How Much Solar Battery Storage Do You Actually Need? A Real-Numbers Sizing Guide

Last updated: 2026-06-30

Here is the short answer: most homeowners need 10–20 kWh of usable battery capacity to cover essential loads through one overnight cycle. If you want two days of autonomy — which is what most power outages actually require — double it.

But "most homeowners" is doing a lot of work in that sentence. A 1,200 sq ft condo in Phoenix has almost nothing in common with a 3,500 sq ft home in Vermont that relies on an electric well pump and a medical CPAP. The only way to get the right number is to do a five-minute load calculation — and that is exactly what this guide walks you through.

Why Battery Size Is the Decision That Determines Everything Else

Picking a battery brand before you know your required capacity is like buying a car before you know how many passengers you need to carry. The price difference between a 5 kWh system and a 20 kWh system is $5,000–$15,000. Getting this wrong in either direction costs you real money.

Go too small and your battery drains before sunrise, leaving you back on grid power — or in the dark during an outage — at the exact moment you needed it most.

Go too large and you pay for storage capacity you will never use, pushing your payback period out by years.

The sizing calculation below takes about five minutes. Do it before you talk to a single installer.

Step 1: Calculate Your Daily Essential Load

Start by listing the appliances you genuinely cannot go without during an outage. Not everything in your house — just the essentials. Here are the real average wattages for common loads:

|---|---|---|---|

| Refrigerator | 150 | 24 (cycles) | ~1.5 |

| LED lighting (whole house) | 200 | 6 | 1.2 |

| Phone + router + modem | 50 | 24 | 1.2 |

| CPAP machine | 30–60 | 8 | 0.3–0.5 |

| Ceiling fans (2) | 120 | 8 | 1.0 |

| Window AC unit (12,000 BTU) | 1,200 | 6 | 7.2 |

| Electric well pump | 1,000 | 1 | 1.0 |

| EV charger (Level 1) | 1,440 | 4 | 5.8 |

Notice how a single window AC unit chews through more energy in a day than a refrigerator runs all week. Central air is typically 3,000–5,000 watts and is the single biggest variable in any battery sizing calculation.

The honest guidance: If you want air conditioning during an outage, plan for significantly more capacity — or plan to run it sparingly. Most homeowners doing a whole-home backup calculation land somewhere between 15–30 kWh daily for essential loads with cooling, or 4–8 kWh daily without it.

Your Essential Load Worksheet

Add up your required daily kWh using the table above as a reference. Use your actual appliance labels or a $15 Kill-A-Watt meter for precise numbers. Once you have your daily total, move to Step 2.

Step 2: Decide How Many Days of Backup You Want

This is a lifestyle and risk tolerance question, not a math question. Ask yourself: what is the longest power outage you realistically want to weather without running a generator?

According to U.S. Energy Information Administration data, the average American experiences about 1.2 outages per year lasting roughly 3–4 hours each. But averages hide the tail events — ice storms, hurricanes, and grid failures that can knock out power for 3–7 days.

Most homeowners land in one of three categories:

Basic overnight coverage (1 day): You want to keep the fridge cold, run lights, and sleep comfortably. You are okay calling a hotel or a relative if the outage goes past 24 hours. Target: your daily essential kWh × 1.

Weekend resilience (2–3 days): You want to handle a long weekend outage without disrupting your household significantly. You may have a home office, small children, or a medical device user in the home. Target: your daily essential kWh × 2–3.

True off-grid buffer (5+ days): You live in an area with frequent extended outages, have a well pump, or simply want maximum independence from the grid. Target: your daily essential kWh × 5, offset by expected solar production.

Step 3: Factor In Solar Charging

If you have solar panels — or are buying a battery as part of a new solar install — your panels will recharge the battery every day the sun shines. This significantly changes the math.

A 10 kW solar system in a medium-sun state like North Carolina produces roughly 35–40 kWh on a good day. Even in winter or partially cloudy conditions, you might see 15–20 kWh. That means a battery that covers your overnight draw (say, 8 kWh of essentials) gets fully recharged by mid-morning.

In this scenario, even a single 13.5 kWh battery gives you multi-day resilience as long as the sun comes out each day.

The calculation becomes:

Net daily battery draw = Daily essential load − Expected daily solar production

During extended cloudy weather or winter in northern states, factor in 25–50% of your peak solar production rather than the full figure.

The Right Battery Size for 3 Common Scenarios

Rather than leaving you with a formula, here are three real homeowner profiles and the battery sizes that actually make sense for each.

Scenario 1: Outage Insurance (No Cooling Needed)

Profile: 2,000 sq ft home, no well pump, no medical devices, mild climate. You want to run the fridge, lights, internet, and ceiling fans through a 24-hour outage.

Daily essential load: ~5 kWh

Target capacity (1.2 days × efficiency buffer): 8–10 kWh usable

Good fit: A single Tesla Powerwall 3 (13.5 kWh), Enphase IQ Battery 5P pair (10 kWh), or an EcoFlow DELTA Pro 2 (4 kWh, expandable to 12 kWh with an extra battery).

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The limitation is obvious: you cannot run your HVAC, electric stove, or well pump from a portable unit. But for a significant portion of homeowners reading this, the portable path is the honest right answer — not a compromise.

What Battery Specs Actually Mean (And Which to Ignore)

Manufacturers love to advertise "capacity" numbers that require a footnote to understand. Here is a quick translation guide:

Total kWh vs. usable kWh: Most lithium iron phosphate (LFP) batteries are rated at 80–100% depth of discharge, meaning usable kWh is close to the rated number. Older lithium NMC batteries sometimes have usable capacity 80% of rated. Always ask for usable kWh.

Round-trip efficiency: Expect 90–95% for LFP batteries. For every 10 kWh you put in, you get 9–9.5 kWh back out. Build a 10% buffer into your sizing to account for this.

Power output (kW) vs. storage (kWh): A battery's kW rating tells you how many appliances it can run simultaneously. A 5 kW continuous output means you can run a well pump (1 kW) and a microwave (1.2 kW) and a refrigerator (0.15 kW) at the same time without issue. If you have a 240V appliance like a heat pump or electric dryer, confirm the battery supports split-phase 240V output — not all do.

Cycle life: LFP batteries typically last 3,000–6,000 cycles before degrading to 80% capacity. At one cycle per day, that is 8–16 years. This is why LFP has effectively become the standard for home storage.

Getting Your Accurate Sizing Recommendation

The calculation above will give you a solid estimate, but your specific roof, utility rate structure, and installer options will refine it further. Time-of-use rates in particular can shift the optimal battery size — if your utility charges 3× as much during 4–9 PM, a larger battery that captures midday solar production and discharges during peak hours can dramatically improve your payback.

The most efficient way to get accurate, customized sizing recommendations is to get competing proposals from certified solar installers. EnergySage lets you post your project once and receive multiple proposals from pre-vetted installers — each proposal includes system size, battery capacity, and projected savings based on your actual utility rate.