Where This Guide Comes From

This guide follows Andy from SolarReviews, who walks through sizing a home solar battery using their free online solar calculator — the same estimating software installers use, made available to consumers without registration. He uses a real New Jersey example and shows the three different ways you can size storage. The video is right here:

SolarReviews — "What Size Solar Battery Do You Need?"
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150,000+ views. Andy runs a real New Jersey home through a solar-plus-battery sizing exercise — surplus storage, whole-night backup, and the load limits nobody mentions.

The core message is worth stating up front because it contradicts what most people assume: there is not a lot of point to a solar battery that you can never fully charge. Your battery size has to be matched to the surplus energy your solar system can actually produce — or you're carrying thousands of dollars of idle capacity.

Why Batteries Now: Blackouts and Net Metering

Until a couple of years ago, 90–95% of homes with solar panels had no battery at all — just panels and an inverter. Here's the catch most people didn't realize: under the NEC electrical code, grid-tied inverters have to switch off during an outage to protect utility workers on the lines. So you could have a full solar system on your roof and zero power in your home during a blackout.

Modern battery systems use an inverter that can island the solar system from the grid during an outage, so panels plus battery keep powering the home. People are also buying batteries for a second reason: regulators are signaling that one-for-one net metering won't be around much longer, which changes the economics of selling surplus solar back to the grid.

The Golden Rule: Match the Battery to Your Surplus

Solar produces most of its energy mid-day — roughly 9am to 3pm — when a typical home isn't using much power. That's the surplus you're storing. Two constraints frame the whole decision:

In the video's example, the New Jersey home (about $200/month electric bill) needs roughly a 14kW solar system to cover its usage. That's the starting point; then you decide what the battery is for.

Sizing Method 1: Store Your Excess Solar (Most Economic)

The most economic battery size is simply one that stores the surplus your solar system produces on a typical day. For the example home, that worked out to about a 32kWh battery bank — roughly three Tesla Powerwalls — filled by the solar surplus shown in the calculator's 15-minute-interval chart.

Why this is the most economic option: every kWh of battery is paid for by energy you'd otherwise sell back to the grid at a low rate (or, with weakening net metering, not be paid for at all). You're not buying backup insurance you don't need — you're buying storage sized to what your panels actually make.

Sizing Method 2: Run All Night on Battery

The second sizing philosophy: you want to run an average day's evening and night loads from the battery, with the grid as backup for the rare bad-weather week. This is the "run all night off my battery" option.

This raises the bar. Instead of matching storage to leftover mid-day surplus, you're matching it to your evening and night consumption — typically larger, and it assumes your solar array is big enough to refill the battery every day. The calculator lets you adjust loads (lights, fridge, central AC) and recalculates the required capacity. This is where "I want my central AC backed up for six hours" leads to a very large system — which is the moment most people reconsider their goals.

Sizing Method 3: Back Up Specific Loads (The AC Question)

The third and most realistic approach: decide which circuits actually need backup, and size for those. This is where a good battery designer earns their fee.

The constraint most buyers don't know: in backup mode, most home batteries can only serve 5–7kW of instantaneous load. You can't run your central AC and electric stove simultaneously off battery. So the honest conversation with an installer is:

Most systems use a backup gateway that wires selected circuits to be backed up, not the whole house. And most people, the video notes, accept the AC limitation — because accepting it can save roughly $20,000 of battery storage. That's the real trade at the heart of every sizing decision.

The practical sizing shortcut: list your must-run loads during an outage (fridge, lights, router, phone charging), add their running watts, multiply by the hours you want backup, and that number is your minimum battery size. Then check whether your solar surplus (or off-peak grid charging) can fill it in a day. If not, shrink the battery or grow the array.

Flat Rate vs Time-of-Use: Does the Battery Pay for Itself?

Batteries are expensive, and whether they ever pay for themselves depends on your utility's rate structure:

That's why the video's advice is to talk to a good local installer who knows your utility's tariff options — not a national sales operation — before committing to a size.

Bottom Line: How to Decide Your Solar Battery Size

  1. Start from your solar surplus, not from "bigger is better." A battery you can never fully charge is wasted money.
  2. Decide what the battery is for first: surplus shifting, whole-night running, or specific-circuit backup. Each has a different size and a different price.
  3. Remember the 5–7kW backup load limit. The AC question alone can swing the system cost by $20,000 — decide early whether you can live without it.
  4. Check your rate structure. Time-of-use billing can make a battery pay its way; flat rate makes it insurance.
  5. Let the roof set the ceiling. Panel space limits daily surplus, which limits useful battery size.
  6. Get a local installer's opinion before you buy. Small family-owned companies tend to be far more honest about the pros and cons than big sales-driven operations.

The honest summary: a solar battery turns an intermittent solar system into something closer to a baseline power source — you can use the energy whenever you need it, not just when the sun is out. But sizing it right is a goals exercise, not a wattage exercise. Decide what you need to keep running, and let that number drive the size.

