Getting the math right on solar-plus-storage sizing — and why the direction your panels face changes everything during an outage.
When homeowners ask us about pairing solar with battery storage, the conversation usually starts in the same place: “How many panels do I need?” But for homeowners who want real backup power — the kind that keeps the lights on through a nor’easter — the more important question is the ratio between your solar system and your battery bank.
Get it right and your home can ride out a multi-day grid outage with confidence. Get it wrong and you’ll have a beautifully sized solar array that runs out of stored power by 10pm every night.
The Baseline Ratio
For a typical Northeast home with average sun exposure — around 4 to 4.5 peak sun hours per day — the optimal starting point for backup focused sizing is:
1:23kw AC of solar to kWh of storage
A 10 kW AC solar system pairs well with 20–30 <br>kWh of battery capacity.
Here’s the logic behind that number. During an extended outage, your battery has two jobs: carry your overnight load, and recharge fully from solar the next day so you can do it again. In the Northeast, a 10 kW system might look like it generates 40 kWh on a good day — but real world yield after inverter losses, temperature derating (panels lose efficiency in cold), and partial shading typically lands closer to 28–34 kWh.
Pairing that with 20–30 kWh of storage means you have 12–18 hours of typical whole-home backup at a 1.5–2 kW average draw, with enough morning generation headroom to refill before the next night cycle begins.
“The battery doesn’t just need to last the night — it needs to be full again by sundown the next day.”
East vs. West: Why Array Orientation Changes the Equation
Most solar sizing guides assume south-facing arrays. But a significant number of homes in the Northeast — especially those on wooded lots or with particular roof lines — end up with east or west-facing arrays. Orientation has almost no effect on your annual production numbers (both directions yield roughly 80–85% of a true south array), but it changes when your solar generates during the day. And during an outage, timing matters enormously.
East-Facing Array
Generation peaks in the morning and tapers off by early afternoon. By the time your household’s evening load kicks in — cooking, lighting, heating — your panels are largely done for the day.
This means your battery has to carry a longer overnight gap before the next morning recharge begins. You’re effectively asking more of your storage.
RECOMMENDATION: +15–20% STORAGE. TARGET 2.4–3.5 KWH PER KW AC.
West-Facing Array
Generation peaks in the late afternoon, which naturally aligns with peak household load — exactly when you need it most. Your battery enters the night more fully charged.
The vulnerable window is morning, before your west array wakes up. For most households this is manageable, but worth planning for.
RECOMMENDATION: NO RATIO CHANGE. ADD 10% IF MORNING LOADS ARE CRITICAL (MEDICAL EQUIPMENT, EV).
BOTTOM LINE ON ORIENTATION
West-facing systems have a hidden advantage for outage resilience — their production timing aligns better with household demand patterns. East-facing systems require more storage to compensate for the longer overnight gap between production windows.
Northeast-Specific Factors That Change the Math
WINTER DERATING
Cold temperatures actually improve panel efficiency slightly, but snow cover is the real variable. A nor’easter can blanket an east-facing array for 48–72 hours with zero solar generation. For homes relying on battery backup in winter, we recommend sizing storage to bridge at least two to three days at minimum load — typically 800W to 1.2 kW for critical circuits only.
HEATING LOAD
This is where sizing gets serious. Running a mini-split heat pump on backup power can add 1–2 kWh per hour of runtime. If heating is part of your backup plan, your storage needs can double. A system sized for a “lights and fridge” load profile looks very different from one that keeps a bedroom warm through a January outage.
INVERTER ARCHITECTURE
This detail is often overlooked: hybrid inverters from Enphase, SolarEdge, and other manufacturers handle solar-to-battery charging
differently when operating in island mode (off-grid during an outage). Some systems limit the charging rate from solar while islanded. Before finalizing your system design, verify that your inverter’s self-supply specification can actually absorb your full array output — otherwise your solar may be producing more than your battery can accept.
Quick Sizing Reference
Critical loads only (lights, fridge, a few outlets): 8–10 kW solar / 20–24 kWh storage → 48–72 hrs coverage on a cloudy day
Partial home (+ one mini-split zone): 10–12 kW / 27–34 kWh → 36– 48 hrs coverage
Whole-home backup (+ EV trickle charge): 14–18 kW / 40–54 kWh → 36–48 hrs coverage
When Bigger Storage Stops Helping
There’s a point of diminishing returns around 3 kWh of storage per kW AC. Beyond that ratio, you’re essentially sizing for multi-day storm scenarios with little to no solar recharge — a completely valid goal if you’re in an area prone to extended outages, but a different design problem that often warrants a whole-home generator as a complement rather than ever-larger battery banks.
For most Northeast homeowners, the 1:2–3 ratio with orientation adjusted tweaks gets you to a system that can handle the vast majority of grid outage scenarios: a couple of winter storms, the occasional summer line fault, and the multi-day events that have become more common across the region.
The goal isn’t to build a bunker — it’s to build a home that keeps going when the grid doesn’t.
Questions about sizing a solar-plus-storage system for your home? Venture Home’s energy advisors work with homeowners across the Northeast to design systems built for real-world resilience — not just payback calculators. Get in touch for a free energy assessment.