Where This Guide Comes From
This guide is based on the University of Maryland extension series on solar PV design — specifically the episode where extension specialist Drew Schiavone walks through battery bank sizing from scratch. It's academic in the good sense: every step is explained, every factor has a number, and the math is shown in full. If you'd rather watch the lesson, the video is right here:
The method matters because it's the same one used to size real off-grid systems, and it applies whether you're building a cabin system, an RV setup, or specifying a battery bank for an OEM product. It's also the closest thing to a textbook answer for the question everyone asks: how big a battery do I actually need?
Battery Chemistry Basics: Lead-Acid vs Lithium
Before the math, the video sets up the two battery families you're choosing between. Most common options are lead-acid — flooded, gel and AGM — plus lithium-ion, which in the solar world today almost always means LiFePO4.
- Energy density: lithium stores almost four times more energy per kilogram — around 150 Wh/kg vs 40 Wh/kg for lead-acid.
- Usable capacity: it's recommended to use around 85% of a lithium battery's capacity, but only 50% of a lead-acid battery. Go deeper and you damage the cells or shorten their life.
- Lifespan: lead-acid typically lasts 7–10 years or ~1,000 cycles when properly maintained; lithium lasts ~20 years or 3,000–5,000 cycles.
- Cost: lithium costs roughly 2.5–3x lead-acid up front — but the longer lifespan can largely offset that difference over the system's life.
That cost-per-cycle comparison is exactly why we build with LiFePO4: over 6,000 cycles in our spec, the per-cycle cost lands below lead-acid, and the usable capacity is nearly double for the same rated size. The chemistry decision is a total-cost decision, not a sticker-price decision.
Step 1: Know Your Daily Energy Use
The sizing formula only works if you know how much energy your loads actually consume each day. The video covers two ways to get that number:
- Grid-tied: the simplest method — read your electric utility bill. In the example, 617 kWh in the billing period, and 5,592 kWh over 12 months.
- Off-grid / mobile / battery-integrated: build an electrical loads list. Record each device's power rating in watts and the hours per day you run it, then multiply: watts × hours = watt-hours per day.
The worked example: a small motor rated at 1,100W, used about 15 minutes (0.25 hours) daily → 275 Wh/day. Repeat for every load. For duty-cycle devices like fridges and air conditioners, either measure the baseline and the compressor-active demand separately, or divide the device's estimated annual energy use by 365 days (the example fridge: 640,000 Wh ÷ 365 ≈ 1,753 Wh/day). Account for seasonal loads too — size for your highest-use season, not the annual average. In the example, all loads add up to 19,528 Wh/day.
Step 2: Estimate System Losses
No battery system is 100% efficient. The video combines three loss factors into one "load subsystem efficiency":
| Loss source | Typical value | Notes |
|---|---|---|
| Battery round-trip efficiency | Lead-acid ~85% / lithium ~95% | Energy retained after a charge–discharge cycle; chemistry-dependent. |
| Wiring losses | ~97% | Keep voltage drop under 2–3% across the circuit. |
| Inverter conversion | 92–94% | Battery inverters are less efficient than solar inverters; 92% used here. |
Multiplying the example's factors — 0.95 × 0.97 × 0.92 — gives an overall load subsystem efficiency of about 85%. We'll flag an inconsistency in these numbers shortly, because the comment section caught it too.
Step 3: Pick a System Voltage
Most off-grid battery banks run at 12, 24 or 48 volts. How do you choose?
- Match your loads: if you're directly powering a 12V DC load, you need a 12V bank; a 24V system can't run 12V appliances without a converter.
- Match your inverter: bigger AC outputs usually need higher DC input — a 200W inverter might come in 24V DC, a 6,000W inverter typically wants 48V DC.
- Match your solar array: the array must produce higher voltage than the battery bank, and panels can be wired in series to reach it if the charge controller handles it.
- Think about wire cost: lower voltage means higher current, which means thicker, more expensive copper. Panels far from the bank? A 24V or 48V system lets you use smaller wire.
