In the early days of the solar energy system, people did not use batteries to store extra power generated by solar panels; rather, their panels were connected directly to the grid stations. The houses used the power their system generated and sent the extra power to grid stations. And at night and during the days when the sun dimmed, they had to buy expensive units from power companies and grid stations.
There were many reasons why people did not use batteries. The most important reason was the complexity of the system due to lead-acid type batteries. Moreover, they were expensive, heavy, had a short life span, and needed constant maintenance. Another reason to avoid batteries was net metering. Giving extra power to grid stations gave you credits which were used to reduce the utility bills.
But now, the complex and high-maintenance batteries are replaced by lithium-ion batteries that are effective and efficient and have a higher life span. Moreover, due to extreme shifts in climate, power outages are increasing in the US day by day. Big events like the Texas 2021 winter freeze, the California wildfire seasons, and hurricanes in states like Florida and Louisiana caused millions of people to lose power for long periods.
Currently, instead of giving the extra power to grids, homeowners are using batteries to store extra power for nights, and then any additional power is sent to grid stations.
Now, as people have started using batteries, the most important question is:
WHAT IS THE EXACT SIZE OF THE SOLAR BATTERY THAT IS NEEDED BY THE HOMEOWNERS?
So the simple and easy answer to this question is: THE DAILY NEED OF YOUR HOME DECIDES THE SOLAR BATTERY SIZE!
In this post, we will find out how to find the exact solar battery sizing for our homes and how we can calculate the average power usage.
Why Getting Solar and Battery Sizing Right Is a Game-Changer
A mismatched solar and battery system is like a bike with flat tires—pedaling gets nowhere fast. The idea behind solar is power on demand, lower costs, and maybe a smug nod to the neighbors. But it’s a letdown if the system can’t handle the load or store enough for a cloudy night. Oversizing? That’s just tossing money into a fancy roof ornament.
Hitting the sweet spot means syncing energy production with storage. Panels soak up enough sun to cover daily use while batteries bank the surplus for after-hours or outages. Done well, it boosts efficiency, trims utility tabs, and keeps the fridge humming when the grid crashes. Screw it up, and it’s either blackouts or buyer’s remorse.
Read More: Trusted Solar Panel Repair and Maintenance Services Near Me in Florida
Step 1: Find Out How Much Power You Use Per Day
If you want to know about total energy you used during the billing period, look at your energy bill. It shows how much total energy you consumed per month in kilowatt-hours (kWh).
So, from the figure given on the bill, you can find your home’s energy use per day. To get it, divide the total units by 30 or 31(days in month). That gives you a daily average.
For example if your total units are 163 kWh during the billing period. Then your daily average use is 5.44 kWh per day.
For more accurate measure, use 12 months of past bills (if available), sum all kWh, divide by 365. That gives an exact daily average regardless of summer/winter swings.
If you don’t have a bill, you can use another method. In this method you have to list down all the appliances that use electricity. Check how many watts each appliance is using. Now find out how many hours they are used per day. To calculate kWh per day of each, use the formula:
Daily kWh =(Watts × Hours used) ÷ 1,000
Finally, add consumption by all the appliances to get overall usage per day. To get monthly usage, multiply your answer with 30 or 31.
Let’s understand this method with the help of following example:
| Appliance | Wattage (W) | Hours Used Per Day | Formula | Daily kWh |
| Fridge | 150 W | 24 hours | (150 × 24) ÷ 1,000 | 3.6 kWh |
| Laptop | 60 W | 5 hours | (60 × 5) ÷ 1,000 | 0.3 kWh |
| Ceiling Fan | 70 W | 8 hours | (70 × 8) ÷ 1,000 | 0.56 kWh |
| LED Lights (5 bulbs) | 50 W | 6 hours | (50 × 6) ÷ 1,000 | 0.3 kWh |
| TV | 120 W | 4 hours | (120 × 4) ÷ 1,000 | 0.48 kWh |
| Microwave | 1000 W | 0.2 hours (12 minutes) | (1000 × 0.2) ÷ 1,000 | 0.2 kWh |
Per day usage = 3.6 + 0.3 + 0.56 + 0.3 + 0.48 + 0.2
= 5.44 kWh per day
Monthly usage = 5.44 kWh × 30 days
= ≈ 163 kWh per month
Because electricity use changes through the year (for example more AC in summers and more heating in winter), using an annual bill history or several months of bills gives a more realistic average than just one month. Many electric bills track “kWh used per month over the last 12-18 months,” which helps spot seasonal differences.
Step 2: Add a Safety Buffer
Safety buffer means that you won’t design your solar system exactly equal to your daily usage, rather you design it to generate and store 20% to 30% more than your daily usage to cope with inefficiencies in the system.
