Home Battery Backup Calculator
Size a home battery from your essential loads: daily kWh, usable capacity after depth of discharge and inverter losses, days of backup and batteries needed.
Last reviewed
Sizing a home battery backup starts with your essential loads, not your whole house. It's calculated as Daily Need (kWh) = Σ(Watts × Hours/Day) ÷ 1000, then compared against your battery's usable capacity after accounting for depth of discharge and inverter efficiency. This shows how many hours or days of backup you actually get, and how many batteries a target number of days requires.
Backup Duration
2.8d
Batteries for 3-Day Target
2
Peak Load vs. Inverter
2,565W
Essential Loads
Daily need: 4.32 kWh · Peak: 2,565W
Battery System
Solar Recharge
Generates ~24.0 kWh/day at an 80% real-world derate. This covers your daily essential need. The battery can recharge during daylight with a compatible backup inverter.
Essential Load Breakdown
| Appliance | Watts | Hrs/Day | Wh/Day |
|---|---|---|---|
| Refrigerator | 150W | 8h | 1,200 Wh |
| LED Lighting | 60W | 5h | 300 Wh |
| WiFi Router & Modem | 15W | 24h | 360 Wh |
| Phone/Laptop Charging | 40W | 4h | 160 Wh |
| Well/Sump Pump | 800W | 1h | 800 Wh |
| Window AC / Space Heater | 1500W | 1h | 1,500 Wh |
| Total Daily Need | 4,320 Wh (4.32 kWh) | ||
Usable Capacity Math
Size the Backup, Not the House
Backing up an entire home (including central air conditioning and an electric range) takes a battery bank most homeowners don't install. A far more common approach is a "critical loads" panel that backs up a short, deliberate list: refrigeration, lighting, internet and phone charging, and anything safety-related like a sump or well pump. This calculator is built around that list-first approach: you name each load's wattage and how many hours a day it actually needs to run, and the daily energy requirement follows from that, rather than from a single "average home" estimate.
The Core Formulas
A Worked Example
This calculator's default six loads (a 150W refrigerator for 8 hrs, 60W LED lighting for 5 hrs, a 15W router and modem for 24 hrs, 40W of phone/laptop charging for 4 hrs, an 800W well or sump pump for 1 hr, and a 1,500W window AC or space heater for 1 hr) add up to 4,320 Wh (4.32 kWh) per day, with a combined peak draw of 2,565 watts if everything ran at once.
A single 13.5 kWh battery (a common residential size) at 95% depth of discharge and 94% inverter efficiency delivers about 12.06 kWh of usable energy; this is enough for roughly 67 hours, or 2.8 days, of these essential loads. Reaching a 3-day backup target takes 2 batteries (24.11 kWh usable), and the 2,565W peak draw sits comfortably under a typical 5,000W continuous inverter rating.
Adding a 6 kW solar array at 5.0 peak sun hours/day (the U.S. national average per NREL) generates roughly 24 kWh/day at an 80% real-world derate, far more than the 4.32 kWh/day need. In principle that lets the battery recharge fully during daylight and extend backup indefinitely, but only with a battery and inverter combination built for "backup" or "islanding" mode, and only on days with something close to full sun (see limitations below).
What This Doesn't Cover
Every grid-tied solar inverter is required to disconnect automatically during a grid outage (anti-islanding is a safety requirement so the system can't feed power into lines utility crews may be working on). Solar only recharges a battery during an outage if the system is specifically designed for it. Outages are also often caused by the same storms that reduce solar output, so treat any "solar covers the need" result as a clear-sky best case, not a guarantee for the exact days you'll need it. This calculator also assumes a conservative "everything could run at once" peak load, doesn't model battery degradation over years of cycling, and doesn't include installation, permitting, or equipment costs. The 30% federal Residential Clean Energy Credit for battery storage and solar (Section 25D) is not available for property placed in service after December 31, 2025 per the IRS. This calculator does not assume or apply any tax credit.
Battery Backup Inquiries
?How much battery capacity do I actually need?
It depends entirely on which loads you're backing up and for how long, not a fixed number. This calculator multiplies each essential load's wattage by the hours per day you'd actually run it, sums that into a daily kWh need, then divides your usable battery capacity by that number. Backing up only true essentials (fridge, lights, internet, a well pump) needs far less than trying to run a whole house including air conditioning.
?Why is 'usable' capacity lower than the battery's rated capacity?
Two real losses apply. Depth of discharge (DoD) is the percentage of a battery's total capacity that can be safely cycled without materially shortening its lifespan. Lithium batteries typically allow 90-100%, but manufacturers often specify a usable percentage below the rated size. Inverter efficiency accounts for the AC/DC conversion loss between the battery and your home's outlets, typically 90-95% for modern inverters. A 13.5 kWh rated battery at 95% DoD and 94% inverter efficiency delivers about 12.06 kWh of real, usable energy.
?Do solar panels keep working during a power outage?
Not by themselves. Every grid-tied solar inverter sold in the U.S. is required to shut off automatically when the grid goes down. This safety feature, called anti-islanding, prevents your system from feeding electricity into power lines utility crews may be working on. Solar can only recharge your battery during an outage if you have a battery and inverter combination specifically designed to disconnect from the grid and keep running on its own (often called 'backup' or 'islanding' mode).
?Why does the peak load check matter?
Your battery's inverter has a maximum continuous power rating (how many watts it can deliver at once, not how much total energy it stores). If every essential load could switch on simultaneously and their combined wattage exceeds that rating, the inverter can trip or shut down even with plenty of stored energy left. This calculator adds up every load's wattage (a conservative, all-at-once assumption) and checks it against your inverter's continuous rating.
?Does more solar mean unlimited backup?
Only under favorable conditions. If daily solar generation exceeds your daily essential load, the battery can theoretically recharge indefinitely during an outage. But outages are frequently caused by the same storms that reduce solar output (heavy cloud cover, snow-covered panels, or extended overcast conditions) which can cut generation well below a clear-sky estimate exactly when you need it most. Treat any 'indefinite backup' result as a best-case, not a guarantee.
About this calculator. Results are estimates for education and planning, based on the inputs you enter and the published formula described above. Everything runs in your browser; nothing you type is sent to us or stored. It is not financial, tax, legal or medical advice. Read the disclaimer and our methodology.
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