Sizing an Off-Grid Battery Bank for Wind and Hydro, Not Just Solar

My own property runs on micro-hydro with solar as a supplement, and the battery bank behind that system was sized completely differently to how I'd size one for a friend running solar with a wind turbine backup. Different generation source, different production pattern, different sizing logic. Treating them the same is where I see off-grid systems go wrong.

Solar's daily cycle is the easy case

Solar production follows a predictable daily curve, zero overnight, ramping up through the morning, peaking around midday, tapering off by evening, with the main variable being cloud cover and season. Battery sizing for a solar-only off-grid system is largely about bridging that predictable overnight gap plus enough extra autonomy days to cover a multi-day cloudy stretch. It's the reference case most off-grid sizing guides are actually written around, which is part of why applying the same logic unmodified to wind or hydro causes problems.

Hydro's steadier baseload changes the calculation

A micro-hydro system with a reasonably consistent water source produces something much closer to a flat, continuous output around the clock, rather than solar's peak-and-trough daily curve. That steadier baseload can mean a smaller battery bank is genuinely sufficient. You're not trying to store a full day's generation to release overnight, because generation is already roughly matching a flatter consumption pattern much of the time. The real risk with hydro isn't a daily gap, it's seasonal: a dry season or drought dropping flow rates well below the level the system was sized against, which is a multi-week or multi-month problem rather than an overnight one, and needs to be sized for separately using your actual seasonal flow data, not an annual average that hides how bad the worst month gets.

Wind's gustiness demands the opposite approach

Wind output is far less predictable minute-to-minute and day-to-day than either solar or hydro. Genuinely still days can follow genuinely windy ones with little warning, and even within a windy day, output can swing hard as gusts come and go. A battery bank behind a wind-primary off-grid system needs to handle a real prospect of several consecutive low-wind days, which generally means more autonomy days built in than the solar-only reference case, not fewer. I size wind-backed systems assuming a longer stretch of underperformance than I'd assume for solar, based on the specific site's measured wind variability rather than a generic assumption.

Depth of discharge and chemistry matter more off-grid

Off-grid batteries get cycled harder and relied on more completely than a grid-tied battery that's mostly there for arbitrage or occasional backup, so cycle life and how deeply you're comfortable discharging the bank on a bad week both matter more here. I lean toward LFP chemistry for off-grid banks specifically because of its longer cycle life under the kind of frequent, sometimes deep cycling an off-grid system puts a battery through. The chemistry tradeoffs are the same ones that apply to grid-tied batteries, just with higher stakes if you get the sizing wrong.

Multi-source systems are usually the right answer

Most of the well-designed off-grid systems I've seen combine at least two generation sources specifically because their production patterns complement each other, solar's daily peak paired with hydro's steady baseload, or wind's more available output in seasons when solar is weaker. A well-matched combination can reduce the total battery bank size needed compared to any single source carrying the whole load, because you're not trying to store enough of one intermittent source to cover every gap alone.

A generator as the honest backstop

I'll say something that isn't always popular in off-grid circles: a small backup generator, sized to cover genuine worst-case gaps rather than daily use, is often the more honest and cost-effective answer to a rare, extreme low-generation stretch than oversizing a battery bank to cover it. Building a battery bank large enough to handle a once-in-a-decade dry spell or dead-calm fortnight on its own can cost more than the generator plus fuel would over the system's entire life. I'd rather size the battery bank around a realistic, regularly recurring worst case and let a generator handle the genuine outlier.

What I'd actually do

Get real production data for your specific site and generation source, measured, not assumed, before sizing a battery bank, and size the autonomy days around your worst realistic stretch for that specific source, not a generic industry rule of thumb built around solar. A hydro-primary system can often run leaner on battery capacity than people expect; a wind-primary system usually needs more headroom than people initially budget for.

See our existing piece on micro-hydro for off-grid properties and how wind turbines generate electricity for the generation side of this, and our battery chemistry comparison for the storage side. Our piece on micro-hydro permitting covers what needs sorting before you even get to sizing equipment.

The US Department of Energy's hydropower basics resources and the Australian Renewable Energy Agency's hydro program pages both have genuinely useful background on small-scale hydro resource assessment.

Inverter and charge controller selection for mixed sources

Combining solar, wind and hydro into one battery bank generally means each source needs its own charge controller matched to its specific voltage and current characteristics, feeding into a shared DC bus or a hybrid inverter designed for multiple input types. Not every inverter on the market handles a genuinely mixed-source setup gracefully, and I've seen systems where a controller designed primarily for solar was pressed into service for a hydro input it wasn't really specified for, with the mismatch showing up as inefficient charging rather than an outright fault. This is a case where I'd lean on an installer or system designer with genuine multi-source off-grid experience rather than someone whose background is purely grid-tied solar.

Monitoring what you've actually got

Off-grid systems benefit enormously from genuine production and battery-state monitoring, not just a voltage reading on the inverter's front panel. Knowing your actual daily generation by source, not just total battery state of charge, is what lets you catch a degrading turbine bearing or a partially blocked hydro intake before it becomes a real supply problem. I check my own system's per-source production weekly, and it's caught more than one developing issue early enough to fix cheaply rather than after a full outage forced the question.

Budgeting realistically for a multi-source system

A well-integrated multi-source off-grid system, proper controllers, a hybrid inverter genuinely rated for the combination you're running, and a battery bank sized to the specific generation pattern rather than a generic rule of thumb, costs more upfront than a solar-only system of similar total capacity. I'd rather a client understand that cost difference honestly at the planning stage than be surprised by it partway through a build, because the payoff, genuinely reliable power through conditions that would leave a solar-only system short, is real, but it isn't free.

— Grace Lindqvist, Wind, hydro & off-grid