Size the bank from your evening load, not from your panel size.
Fix the system voltage first, because amp-hours mean nothing until volts are set.
A 12V 250Ah battery holds about 3kWh, but usable energy is less after depth of discharge.
LiFePO4 lets you use most of the bank every day, which lead-acid cannot.
A sets the voltage and current your bank must match.
Introduction
Picking feels like guesswork until you use a worksheet. The mistake most people make is starting from the panel size. The right start is your evening load: what you want to run after the sun drops. This guide gives you a step-by-step worksheet so you can size a solar battery bank in kilowatt-hours instead of vague "bigger is better" thinking.
Start With Your Evening Load, Not the Panels
Panels decide how fast you refill. Batteries decide how long you run. Those are different questions. If you size the bank to the panels, you may buy storage you never drain or, worse, too little to cover the night.
Write down the things you want on battery: fridge, lights, Wi-Fi, a few sockets. That list is your true target. Everything else builds from it.
Step 1: List Every Device You Want on Battery
Walk the room and note each device with its watt draw. A fridge might pull 150W while running. Lights might be 40W. A router and phone charging are another 20W. Be honest about what must stay on, and leave out the air conditioner unless you plan to pay for it.
The goal is a single number: total watts you expect to run at the same time.
Step 2: Turn Watts and Hours Into Watt-Hours
Multiply each device's watts by the hours you run it. A 150W fridge cycling for 8 hours is 1,200Wh, or 1.2kWh. Add every device the same way. The sum is your daily evening need in kWh.
Most homes land between 3kWh and 15kWh of usable storage. A cabin or van is near the low end. A whole-house backup sits at the high end.
Step 3: Fix the System Voltage First
Battery capacity is often shown in amp-hours at a voltage. A 250Ah label means little until you fix the volts. At 12V, 250Ah is about 3kWh. At 48V, the same 250Ah is roughly 12kWh. Always think in watt-hours, because that is what your appliances use.
Your inverter and charge controller set the voltage, not your preference. Small systems often run 12V. Larger home systems move to 48V, because higher voltage means lower current, thinner cables, and less heat. Pick the voltage before you buy a single battery.
Step 4: Apply Depth of Discharge
You should not drain a battery flat every night. Lead-acid likes only the top third of its capacity, so a 100Ah lead-acid bank may give you 30–50Ah of usable energy. LiFePO4 handles 80% or more daily discharge without complaint.
A at 50% depth gives you about 1.5kWh you can use. A LiFePO4 of the same size gives you far more. The usable number, not the label, is what runs your night.
Step 5: Add a Margin for Real Life
Real nights are longer in winter and heavier on weekends. Add 20–30% to your worksheet total so you are not draining the bank flat. A bank you treat gently also lasts longer.
If your evening need is 5kWh, size for about 6.5kWh of usable storage. That margin is the difference between a bank that ages well and one that dies early.
A Worked Example
Say your evening load is a 150W fridge (8h = 1.2kWh), 40W lights (5h = 0.2kWh), and 20W of electronics (8h = 0.16kWh). Total is about 1.6kWh. Add a 30% margin and you need roughly 2kWh usable.
A at 80% depth gives you about 2.4kWh usable, which covers this load with room to grow. The worksheet turns a guess into a number.
How the Charge Controller Affects Sizing
The charge controller sits between panels and battery. A MPPT solar charge controller pulls more from the panels than a PWM unit and suits larger banks. Its battery-voltage setting must match the bank, or you risk undercharging the cells.
A controller that is too small becomes the bottleneck. Match its charge current to the bank size, or the battery never fills in the sunlight window.
Matching the Bank to a Hybrid Inverter
The inverter must handle both the bank voltage and the load current. A covers most homes, but heavy draws like a well pump can outrun a small unit. Also watch the C-rate, which shows how fast the bank can deliver current.
Confirm the inverter's battery-voltage rating before you buy. A 48V inverter will not run a 12V bank without extra gear, and mixing voltages in one system is a mistake.
Which SUOER Battery Fits Your Worksheet
SUOER covers both chemistries and several shapes. A is a practical 12V building block for small systems. The suits home arrays that want more usable energy in less space.
For the bigger picture on pairing storage with your array, see our guide on . For small urban installs, our covers compact footprints.
Conclusion
Sizing a solar battery bank is not about the biggest number on the label. It is about match: capacity to your load, voltage to your inverter, and chemistry to your duty cycle. Use the worksheet, fix the voltage early, and size from real evening need. Do that and the bank becomes the quiet base your whole system rests on.
Frequently Asked Questions
Q: How many batteries do I need for a solar system?A: It depends on your evening load and system voltage, not a fixed count. A 12V 250Ah battery holds about 3kWh; a home wanting evening coverage often needs several times that, so calculate in kWh first.
Q: Is LiFePO4 safe for home use?A: Yes. LiFePO4 is one of the more stable lithium chemistries, with low thermal risk and a long cycle life, which is why it has become standard for residential solar storage.
Q: Can I use car batteries for solar?A: Not well. Starter batteries are built for short bursts, not deep daily cycling, and fail quickly in solar service. Use deep-cycle or LiFePO4 cells made for the duty.
Q: Why does my 250Ah battery not power my whole house?