And if you're not hunting for a consumer brand but need the home battery storage itself — a custom LiFePO4 pack at a specific voltage and capacity, an OEM or ODM project, or wholesale supply for your own battery line — that's what we do. Dajiu Energy is a Chinese manufacturer building LiFePO4 batteries for RVs, homes, golf carts and portable applications since 2017. Our product range covers LiFePO4 drop-in replacement batteries, EV and golf cart batteries, wall-mounted home storage, portable power stations, commercial energy storage and battery swap cabinets — explore the full lineup in our product center, or browse more buying guides. Tell us your voltage, capacity and size, and we'll engineer the pack to hit it.

Reviewed by Dajiu Energy Engineering Team — a Chinese LiFePO4 battery manufacturer building LiFePO4 batteries for RVs, homes, golf carts and portable applications since 2017, serving OEM/ODM projects with CE, UN38.3, MSDS and ISO9001 certified builds.

What Our Readers Asked (Top Comments on the Video)

These are the most-liked comments on the source video, with our practical answers. The original creator did not reply to these threads, so the answers below are ours — written the way we'd answer a customer on the shop floor.

@etiennelouw9244 · 29 likes
"I live in Cape Town, South Africa. I built my own system: 24V 50Ah battery with BMS, 4 x 100W panels in series, Victron MPPT and a 3000W inverter. It runs 2 small fridges and a chest freezer through 2-4 hour load shedding. I'm saving up to upgrade to 200Ah and then 48V."
Our take: This comment is a textbook example of sizing done right — and it's exactly the route we'd recommend for anyone dealing with scheduled outages. Start small and proven (24V, one battery), verify it covers your real loads, then scale capacity and voltage as budget allows. The 48V upgrade matters because at 200Ah the current gets serious: 48V halves the wiring losses and opens up bigger inverters. Also worth noting: chest freezers hold cold for hours, so a modest battery that cycles through load shedding beats a giant battery you wait years to afford.
@lordnetsplits3192 · 4 likes
"Are the amps on a power tool label the starting amps or the running amps?"
Our take: Good question, and the answer matters for sizing. The amp rating on a tool's label is typically the running current — what it draws during normal operation. But motors (drills, saws, compressors, fridges) draw a much higher surge current for a fraction of a second when they start — often 2–5x the running current. That's why inverters and batteries quote both continuous and surge ratings: the surge handles the start, the continuous handles the run. When you size a battery, use running watts for runtime math, but make sure your inverter's surge rating covers your biggest motor's startup draw, or the inverter trips on every start.
@bingbangboom1239 · 3 likes
"Rule of thumb: calculate your worst 5 days of winter (consumption minus generation) and that's your storage requirement. Then buy a 3kW generator to cover any overrun."
Our take: That's a genuinely sensible rule, and it matches how we'd approach a cold-climate off-grid home. Worst-5-days sizing covers the longest realistic no-sun stretch without buying 20 days of storage you'll never use. And the 3kW generator as a backstop is the pragmatic middle ground — it converts a 10-day weather event from "battery starvation" into "a few hours of generator run per day." The one refinement we'd add: after you size, make sure your solar array can refill that storage in a normal winter day; otherwise even the 5-day margin slowly erodes.
@johnsteed265 · 2 likes
"I was hoping for a straightforward answer: e.g., '3 Powerwalls will cover an average home.' Instead the video just tells me to get a quote from an installer. Disappointing."
Our take: Fair criticism, and here's the direct answer the video dances around. An average US home uses roughly 30kWh/day. To run a full day from battery you'd need ~30kWh of storage (about 3 Powerwalls or 3–4 of the 9–10kWh class batteries), and a solar array big enough to refill it — around 10–14kW depending on your climate. But that's the "run everything, no solar for a day" answer. If your real goal is outage protection for fridge, lights and Wi-Fi, you're looking at a far smaller system — 10kWh or less. The honest reason installers quote rather than give a number is that "average" never matches your actual loads. The direct math is: list your must-run loads, multiply by hours, add 25% margin.
@freddy1571 · 2 likes
"I have a hybrid solar system and I monitor everything from my phone. It's the best feeling to see the battery at 100% and know I'm covered."
Our take: "The best feeling" is the part people don't put in the spec sheet — and it's real. Monitoring changes how you use the system: you see exactly when the battery fills, when the house starts drawing, and how much margin you have before a storm. Modern LiFePO4 batteries with BMS communication make this easy — CAN/RS485/Bluetooth interfaces let you see state of charge, cell voltage and cycles from your phone. If you're buying a system, ask for a battery that talks to your monitoring app, not just a dumb box.

Source video: SolarReviews — "What Size Solar Battery Do You Need?" (youtube.com/watch?v=Efzy8aNuuXw). Comment excerpts are quoted verbatim from the video's top comments; the original creator did not reply to these threads, so all answers are Dajiu Energy's own.