EV energy (kWh) ÷ (peak sun hours × 365 × 0.80 system efficiency). For 146,000 kWh/year in a location with 4.5 peak sun hours: 146,000 ÷ (4.5 × 365 × 0.80) ≈ 111 kWp. Add 15–20% for future fleet growth.

Do you need a battery buffer?

Battery storage adds cost — typically $200–350/kWh installed for LFP systems — but it solves three problems that are otherwise difficult to manage:

• Timing mismatch: Solar peaks at noon; vehicles often charge early morning and evening. A 150–300 kWh BESS stores midday surplus and releases it during charging sessions, raising solar self-consumption from ~40% to ~75–85%.
• Demand charge spikes: Ten 22 kW chargers activating simultaneously draws 220 kW — a demand event that sets the entire month's demand charge. A BESS can limit the grid draw to a preset ceiling by ramping in as chargers activate.
• Grid connection limitations: Many commercial premises have a grid connection sized for existing building load. Adding EV chargers without a BESS may require costly grid upgrade works. A battery buffer lets you add charging capacity without touching the grid connection.

For sites where vehicles charge exclusively during daylight hours and grid demand rates are below $8/kW/month, a battery-free system is often adequate. For everyone else, the BESS pays for itself through demand charge savings alone — independent of the solar ROI.

Demand charge risk: the silent threat

This deserves its own section because it is the number-one cost surprise in EV fleet electrification. Commercial tariffs bill demand on the highest 15-minute average draw of the month. Ten 22 kW chargers activating at 08:00 when the building HVAC also starts up produces a 280–320 kW demand event. At $18/kW/month that is $5,400–5,760 per month in demand charges alone — before any energy charges.

Smart charge management software can stagger charger activation to keep simultaneous draw below a ceiling. Combined with a BESS (see our peak shaving guide), the ceiling can be held precisely regardless of how many vehicles plug in. This transforms demand charge exposure from an unpredictable variable into a fixed, controlled operating cost.

Inverter selection: string vs hybrid

For a straightforward daytime-charging-only installation with no battery and modest size (<50 kWp), a standard commercial string inverter is the most cost-effective choice. The Sungrow SG30CX to SG250HX range covers 30–250 kW per unit with high efficiency (up to 98.9%) and proven reliability.

For any system that includes — or plans to include — battery storage, a hybrid inverter (or an AC-coupled battery inverter) is required. The Sungrow SH series integrates solar MPPT, battery charge/discharge control, and grid interaction in a single unit, simplifying installation and monitoring. For large C&I sites above 50 kW, the SH250CX-V21 handles up to 250 kW AC output with multiple MPPT inputs.

Carport vs rooftop: which mounting works for EV depots?

EV fleet depots typically have large paved parking areas — ideal for solar carport structures. A carport does two things: it generates solar power and it provides covered parking, which extends EV battery life in extreme heat or cold climates. The dual function often makes the higher structural cost easier to justify to finance teams.

Key carport design considerations:

• Column spacing: must allow vehicle manoeuvre — typically 5.5–6 m between rows for vans, 8–10 m for trucks
• Cable routing: DC cabling from panel strings runs down columns to a combiner box; keep DC cable runs under 100 m to manage voltage drop
• Structural load: snow load (for CIS and northern European markets), wind uplift, and dynamic panel replacement access all affect the steel specification
• Module tilt: 5–10° is typical for carport structures — low tilt reduces wind load but slightly reduces annual yield; acceptable at latitudes below 40°

Our Econo Solar mounting systems include flat-roof ballast and custom carport frames engineered for commercial vehicle clearances.

ROI estimate: 100 kWp solar + 200 kWh BESS for a 20-van depot

• Solar generation: 100 kWp × 4.5 PSH × 365 × 0.80 efficiency = 131,400 kWh/year
• EV charging covered by solar: ~105,000 kWh (80% self-consumption with BESS)
• Grid energy avoided: 105,000 kWh × $0.15 = $15,750/year
• Demand charge saved: shaving 80 kW peak × $18/month × 12 = $17,280/year
• Total annual saving: ~$33,000
• Installed cost: 100 kWp solar at $0.65/Wp = $65,000 + 200 kWh BESS at $280/kWh = $56,000 + installation = ~$165,000
• Simple payback: ~5.0 years; IRR over 20 years >15%

These figures are indicative — your tariff structure, grid connection cost, and local labour rates will shift the numbers. Send us your utility bill and fleet data and we will model the project accurately for your site.

Key questions to answer before you design

1. What hours do vehicles charge, and can that schedule be shifted toward midday?
2. What is the existing grid connection capacity, and what demand tariff applies?
3. Is rooftop, carport, or ground-mount installation available?
4. Does the site have future fleet expansion plans that should be sized into the system now?
5. Is the site grid-connected or are there reliability concerns that require battery backup capability?