The solar + EV sizing worksheet I hand every client

"How much solar do I need to charge my EV?" is the most common question I get. The answer is almost always "less than you think," but the right answer requires a few numbers. Let me walk through how to actually calculate this.

Step 1: Know your EV's consumption
Find your car's real-world efficiency in miles per kWh (not the EPA estimate — check your car's lifetime average in the energy screen). Common ranges:

• Economy EVs (Leaf, Bolt): 3.5–4.5 mi/kWh
• Midsize (Model 3 SR, Ioniq 5): 3.5–4.0 mi/kWh
• Larger/AWD (Model Y, Mach-E): 2.8–3.5 mi/kWh
• Trucks (F-150 Lightning, Cybertruck): 1.6–2.2 mi/kWh

Step 2: Calculate your daily charging need
If you drive 37 miles/day (national average) in a Model Y at 3.2 mi/kWh, you need about 11.6 kWh per day to replace what you use. Add 10–15% for charging losses (heat, AC-to-DC onboard conversion) and round up: call it 13 kWh/day.

Step 3: Know your sun hours
"Peak sun hours" is the key metric — it's not total daylight, it's equivalent hours at 1,000 W/m² irradiance. Varies significantly by location:

• Phoenix: ~5.5–6.5 PSH
• Denver: ~4.5–5.5 PSH
• Chicago: ~3.5–4.5 PSH
• Seattle: ~3.5–4.0 PSH
  Look up your specific location on the NREL PVWatts calculator — it uses actual historical weather data.

Step 4: Size your array
Needed array size = Daily kWh need ÷ PSH × efficiency factor (typically 0.8 for system losses).
For our example: 13 kWh ÷ 4.5 PSH ÷ 0.8 = 3.6 kW of panels.
A 4kW system (8–10 standard panels at modern 400–500W ratings) will comfortably cover average EV charging for most drives in a mid-latitude location.

Step 5: Do you need batteries?
This is where it gets interesting. If you can charge during the day (work from home, Level 1 at home during daylight), you don't need batteries — just direct solar-to-charge. But if you charge at night, you need storage to bank the day's solar.
Rule of thumb: size your battery at 1–1.5× your daily charging need to handle a cloudy day. For our example: 13–20 kWh of storage.

The economics
A 4kW solar array + 15kWh battery system runs roughly $12,000–18,000 installed (post-federal tax credit, based on ~$2.50–$3.50/W for panels plus ~$500–$800/kWh installed for battery). At the 2024 national average of ~$0.165/kWh, charging 13 kWh/day saves about $783/year. Payback on EV charging alone: roughly 15–23 years. That's not a great standalone investment for EV charging alone.

But here's what changes the math: if you're also offsetting household electricity (which most people are), the system economics improve dramatically. Most homeowners with solar see 70–80% of their bill offset, which often means payback of 7–10 years on the whole system.

What size system are you running, and is your EV charging a significant part of it? I'm curious about people who sized specifically around EV charging vs. those who already had solar and just added EV charging.