This is the question I get asked on almost every battery job I do in New Jersey. And honestly, it’s the right question to start with — because the answer determines everything else: which battery you buy, how many, and whether your system actually does what you’re expecting it to do when the grid goes down at midnight in January.

I’ll give you the same calculation I use with my customers. No guessing, no upselling to more batteries than you need. Just the math.

For New Jersey homeowners: The stakes here are real. NJ averages over 2 hours of power outage per customer per year — and that average is dragged down by summers and winters with much longer events. PSE&G’s territory saw multi-day outages after Ida in 2021. JCP&L customers in Ocean County went days without power after Sandy. A battery system that’s right-sized for your home is the difference between riding out those events and suffering through them.

Step 1: Understand the Two Numbers That Define Every Battery

Before we get to how many batteries you need, let’s make sure you understand the two specs that actually matter — because I’ve seen too many homeowners focus on the wrong number and end up with a system that disappoints them.

Capacity (kWh) is how much energy the battery can store. Think of it like a gas tank. A Tesla Powerwall 3 has a 13.5 kWh tank. An Enphase IQ Battery 5P has a 5 kWh tank. This tells you how long the battery lasts at a given draw rate.

Power output (kW) is how fast the battery can deliver that energy. Think of it like the engine. A Powerwall 3 delivers 11.5 kW continuously. An Enphase IQ 5P delivers 3.84 kW. This tells you what loads the battery can actually run at one time.

You need both numbers to answer the real question. A battery with plenty of capacity but low power output can’t run your central AC even if it has hours of energy stored. A battery with high power output but small capacity can run everything — but only for a couple of hours before it’s depleted.

Step 2: Decide What You Actually Want to Back Up

This is where most homeowners need to stop and make a real decision, because the answer changes the battery count dramatically.

There are three common backup tiers I see in NJ homes:

Critical loads only: Refrigerator (150W), key lights (200W), internet router (20W), phone chargers, maybe a TV. Total continuous draw: 500–800 watts. At this level, a single battery lasts 12–18 hours or more. This is the “keep the food cold and stay connected” setup.

Partial home backup: Everything above plus central AC (3,000–4,000W continuous), well pump if you have one (750–1,500W), and some additional outlets. Total continuous draw: 4,000–6,000 watts. This is what most NJ homeowners actually want — comfortable through a summer night or a winter storm. One Powerwall 3 gets you 2–3 hours of AC backup without solar recharge. Two Powerwalls gets you through the night.

Whole-home backup: Everything running normally — multiple AC zones, electric range, EV charger, hot water if electric, pool equipment. Total continuous draw: 8,000–15,000+ watts for a large NJ home. This requires three or more batteries and a properly sized solar array to sustain it beyond one night.

Most NJ homeowners I work with land in the partial home backup category. They want to sleep comfortably during a summer outage, keep the refrigerator running, and stay connected. Two Powerwalls or three Enphase IQ 5P units typically covers this well.

Step 3: Pull Your Actual Energy Usage Numbers

Look at your last 12 utility bills. Every PSE&G, JCP&L, and ACE bill shows your monthly kWh usage. Add up all 12 months and divide by 365. That’s your average daily consumption in kWh.

For reference: a typical 2,000–2,500 sq ft NJ home with central AC uses 25–35 kWh per day in summer, 15–20 kWh per day in shoulder seasons, and 20–30 kWh per day in winter (more if electric heat). Your summer number is usually your planning number for battery sizing, because that’s when outages are most uncomfortable and when your daily consumption is highest.

Let’s say your summer daily usage is 30 kWh. You want partial home backup — AC, fridge, lights, internet. That’s roughly half your full consumption: 15 kWh per night at reduced AC runtime. One Powerwall (13.5 kWh) gets you most of the way through an overnight outage. Two Powerwalls (27 kWh) gives you comfortable margin and covers a full 24-hour outage if solar recharges them during the day.

Step 4: Factor In Solar Recharge

This is the calculation that most battery sales conversations skip, and it’s arguably the most important one for multi-day outages.

In New Jersey, a solar system produces roughly 1,100–1,200 watt-hours per day per installed kW of capacity. A 10 kW solar system in central NJ produces about 11,000–12,000 Wh (11–12 kWh) on an average day in October. In July, more like 13–14 kWh. In December, maybe 8 kWh.

Here’s why this matters: if your battery runs your home overnight and depletes, solar needs to recharge it the next day while also running your home during daylight hours. The math has to work in both directions.

Let’s say you have two Powerwalls (27 kWh) and a 10 kW solar system. During a summer outage: overnight, you draw down 15 kWh running partial home loads. At sunrise, your panels start producing. By noon you’ve produced 7 kWh. By 4 PM, another 7 kWh. Total day production: 13 kWh. Minus 5 kWh used for daytime loads, you’ve added 8 kWh back to the battery. You start the second night with 20 kWh. That’s comfortable.

Now do the same math for a December outage with 8 kWh daily production and heating loads instead of AC. The numbers change significantly. This is why battery sizing in NJ requires thinking about both summer and winter scenarios, not just the most common outage season.

Step 5: Check Peak Power Demand — Not Just Daily kWh

Your battery’s power output (kW) determines whether it can run your loads simultaneously — not just whether it has enough stored energy overall.

Here are the real power draws of common NJ home loads:

Central AC (3-ton): 3,500W continuous, up to 75A surge on startup. Central AC (5-ton): 5,000W continuous, up to 185A surge on startup. Well pump (1 HP): 750W continuous, 2,250W surge. Electric water heater: 4,500W. Refrigerator: 150W continuous, 500W startup. Sump pump: 800W continuous, 2,400W surge. Level 2 EV charger: 7,200W.

