How to Size Your Home Battery System (2026 Guide)
By Rowan Watts | Updated September 2026
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Buying a home battery is one of the few purchases where the wrong size is worse than the wrong brand. Too small and it dies two hours into an outage. Too big and you paid thousands of dollars for capacity you never touch. The good news: sizing a home battery system is four simple steps, and the math fits on an index card. This guide walks through the method, then runs three worked examples and matches them to real, currently available products.
The 4-Step Sizing Method at a Glance
- List the loads you want to back up
- Multiply each load’s watts by the hours it runs to get watt hours
- Add a 20% buffer for surges, aging, and rounding errors
- Divide by the battery’s usable capacity, which depends on its chemistry
That is the whole method. Now let’s do each step properly.
Step 1: List Your Loads
Write down only the devices you actually want to run during an outage. Not your whole house, unless you genuinely have the budget for that. A typical essentials list looks like this:
- Refrigerator
- A few LED lights
- Internet router and modem
- Phone and laptop chargers
- Maybe a CPAP machine, a table fan, or a microwave for reheating food
- Inverter efficiency losses (roughly 5 to 10% of energy is lost converting DC battery power to AC)
- Battery aging, since capacity fades a few percent per year
- Cold weather, which temporarily reduces usable capacity in lithium batteries
- The OUPES Mega 5 stores 5,040 Wh (roughly 4,500 Wh usable) with a 4,000W inverter. That covers just over 2 days of the full essentials list, or closer to 3 days if you raise the fridge to its warmest safe setting and skip the microwave.
- The Bluetti AC200L stores 2,048 Wh with a 2,400W inverter and accepts heavy solar input, so it covers about one full day and recharges from panels between outages.
For each item you need two numbers: running watts and hours per day you expect to run it. Running watts are on the appliance label or in the manual. The “Lighting” section of the best home energy monitors can also measure real-world draw, which beats guessing because labels often overstate or understate actual use.
Watch out for startup surge. Compressors in fridges and well pumps can pull 2 to 3 times their running wattage for a second when they kick on. Your battery’s power rating (in watts) has to clear that spike, not just the steady draw.
Step 2: Multiply Watts x Hours
Energy is power over time. A 100 watt device running 5 hours uses 500 watt hours (Wh). Utilities measure in kilowatt hours, so 1,000 Wh equals 1 kWh. Multiply every item on your list, then add the column up for a daily total.
One honest correction to old sizing advice: a refrigerator does not run 24 hours a day. The compressor cycles, so most fridges average 1.0 to 1.5 kWh per day, not the 3 to 4 kWh you get if you naively multiply the nameplate watts by 24. Sizing around cycling reality saves you from oversizing.
Step 3: Add a 20% Buffer
Multiply your daily total by 1.2. This buffer covers three things people forget:
The buffer is also what turns “the math says it fits” into “it actually fits on a bad day.”
Step 4: Divide by Usable Capacity (Depth of Discharge)
Here is where chemistry matters. Depth of discharge (DoD) is how much of a battery’s rated capacity you can actually use without hurting it.
LFP Usable Capacity vs Lead-Acid
Modern home batteries use lithium iron phosphate (LiFePO4 or LFP) chemistry, and most manufacturers rate it for 90% or better depth of discharge. A 1,000 Wh LFP battery delivers roughly 900 usable watt hours.
Traditional lead-acid batteries are rated for about 50% DoD. Cycle them deeper than that regularly and their lifespan collapses. So a “100 Ah, 12V” lead-acid battery (about 1,200 Wh on paper) gives you roughly 600 usable watt hours. To match one 1,000 Wh LFP unit you would need two lead-acid batteries of the same paper rating, which then live in your garage, weigh more than a washing machine combined, and need occasional maintenance.
For a 12 volt DIY bank, the math is simple with LFP. A 12V 100Ah LiFePO4 Group 31 battery stores 100 Ah x 12.8V = 1,280 Wh, with essentially all of it usable and no venting or watering. Two of them in series make a 24V bank; four make 48V. To match that same usable energy with flooded lead-acid, which only delivers about half its rated capacity, you would need roughly eight 12V 100Ah batteries.
So when you compare products, always divide by usable capacity, not the number on the box.
Worked Examples
Example 1: Essentials Only (Fridge, Lights, Router)
| Load | Watts | Hours/Day | Wh/Day |
|---|---|---|---|
| Refrigerator (cycling) | 150 | 8 | 1,200 |
| LED lights | 40 | 6 | 240 |
| Router + modem | 15 | 24 | 360 |
| Phone and laptop charging | 100 | 1 | 100 |
| Daily total | 1,900 |
Add the 20% buffer: 1,900 x 1.2 = 2,280 Wh per day.
