Direct-drive solar: loads that can run without a battery

Almost all consumer solar advice assumes a battery. Panels charge it, it powers the house, and when it wears out you buy another one. The battery is usually the most expensive part, the part that degrades fastest, and the part that decides when the whole system needs replacing.

There is an older arrangement that skips it. Wire a panel straight to a motor, a pump, a compressor or a heating element, and the thing runs while the sun is up. Nothing stores electricity, so nothing degrades on a five-year clock. This is called direct drive, and it is common in agriculture and development engineering and almost absent from consumer comparison sites.

The short answer. Direct drive only works for loads that can wait for sunshine. Where the load can wait, it is dramatically cheaper and lasts much longer, because the expensive consumable is gone: a Lorentz PS2-100 moves over 20,000 litres a day from one 205 watt panel with no battery, and a pond aerator costs $199.99. Where the load cannot wait, adding a small battery for that one job is the right answer, and a full battery system costs roughly $11,000 to $15,000 more than solar alone. If mains power already reaches your property, staying on it is usually correct: a new line extension runs about $20,000 a mile, and below roughly a sixteenth of a mile the grid wins.

The trick is storing the work, not the electricity

Direct drive is not about doing without storage. It is about storing something cheaper than electrons.

A solar pump fills a tank during the day, and the tank feeds the trough all night. A solar refrigerator freezes a block of material during the day, and that block holds the temperature after dark. The storage is water, or ice, or heat, and all three cost far less per unit than a battery.

The National Center for Appropriate Technology puts the principle plainly in its guide to livestock watering: storing water is almost always cheaper and simpler than storing electricity. The same guide says batteries are usually avoided in these systems because they reduce overall efficiency and add maintenance and cost.

That is the whole idea, and it tells you exactly which jobs suit it.

What it costs, and what the alternative costs

The battery figure comes from the National Renewable Energy Laboratory's Q1 2021 benchmark, which put residential solar without storage at $2.65 per watt DC, and a 7.15 kW system with 5 kW and 12.5 kWh of storage at $30,326 to $33,618 all in. That gap is what a battery buys you, and what direct drive declines to spend.

The line extension figure comes from University of Wyoming Extension, which also did the arithmetic in the other direction: solar pumping beats a new grid connection on annual cost unless the extension would cost under about $1,348, which is roughly one sixteenth of a mile. If the poles are already close, connect to them.

The limit, stated plainly

A direct-drive system produces nothing after sunset and little under heavy cloud. Every straight description of the approach says so, and any seller who leaves it out is selling you a disappointment.

The equipment in the explorer says it in its own words. The Missouri Wind and Solar aerator works from approximately dawn until dusk. The Remington attic fan is ready for an optional 110 volt hybrid adapter, sold separately, for nighttime operation, which means the standard unit ventilates only in daylight. Nobody is hiding this. It is simply the deal.

Two things soften it without buying a battery. The first is oversizing the panel: Living Energy Lights describes pairing a refrigerator that needs 80 watts with a 400 watt panel specifically to cover cloudy days. The second is choosing equipment that stores the result, like the SunDanzer BFRV55, which uses phase change material to eliminate cold spots and hold temperature, a design borrowed from vaccine refrigeration.

Adding a battery for one job, rather than for the house

If exactly one load cannot wait, the sensible move is a small battery for that load, not a whole-house system.

NCAT names the three situations where a battery earns its place in a watering system: the storage tank does not sit high enough for gravity flow, there is not enough tank capacity so water must be pumped at night or in cloudy weather, and continual flow is needed. That is a good general test. A battery is a fix for a named problem, not a default component.

One planning note that matters in the United States: the IRS's Residential Clean Energy Credit page states that battery storage has qualified for a 30 percent federal credit since 2023, and that the credit is not available for any property placed in service after December 31, 2025. Anyone budgeting a battery on the assumption of that credit should check the current position before signing anything.

The high-voltage version, and why it needs an electrician

Most direct-drive equipment runs at low voltage. Living Energy Farm in Virginia runs a different arrangement, documented by Living Energy Lights, that removes even more of the hardware: with direct drive, not only is the inverter removed from the circuit, but also the battery, with panels connected directly to loads and little or no controllers or other electronics.

The results are striking. Their Roxy Oven runs on 300 to 800 watts, against the 5,000 to 10,000 watts a typical electric oven draws. But their page repeatedly warns that professional electrician installation is required, because these systems typically run at 90 to 120 volts DC, and 180 to 200 volts DC for larger arrays. That is well above ordinary low-voltage solar and it is not a weekend project.

The same page states the constraint that governs the whole approach: not all DC appliances can run direct drive.

Deciding, in six questions

1. Can this load wait for the sun? If yes, direct drive is probably cheapest over any long period. If no, stop here and price a battery or the grid.
2. What can store the work instead of the electricity? A tank, a cold mass, a warm floor. If nothing can, direct drive is the wrong tool.
3. How far away are the poles? Under about a sixteenth of a mile, a grid connection usually wins on cost.
4. Does this pump need a controller? Dankoff says controllers are required for all positive displacement pumps used without a battery, and add around 30 percent to annual performance. Budget for it.
5. What voltage does this run at? Above roughly 60 volts DC you are hiring an electrician, and that changes the cost.
6. What happens on the third cloudy day? Oversize the panel, size the tank, or accept the outage. Choose deliberately rather than finding out.

What this guide will not tell you

It will not tell you that direct drive is a general replacement for household solar. It is not. It powers specific machines during specific hours, and a house full of lights, laptops and a freezer is not that.

It also will not tell you that skipping the battery is always greener. A generator is worse: the EPA's rules for nonroad spark-ignition engines list generators by name because they burn fuel and produce exhaust. But an existing grid connection carries no new manufacturing at all, and the greenest system is often the one you do not build.