How much battery storage is enough?

A home battery stores energy so you can use it later: solar power from the afternoon used in the evening, or stored power that keeps the lights on during an outage. Whether you need one, and how big, depends on what you want it to do. A battery sized for backup and a battery sized to save money on your bill are sized in different ways.

This guide covers both, with worked examples, and helps you judge when storage pays off and when it doesn’t.

First, a reality check about outages

Many people assume solar panels keep the power on when the grid goes down. A standard grid-tied solar system shuts off during an outage. This is a safety requirement, so your panels don’t send power into lines that utility crews are working on.

To have power during an outage, you need equipment designed for it, usually a battery with an inverter set up for backup. Even then, many systems back up only a selected group of circuits, not the whole house. If backup matters to you, ask installers exactly which circuits would stay on.

Know the four numbers on a battery spec sheet

Usable capacity (kWh): how much energy the battery can actually deliver. This is the number to size with. Some spec sheets also list a larger “total” capacity, so make sure you’re reading the usable one.
Power rating (kW): how much it can deliver at once. Capacity decides how long things run, and power decides how many things run at the same time. There is often a continuous rating and a higher short-term peak rating.
Round-trip efficiency (%): some energy is lost going into and out of the battery. If the spec says 90%, you get back about 90 kWh for every 100 kWh you put in.
Warranty: usually a number of years, a total amount of energy cycled through, and a promise about how much capacity remains at the end. Read all three.

Goal 1: Backup during outages

Step 1: List what you need to run

Pick the essentials, not the whole house. For each item, estimate how much energy it uses in a day:

Daily energy (watt-hours) = watts × hours used per day

Find the watts on the appliance label or in its manual. For refrigerators and other appliances with an energy label, the yearly kWh figure divided by 365 gives a good daily number.

Here is an example list. These are illustrative numbers, not measurements, so check your own appliances:

Item	Example use	Example daily energy
Refrigerator	label shows about 550 kWh per year	about 1.5 kWh
LED lights	5 bulbs × 10 W × 5 hours	0.25 kWh
Internet modem and router	20 W × 24 hours	about 0.5 kWh
Phone and laptop charging		about 0.2 kWh
Furnace fan or well pump	500 W × 1 hour	0.5 kWh
Total		about 2.95 kWh per day

Step 2: Decide how long you want backup to last

Multiply your daily total by the number of days you want covered. Think about how long outages usually last where you live, not the worst case you can imagine, since every extra day of backup adds cost.

If your solar can recharge the battery during an outage, you may need fewer days of storage. That only works if the system is designed to keep the panels running in backup mode, so confirm it with your installer.

Step 3: Allow for efficiency and reserve

Divide by the round-trip efficiency, then add some margin. Many batteries also keep a reserve that you either can’t use or shouldn’t use regularly.

Usable capacity needed = daily energy × days ÷ round-trip efficiency

Example: 2.95 kWh per day × 2 days ÷ 0.9 = about 6.6 kWh of usable capacity, before any extra margin.

Step 4: Check the power, not just the capacity

Add up the watts of everything that might run at the same time, and compare that to the battery’s continuous power rating.

Motors are the catch. Well pumps, air conditioners, and some refrigerators draw a short surge when they start, often several times their running watts. If the battery or inverter can’t supply that surge, the appliance won’t start even when the battery is full. Check the starting (surge) rating of any motor you plan to back up, and compare it to the battery’s peak power rating.

Goal 2: Saving money on your bill

A battery saves money when power you use later is worth more than power you send to the grid now. Two situations create that gap:

Time-of-use rates: electricity costs more during peak hours, often late afternoon and evening, when solar output is falling.
Low export credits: your utility pays less for the solar power you send back than it charges for the power you buy. See our net metering guide for how these rules work.

How to size for savings

Look at how much electricity you use in the expensive hours, usually after the sun goes down. Your utility may show hourly or time-of-use usage in your online account.

A battery sized to cover that amount, and no more, captures most of the savings. Extra capacity sits unused on most days.

Worked example

These numbers are made up to show the method. Use your own rates and usage:

Evening peak usage: 8 kWh per day
Peak electricity price: $0.40 per kWh
What your utility credits for exported solar: $0.10 per kWh
Round-trip efficiency: 90%

Value of the peak power you avoid buying: 8 × $0.40 = $3.20 per day
Solar needed to charge it: 8 ÷ 0.9 = about 8.9 kWh, which you would otherwise have exported for 8.9 × $0.10 = about $0.89
Daily savings: $3.20 − $0.89 = about $2.31
Yearly savings: $2.31 × 365 = about $840

To estimate payback, divide what the battery would cost you installed, after any incentives, by the yearly savings. For example, a battery that cost $10,000 installed would pay back in about 12 years ($10,000 ÷ $840). Compare that to the warranty: if payback takes longer than the warranty lasts, the savings case is weak.

Your real savings may be lower. Some rate plans only charge peak prices on weekdays, and you won’t use exactly the same amount every evening.

When storage earns its cost, and when it doesn’t

A battery is more likely to make sense if:

your utility pays much less for exported solar than it charges for power
you’re on a time-of-use plan with a big gap between peak and off-peak prices
outages are frequent or long where you live, and backup has real value to you
incentives significantly lower the installed cost

A battery is less likely to pay for itself if:

your utility credits exported solar at the full retail rate. In that case the grid already works like a battery, and a real one mostly adds cost and efficiency losses.
you have a flat rate that’s the same at all hours
outages are rare and short

Backup can still be worth it even when the savings math doesn’t add up. That’s a judgment about how much you value keeping essentials running, not just a financial calculation.

What this estimate leaves out

battery capacity slowly declining over the years
changes to your rates, rate plan, or export rules
temperature, since very hot or cold locations can affect performance
where the battery can go, and local code, permit, and fire safety requirements
fully off-grid systems, which need much more careful design than this guide covers

Next steps

Decide your main goal: backup, savings, or both.
Gather your numbers: your list of essentials for backup, or your peak-hour usage and rates for savings.
Check your utility’s rules on time-of-use rates and export credits.
If you’re also sizing solar, start with our guide to how many solar panels you need.
Get detailed quotes from more than one licensed installer, and ask exactly which circuits would be backed up and for how long.