A single EcoFlow DELTA 3 Plus stores 1,024 Wh with about 920 Wh usable (LFP) and 1,800W of output, which easily clears a fridge compressor surge. That runs the essentials list for roughly 10 hours, or the fridge and lights alone for most of a day. To cover the full 2,280 Wh target you would add EcoFlow’s expansion battery later. The limit to know about: like every product in this guide, it will not run central air conditioning or an electric water heater.
Example 2: Home Office Backup
| Load | Watts | Hours/Day | Wh/Day |
|---|---|---|---|
| Laptop + monitor | 95 | 9 | 855 |
| Router + modem | 15 | 24 | 360 |
| LED lights | 40 | 6 | 240 |
| Phone charging | 60 | 1 | 60 |
| Daily total | 1,515 |
With the buffer: 1,515 x 1.2 = 1,818 Wh.
The Jackery Solar Generator 1000 Plus stores 1,264 Wh (about 1,140 Wh usable) with a 2,000W inverter and includes a 200W solar panel for recharging, so it covers this laptop-based workday comfortably, though not the full 24-hour total the example computes. Honest limits: swap the laptop for a gaming desktop and a big monitor and the draw roughly triples, which cuts runtime to a few hours. The unit is expandable with Jackery battery packs if your office grows. If you want a deeper dive on how this brand stacks up, see our Jackery vs EcoFlow vs Bluetti comparison.
Example 3: Multi-Day Outage Backup
Start from the essentials list in Example 1 (1,900 Wh per day) and plan for 3 days: 1,900 x 3 x 1.2 = 6,840 Wh.
The tradeoff at this tier is price per usable Wh and weight. Past about 3 days of full home backup, purpose-built whole home battery energy storage systems wired to your panel become the better value, especially if you have solar.
Sized Picks: Matching Products to the Math
| Your goal | Daily need (with buffer) | Pick | Usable capacity | Honest limit |
|---|---|---|---|---|
| Overnight essentials | About 2,300 Wh | EcoFlow DELTA 3 Plus | About 920 Wh | Covers 10 to 12 hours, not a full day |
| One workday of office gear | About 1,800 Wh | Jackery Solar Generator 1000 Plus | About 1,140 Wh | Covers the workday, not the full 24-hour total |
| 2 to 3 day outage | About 6,800 Wh | OUPES Mega 5 | About 4,500 Wh | No central AC, needs load discipline |
| Rechargeable day-one backup | About 2,300 Wh | Bluetti AC200L | About 1,850 Wh | One day per charge without solar |
Read that last column twice. None of these products run a central air conditioner, which alone can draw 3,000 to 5,000 watts for hours. If cooling during outages is your priority, right size a unit to run fans and a window unit instead, or read our guide to home energy storage for power outages for the full strategy.
FAQ
Q: What size battery do I need to run just my refrigerator?
A: Most refrigerators average 1.0 to 1.5 kWh per day with compressor cycling. With a 20% buffer you want about 1,200 to 1,800 Wh of storage, and any battery with at least a 600W output rating to handle the compressor surge. A 1,000 Wh class power station runs the average fridge for roughly half a day to a day depending on how often the door opens.
Q: Is depth of discharge the same as usable capacity?
A: Usable capacity equals rated capacity multiplied by depth of discharge. A 2,000 Wh battery with 90% DoD gives 1,800 usable watt hours. The same paper rating at 50% DoD, typical for lead-acid, gives only 1,000.
Q: How many batteries do I need for a 3 day outage?
A: Take your daily essentials total, multiply by 3, then by 1.2 for the buffer. For a 1.9 kWh per day essentials list that is about 6.8 kWh, which is one 5 kWh class unit plus load discipline, or two 2 kWh class units.
Q: Will a home battery run my central air conditioner?
A: Almost certainly not a portable or plug-in one. Central AC needs 3,000 to 5,000W sustained, which exhausts even large units in under 2 hours and exceeds many inverters entirely. Size your battery for refrigeration, lighting, and devices, and handle cooling with fans or a window unit.
Q: Can I start small and expand later?
A: Yes, and it is the smartest budget move. Pick a system with stackable or daisy-chainable expansion batteries, size the first unit to your essentials list, and add capacity only when a real outage teaches you what you actually missed.
The Bottom Line
Sizing a home battery system is not a mystery and it does not require software. List your loads, multiply watts by hours, add 20%, and divide by usable capacity. For most households the honest answer lands between 1 and 5 kWh: one 1 kWh class unit keeps the food cold overnight, a 2 kWh class unit covers a workday, and a 5 kWh class unit buys you a genuine multi-day outage cushion. Buy for the loads you actually need to run, expand when experience tells you to, and no salesperson’s spec sheet will ever size your system for you again.
Last updated: September 2026