Buffer is considered because over time, solar batteries lose some efficiency, and solar panels produce less energy due to dust, aging, wiring losses, etc. A buffer helps the system stay reliable even after these losses accumulate.
This is useful in conditions when you live off-grid and do not take electricity from grid stations rather solely depend on solar energy. Considering a buffer means you won’t run out of stored power at night or when solar production is poor.
So adding an extra buffer is smart planning. It ensures your solar + battery system works well under different real-life conditions, not just perfect weather or ideal usage.
From the example we discussed in the first step, we came to know that our total monthly usage is 163 kWh per month and daily usage is 5.4 kWh/day.
Now Consider 2 conditions
1. Suppose you are expecting small fluctuations in usage due to extra use, a cloudy or rainy day, power losses. So, you add a 20% buffer that means:
5.4 kWh/day x 0.2 = 1.06
Total kWh/day including buffer = 5.4 + 1.06
=6.46 ≈ 6.5 kWh/day
2. If you plan off-grid living or want maximum reliability for long nights, storms, unpredictable use, you may add 30% buffer:
5.4 kWh/day x 0.3 = 1.62
Total kWh/day including buffer = 5.4 + 1.62
=7.02 kWh/day
So for sizing: you design your solar + battery to produce and store ~6.5–7.0 kWh/day, not just 5.4 kWh/day.
Step 3: Size the Solar Panels
Peak sun hour is a period when one square meter of ground receives 1000 watts of solar energy. Sun rays during peak sun hours are known as full power sunlight. During peak sun hours solar panels are capable of generating maximum electrical energy.
Peak sun hours period only consist of a small portion of day, usually 3 to 7 hours. It only includes the hours with strong, full-sunlight, during which our solar panels work at full capacity.
In the U.S., different states/regions get different numbers of peak sun hours per day. For example, sunny states like Arizona and California often receive 5-7 peak sun hours. Whereas in northern or cloudier states (or winter months), it can be 3–4 peak sun hours.
So before sizing solar panels, it’s important to know how many peak sun hours your area gets on average.
Once you know how much energy your home needs per day (with buffer) and how many sun hours your area gets; you can calculate how the strength of your solar panel system with help of following formula:
System size in kW = Daily energy need (kWh) ÷ Peak sun hours
Your system size in kW tells you how many solar panels you need so that the sunlight they get each day can produce the same amount of electricity your home uses in one day.
Now let’s extend the similar example we have been discussing in our previous steps. Our daily energy usage is 6.5 kWh with 20% buffer and 7.02kWh with 30% buffer. Suppose we are living in California and our peak sun hours are 5 hours/day. So, our system size will be
- In case of ~6.5 kWh/day:
System size = 6.5 ÷ 5 = 1.3 kW - In case of ~7 kWh/day:
System size = 7 ÷ 5 = 1.4 kW
So you need a solar array capable of about 1.3–1.4 kilowatts to meet your daily energy requirements (on an average sun-hour day).
Now the next question is HOW MUCH SOLAR PANELS WE NEED FOR SYSTEM WITH 1.3 to 1.4 kW?
Let’s find out!
Solar panels are usually rated in watts (for example, 350W or 400W). To know how many panels you need, you convert system size to panel count:
1.3 kW = 1,300 W. ( to convert kW in W, we multiply 1.3 with 1000)
- If each panel is 350 W then 1300 ÷ 350 ≈ 3.7 ( round up to 4 panels).
- If you use 400 W panels then 1300 ÷ 400 = 3.25 ( round up to 4 panels).
Similarly, 1.4kWh = 1400 watts
- If each panel is 350 W then 1400 ÷ 350 ≈ 4
- If you use 400 W panels then 1400 ÷ 400 = 3.5 ( round up to 4 panels).
So a 1.3–1.4 kW system may need about 4 solar panels (depending on their wattage).
The formula assumes “perfect sunlight” (full intensity and no shading). In real life, things like shade, panel angle, dust, temperature, wiring losses reduce output. Some calculators apply a “system efficiency factor” (e.g. 70–80%) to adjust for those.
Step 4: Size the Battery
Usually in U.S. homes, from late afternoon around 4 pm to night around 9 pm, electricity is used abundantly and this period is considered as peak hours. This is because people come home from work or school, turn on lights, cooking appliances, TVs, computers, heating or AC, and laundry.
So, according to solar designers about 70% of daily energy is used during peak hours after the sun is weak or gone.
So, if we use 6.5 kWh per day (20% buffer) and about 7 kWh per day (30% buffer), about 70% out of this power is used during peak hours
- night use for 6.5 kWh/day = 6.5 × 0.7 ≈ 4.6 kWh
- night use for 7 kWh/day = 7 × 0.7 ≈ 4.9 kWh
So, our battery has to cover 4.6 kWh to 4.9 kWh.
