Sizing your solar panels is only half the equation — the other half is your battery bank, since that’s what actually keeps your lights, fridge, and well pump running once the sun goes down. Use the calculator below to find out how long your battery bank will last, or work backward to figure out what size battery bank you actually need.

Why You Need a Battery Bank Runtime Calculator

A battery bank that’s too small leaves you in the dark during a stretch of cloudy days — exactly when you need it most. One that’s too large means paying for capacity you’ll rarely use. This calculator turns your battery capacity, battery chemistry, and typical power draw into a concrete runtime estimate, or flips the math around to tell you what size battery bank to buy for a target backup time.

How This Battery Bank Runtime Calculator Works

Enter your battery bank’s total capacity, choose lithium (LiFePO4) or lead-acid since they have very different usable depth of discharge, and set your system’s round-trip efficiency to account for inverter and wiring losses. From there, enter your continuous load in watts to see runtime in hours and days, or enter a desired backup duration to get a recommended battery bank size for that target.

Benefits of Knowing Your Battery Runtime

Knowing your real runtime — not a guess — lets you budget for the right battery bank the first time, avoid nasty surprises during multi-day cloudy stretches, and decide with confidence whether lithium’s higher upfront cost is worth it for your usable capacity versus cheaper lead-acid.

Factors That Affect Battery Runtime

Battery chemistry matters most: lithium (LiFePO4) batteries can typically be discharged to around 90% of rated capacity without damage, while lead-acid batteries should generally stay above 50% depth of discharge to avoid shortening their lifespan — meaning a lead-acid bank needs roughly double the rated capacity of a lithium bank for the same usable runtime. Inverter efficiency, wiring losses, and cold temperatures (which reduce usable battery capacity) also pull real-world runtime below the theoretical number.

Plan Your Full Off-Grid Power Setup

Start with our off-grid solar calculator to size your panels and daily generation, then use this tool to make sure your battery bank can carry you through the night and the occasional cloudy stretch. Also check our off-grid heating calculator and rainwater catchment calculator to round out your homestead plan. For more on how batteries pair with solar, see the U.S. Department of Energy’s solar energy and storage basics guide. Growing your own food matters too — see our garden self-sufficiency calculator to plan that side of your homestead. Check our off-grid cooling guide for DC heat pump options that pair well with a battery bank like this one. Once you have your numbers, check our off-grid budget calculator to see the total cost.

FAQ

How big of a battery bank do I need off-grid?

It depends on your continuous load, how many hours or days of backup you want, and your battery chemistry. Lead-acid banks generally need roughly double the rated capacity of lithium for the same usable runtime, since lithium can be discharged much deeper.

Lithium or lead-acid: which is better for off-grid?

Lithium (LiFePO4) costs more upfront but offers much deeper usable discharge, longer cycle life, and less weight. Lead-acid is cheaper initially but needs a larger rated capacity and more careful maintenance to reach a similar lifespan. Which is “better” depends on your budget and how you weigh upfront cost against usable capacity.

Does cold weather affect battery runtime?

Yes. Cold temperatures reduce usable battery capacity, particularly for lead-acid batteries. If you’re in a cold climate, it’s worth sizing with some extra margin beyond the calculator’s baseline estimate.

Will solar recharging extend my runtime beyond this estimate?

Yes. This calculator assumes no solar input during the backup period, which represents a worst-case scenario like a stretch of cloudy days. Any solar recharging during that time will extend your actual runtime beyond the estimate.

This calculator provides general planning estimates, not an engineering design. Actual runtime depends on battery age and condition, temperature, wiring, inverter efficiency, and manufacturer specifications — consult a qualified solar installer or electrician before purchasing equipment.