Add up the loads you expect to run simultaneously. That’s the power output you need from your battery. A single Powerwall 3 at 11.5 kW continuous handles: AC (3,500W) + well pump (750W) + refrigerator (150W) + lights and internet (500W) = 4,900W. Comfortable margin. But add a second AC zone (another 3,500W) and you’re at 8,400W — still within spec but tighter. Add an EV charger to that and you’re over the limit.

This is exactly why I build a load list with every customer. You can’t guess at this number.

The Real-World Answer for NJ Home Types

Based on 15 years of NJ installations, here’s what I actually see homes need:

1,200–1,600 sq ft home, gas heat, one AC zone: One Powerwall 3 is often sufficient for overnight critical and comfort loads. Consider two if you want reliable backup through a full 24-hour outage without solar dependence.

2,000–2,500 sq ft home, central AC, no EV: Two Powerwalls. This is the NJ sweet spot — comfortable whole-home backup overnight, solar recharges through a multi-day outage in summer. This is what I’d recommend for most homes in Marlboro, Middletown, and Howell.

2,500–3,500 sq ft home, multiple AC zones or well pump: Two to three Powerwalls. Run a load analysis first — the specific combination of heavy loads matters a lot here.

Home with current or planned EV: Add one battery to whatever you’d otherwise need. A Level 2 EV charger at 7,200W is a major load. Even if you don’t plan to charge your EV during an outage, the additional capacity handles the overall system balance better.

Home with electric heat pump or planning to install one: Plan for at least three batteries. Heat pump heating in a cold NJ winter is your most demanding load scenario — it runs almost continuously at 3,000–5,000W when it’s 20°F outside.

Why Oversizing Is Better Than Undersizing (Within Reason)

I never recommend buying more batteries than a home legitimately needs. But between undersizing and oversizing, oversizing by one battery is almost always the better mistake to make.

An extra battery means: longer backup duration during extended outages, a faster full recharge from solar, more comfortable load management (you don’t have to turn the AC off to preserve charge), and the ability to add EV charging or a heat pump without redesigning your system.

An undersized battery means: cutting AC runtime during outages, running out of charge before sunrise, and a frustrated family calling you asking why they bought this thing. I’ve gotten those calls. You don’t want to make that call.

What a Proper Battery Sizing Conversation Looks Like

When I sit down with a customer in Toms River or Bridgewater or Hamilton, here’s what we go through together before I recommend a single battery:

First, I pull 12 months of utility bills and calculate monthly kWh and average daily consumption. Second, I walk through every major load — AC, heat, water heater, well pump, any planned EV — and note whether it’s gas or electric, and what it draws. Third, I confirm the solar system size or propose one that matches both production and battery recharge needs. Fourth, I calculate the power draw of simultaneous loads and verify the battery’s output can handle them. Fifth, I ask about future loads: EV in the next three years? Heat pump? Pool? And I design for those now so the system doesn’t need to be redone.

That conversation takes 30–45 minutes. It’s the most important 30 minutes of the whole project. If an installer isn’t having it with you, ask why not.

Frequently Asked Questions

Can one battery power a whole New Jersey home during an outage?

For a small to medium NJ home with gas heat and one AC zone, a single Powerwall 3 can cover critical and comfort loads for 3–4 hours with AC running, or 12–18 hours on critical loads only. Most NJ homeowners with whole-home backup goals need two batteries minimum. One battery is a starting point for critical loads, not a complete whole-home backup solution.

How long does a battery last during a power outage in New Jersey?

It depends entirely on what you’re running. A single Powerwall 3 (13.5 kWh) lasts 3–4 hours running central AC continuously, or 12–18 hours on critical loads only. Two Powerwalls (27 kWh) gets you through most overnight outages with comfort loads running. With solar recharging during the day, you can sustain indefinitely through extended outages in summer.

How many Enphase batteries equal one Tesla Powerwall?

For equivalent storage: three Enphase IQ 5P units (15 kWh total) or two Enphase IQ 10C units (20 kWh total) versus one Powerwall 3 (13.5 kWh). For equivalent power output: two IQ 10C units (14.16 kW combined) slightly exceed the Powerwall 3 (11.5 kW). Always compare total kWh and total kW — not unit count.

Should I size my battery for summer or winter in NJ?

Size for summer first — that’s when outages are most uncomfortable and your daily consumption is highest due to AC load. Then check the winter math separately, especially if you have electric heat. If your solar production in December is significantly lower than summer, you may need an extra battery to maintain multi-day backup capability in winter months.

What’s the cost of adding a battery in New Jersey after the original solar installation?

Retrofitting a battery onto an existing NJ solar system typically costs $1,000–$3,000 more than installing it at the same time as the solar. The extra cost covers a new utility interconnection application (sometimes required), additional electrical work, and the mobilization cost of a second installation visit. This is one of the main reasons I recommend thinking through battery count at the time of the original solar installation — the incremental cost of a second battery upfront is almost always less than the cost of a retrofit later.

Ready to Size Your Battery System Correctly?

If you want a real load analysis for your NJ home — not a guess, not a upsell, just the right answer for your specific situation — reach out. I’ll walk through the numbers with you and tell you exactly what you need.

👉 Get Your Free Battery Sizing Estimate

📞 Call or text Jon directly at (609) 908-3700