Solar + EV integration

EV solar battery triple sizing calculator

Household use, EV charging, and backup goals need to be sized together, not separately. Add all three for a combined panel count and battery size that actually works as a system.

Your household, EV, and backup needs

1 Project details Optional
2 Household and EV energy use
Check your utility bill's average daily usage, excluding EV charging.
From daily miles times vehicle efficiency, or check the EV charging load addition calculator.
3 Solar panel specs
Check NREL's PVWatts for a US address or NRCan's solar maps for Canada.
4 Battery backup goals
How many full days of outage coverage you want for essential circuits.
Fridge, well pump, some lighting, and internet, not full central AC.
Common home battery products range from about 10 to 16 kWh per unit.

How it works

Three steps to sizing all three components

Step 1

Combine household and EV load for solar sizing

Solar panels need to cover your total annual energy use, so household and EV load are added together in one calculation, not sized separately.

Step 2

Size battery around backup duration, not annual use

Battery sizing answers a different question: how many hours or days of essential load can you cover during an outage, which uses very different math than solar's annual energy sizing.

Step 3

Decide if EV charging matters during an outage

Most households let EV charging pause during a power outage and size the battery around essential circuits only. Including EV charging in backup sizing meaningfully increases the battery capacity required.

Understanding combined solar, battery, and EV sizing

Why solar and battery are two separate sizing questions

Solar panels need to be sized around total annual energy production, modeled by tools like NREL's PVWatts, to offset ongoing yearly consumption, which is a volume question answered in kWh per year. Battery storage, by contrast, needs to be sized around a specific backup duration scenario, a power question answered in kW sustained over a set number of hours or days. Conflating these two calculations is a common sizing mistake that either oversizes one component or undersizes the other.

Why most households exclude EV charging from backup sizing

Including EV charging in battery backup sizing dramatically increases the capacity needed, since a typical daily charging session can rival or exceed a household's entire essential backup load for a full day. Most installers instead size backup batteries around genuinely essential circuits, refrigeration, some lighting, internet, and perhaps a well pump, and let EV charging simply pause during an outage, resuming once grid power or solar production returns.

The EV itself is often already a backup resource

Even without including EV charging in the home battery's backup sizing, the EV retains whatever charge it had before an outage began, which can itself become a backup power source through V2H equipment if installed. This is a separate consideration from the home battery calculation here, covered in more detail by the V2H payback calculator and V2H battery replacement value calculator.

Battery capacity adds up fast for whole-home backup

Sizing for multiple days of backup at a meaningful load quickly requires a substantial number of battery units, which is why most residential backup systems target specific critical circuits rather than the entire home's typical daily draw. Understanding this tradeoff upfront, alongside the loan term comparison tool, helps set realistic expectations before requesting installer quotes.

What to do with these numbers next

Take the panel count to check roof capacity with the usable roof area calculator, and the battery sizing to the solar battery backup priority tool to think through exactly which circuits deserve backup coverage.

Questions

Frequently asked

Yes, sizing solar around household use alone and treating EV charging as a separate add-on later often leads to an undersized system. Combining both into one calculation from the start, as this tool does, gives a more accurate total panel count for full solar offset.

Not necessarily. Most households size battery backup around essential home circuits and let EV charging pause during an outage, since the EV's own battery still holds whatever charge it had before the outage started. Including EV charging in backup sizing significantly increases the battery capacity needed.

For 5 kW of essential backup load over 2 days, roughly 240 kWh of total capacity is needed, which works out to about 18 units of a common 13.5 kWh home battery product. This is a substantial and expensive amount of storage, so most households size backup for critical circuits only, not the whole home.

Solar panels need to be sized for total annual energy production to offset ongoing yearly use, while battery storage needs to be sized for a specific short-duration backup scenario, since batteries store energy rather than generate it. These are fundamentally different sizing questions that use different math.

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