One of the most important things that must be kept in mind is that a battery cannot be emptied to 0% every day. If this happens it would affect the overall efficiency and effectiveness of the battery.
To keep our battery safe and working for a longer period of time we need to consider “depth of discharge” (DoD), which tells how much of the battery we can use daily without damaging it.
Lithium batteries can safely use about 90% of their stored energy, while lead-acid batteries can use about 50% to protect their life.
So, to find the battery size, perform the following calculations:
For lithium
Battery size = night use ÷ 0.9
For 6.5 kWh/day
- 4.6 ÷ 0.9 ≈ 5.1 kWh
For 7 kWh/day
- 4.9 ÷ 0.9 ≈ 5.4 kWh
For lead-acid
Battery size ≈ night use ÷ 0.5
For 6.5 kWh/day
- 4.6 ÷ 0.5 ≈ 9.1 kWh
For 7 kWh/day
- 4.9 ÷ 0.5 ≈ 9.8 kWh
In reality, we also add a 10% buffer for small losses and aging.
- If you choose a lithium battery, you would look for about 5.5–6 kWh of usable storage.
- If you choose a lead-acid battery, you would look for about 10–11 kWh of storage.
Step 5: Match the Inverter to the System
From the above calculations, it’s confirmed that our home needs around 6.5 – 7 kWh per day. Now, our next step is to choose the inverter and charge controller. These two devices make sure the solar system runs smoothly.
Inverter converts battery power that is in the form of direct current into the alternating current your home appliances can use.
As we cannot use direct current to run our home appliances, therefore without an inverter, the whole system is just trash.
We size the inverter based on the highest number of appliances you might use at the same time. This is called your peak load.
In the first step, we discussed how much watts, different appliances in our home use, suppose we are using the following appliance during peak load.
- Fridge = 150W
- LED lights = 50W
- Fan = 70W
- Laptop = 60W
- TV = 120W
Total watts at the load time equals to the sum of watts of all appliances running at same time
150 + 50 + 70 + 60 + 120 = 450 watts
But appliances like the fridge need a short burst of extra power when they start.
So we add 25% buffer for extra safety:
450W × 1.25 = 562 watts
To be safe, choose an inverter around 600–800 watts.
This means your system can run your evening appliances comfortably and safely if the inverter is around 600–800 watts.
Step 6: Match the Charge Controller to the System
A charge controller protects your battery from getting too much power from the solar panels. It makes sure your battery charges smoothly and remains efficient and effective throughout its life span.
In step 3, we found out that our system needs around 1.3–1.4 kW of solar panels.
Let’s use 1,300 watts (1.3 kW) as an example.
To calculate the size of Charge controller, use the following formula:
Required amps = Solar panel watts ÷ battery voltage
If your system uses a 24V battery, then:
1,300W ÷ 24V = 54 amps
We add 25% buffer for safety:
54 × 1.25 ≈ 68 amps
Choose a 70-amp MPPT charge controller.
This ensures your battery gets controlled, safe charging every day.
FAQs about Solar Battery Sizing
1: Why didn’t people use batteries with solar systems in the early days?
Earlier most people were interested in net metering and getting extra credits to pay for utility bills or electricity they take from grids during night time or during cloudy weather conditions. One of the major reasons was inefficiency of old lead-acid batteries that were costly and needed regular maintenance. Moreover, they have higher DoD due to which you have to use high wattage batteries even for homes with less kWh usage
2: Why do people use solar batteries now?
People now use batteries because modern lithium-ion batteries last longer, work better, and need almost no maintenance. Moreover, they have lower DoD, which means you can use up to 90% of the charging capacity during night time or cloudy days without damaging your battery. In addition to this, grid stations usually offer expensive units to homeowners to use when solar power goes out.
3: How do I know what size of solar battery my home needs?
The size of the battery depends on how much electricity your home uses each day. First, you calculate your daily kWh use. Then you add a buffer (20–30%) to stay safe. After that, you calculate how much of this energy you need at night. This night-time energy decides how big your battery should be.
4: What is a safety buffer, and why do we add it?
A safety buffer means adding extra capacity on top of your daily use. This is done because reality is not perfect and there are ups and downs, too. Some days are cloudy, some days you use more electricity, and over time solar panels and batteries lose a bit of their efficiency. Adding 20–30% extra makes sure your system still works smoothly even when things change.
5: How do I calculate my daily electricity use if I don’t have a bill?
If you don’t have an electricity bill, you can still find your daily use by listing all the appliances you use at home. Write down how many watts each appliance uses and how many hours you use it per day. Then use this formula:
(Watts × Hours) ÷ 1,000 = daily kWh.
Do this for each appliance and add all the results. This gives your total electricity use per day.