Batteries
Your napkin math from Know Your Numbers gave you a rough capacity target. Here’s how to refine it and turn it into an actual battery purchase.
Why batteries matter
In a standalone system, batteries are the bridge between when the sun shines and when you need power. Panels charge them during the day. You draw from them at night. If you’re designing for autonomy — getting through cloudy days without running a generator — the battery bank is where that spec lives.
Sizing your battery bank
The formula: overnight load (watts) × hours of coverage ÷ usable depth of discharge = battery capacity needed.
Overnight coverage is where most people start. If your overnight load averages 500 watts over 10 hours (8 PM to 6 AM), that’s 5,000 Wh. At 85% usable DoD on LiFePO4, you need about 5,900 Wh of rated capacity — call it a 6 kWh bank.
Autonomy means covering a full no-sun day: overnight load plus daytime essentials. If you run 800 watts of essential loads during the day, that’s another 8 kWh on top. Now you’re looking at 14+ kWh for a real autonomy day. That’s a serious battery bank — probably not your starting point, but useful to know your ceiling.
Most people start with overnight coverage and expand when they feel they need more autonomy. That’s a reasonable approach.
Form factors
Server rack batteries (typically 5 kWh, 48V per unit) are the workhorse of DIY solar. Modular, stackable, and widely compatible with prosumer inverters. Each unit weighs about 100 lbs — heavy but manageable. They connect in parallel to increase total capacity. When you want more capacity, you wire in another unit.
All-in-one units (Bluetti, EcoFlow, and similar) pair with specific inverters from the same brand. Setup is simpler and the software integration is tighter. The trade-offs: higher cost per kWh and you’re locked into that brand’s ecosystem for expansion.
If you went the prosumer inverter route (Victron, EG4, Growatt), server rack batteries give you more flexibility and better cost per kWh. Consumer inverter path means you’ll likely use the matching brand’s batteries.
Expansion planning
Battery expansion is the easiest upgrade in the system. Slot in another unit, wire it to the bus, tell the inverter about it. Done.
Plan for it even if you’re starting small: leave room in the rack, size your wiring for future capacity, and confirm your inverter supports the total bank size you might eventually want.
Communication compatibility
Modern LiFePO4 batteries communicate with the inverter digitally — usually over CAN bus or RS485. This lets the inverter know the battery’s state of charge, cell voltages, temperature, and health. It also lets the inverter manage charging precisely: the right voltage, the right current, the right cutoffs.
“Compatible” doesn’t just mean the voltage matches. Battery and inverter must speak the same digital protocol. Victron uses CAN bus or VE.Can. EG4 and Growatt typically use RS485. Some batteries support both.
The best source for verified combinations isn’t spec sheets — it’s forums. Check before you buy.
Where to go next
Once you know your battery bank size and form factor, the next question is how it all connects. Head to Wiring and Safety for the physical side. If you’re still finalizing your inverter choice, Inverters covers how inverter selection drives battery compatibility.
DATA SOURCED FROM: Standalone Solar Design Guide, Section 3D (primary source for sizing methodology and form factor guidance). Battery specifications and DoD ratings from manufacturer datasheets (Victron, EG4, SOK). Protocol compatibility guidance from manufacturer documentation and diysolarforum.com community reporting.