- Bonus: a 24V charge controller can be half the size of a 12V unit — a small system cost saving.
The example picks a nominal 24V bank for a ~19.5 kWh/day demand.
Step 4: Choose Days of Autonomy
Days of autonomy = how many days you plan to run your loads with no or partial sun. The video's guidance:
- A good figure is typically 3 to 5 days.
- Check historical weather for your site — if you get 3, 4 or even 5 days of inclement weather in a row, that's your starting point.
- Fewer days means you lean on a generator as a second source; more days means a much more expensive battery bank.
The example selects 3 days of autonomy.
Step 5: Temperature and Depth of Discharge
Two more correction factors before the final calculation:
- Temperature compensation: batteries are rated at 77°F (25°C), and capacity drops in the cold. The example stores a lead-acid bank in a room averaging 50°F in winter, which maps to a correction factor of 1.19.
- Depth of discharge (DoD): lead-acid maxes out around 50% DoD; lithium can go to about 85% without significant degradation. A 1,000Wh lead-acid battery delivers ~500Wh; a lithium one ~850Wh. The example assumes a factor of 0.5 for its lead-acid bank.
There's a trade-off the video points out: a larger battery costs more but lasts longer, because a smaller percentage of its capacity is depleted each cycle. Sizing slightly up is often cheaper over the bank's life than cycling it hard.
Step 6: The Sizing Calculation — Worked Example
Here's the full formula from the video:
19,528 Wh/day × 3 days autonomy × 1.19 (temperature) ÷ 24V (system voltage) ÷ 0.85 (load subsystem efficiency) ÷ 0.5 (max DoD) ≈ 6,835 Ah of usable battery capacity.
Then translate amp-hours into batteries. Choosing 12V 215Ah flooded lead-acid units: 6,835 Ah ÷ 215 Ah ≈ 32 batteries. That's a big, expensive bank — which is exactly the point of the exercise. The video's advice: batteries are significantly more expensive than solar panels, so avoid oversizing the bank, and consider adding solar capacity instead. Extra solar also helps the bank recover faster after bad weather.
What the Math Tells You
The worked example is deliberately extreme — 32 batteries is a serious investment, and that's the point. Three takeaways that survive the calculation:
- Lithium changes the economics. At 85% usable capacity, 95% efficiency and 3,000–5,000 cycles, the same energy demand needs roughly half the rated capacity of lead-acid — and the "32 batteries" problem largely disappears.
- Autonomy days are the biggest lever. Going from 3 to 5 days adds 67% to the battery requirement. Before you spend on more batteries, check whether your location really has 5-day no-sun stretches.
- More solar beats more battery. When panels are cheaper per watt-hour than batteries, the smart system trades autonomy days for generation capacity.
That last point is the one we'd underline for anyone building or buying an off-grid system: size the battery for your worst night, not your worst week — and let solar carry the week.
And if you're not hunting for a consumer brand but need the solar battery bank itself — a custom LiFePO4 pack at a specific voltage and capacity, an OEM or ODM project, or wholesale supply for your own battery system line — that's what we do. Dajiu Energy is a Chinese manufacturer building LiFePO4 batteries for RVs, homes, golf carts and portable applications since 2017. Our product range covers LiFePO4 drop-in replacement batteries, EV and golf cart batteries, wall-mounted home storage, portable power stations, commercial energy storage and battery swap cabinets — explore the full lineup in our product center, or browse more buying guides. Tell us your voltage, capacity and size, and we'll engineer the pack to hit it.
What Our Readers Asked (Top Comments on the Video)
These are the most-liked comments on the source video, with our practical answers. The original creator did not reply to these threads, so the answers below are ours — written the way we'd answer a customer on the shop floor.
Source video: EnergyUME (University of Maryland Extension) — "How to design and size a solar battery system" (youtube.com/watch?v=FtO1sunTHOE). Comment excerpts are quoted verbatim from the video's top comments; the original creator did not reply to these threads, so all answers are Dajiu Energy's own.