Solar · Controllers and inverters
A charge controller decides how much of what the array makes reaches the bank. An inverter decides what the house can actually run. Neither is glamorous, and getting either one wrong is how a correctly sized array still fails to start a well pump.
The two jobs
Panels produce direct current at a voltage that swings with sunlight and temperature. A battery bank wants a controlled voltage that changes with its state of charge. The charge controller sits between them and manages that mismatch.
On the other side, the bank stores direct current and the house runs on alternating current. The inverter handles that conversion, and its rating determines which appliances the property can run at all.
Both devices lose a little energy doing their work. Those losses are already inside the efficiency figure used on the array sizing page, which is why they do not get counted a second time here.
The controller is sized from the array and the bus voltage. The inverter is sized from the household's loads, not from the array. Those are separate calculations that happen to meet at the battery bank, and treating them as one number is a common source of mismatched systems.
Controller type
Two designs, and for a property-scale array the choice is not close.
Acts essentially as a switch, pulling the array down to whatever the battery is sitting at. If panels are producing at a much higher voltage than the bank, that difference is simply lost.
Cheap, simple, and reasonable on a small system where panel voltage closely matches bank voltage.
Continuously finds the array's most productive operating point, then converts surplus voltage into extra charging current. The energy PWM discards becomes amps into the bank.
The gain is largest in cold weather and whenever array voltage sits well above bank voltage, which describes most winter mornings.
There is a second reason MPPT wins at this scale. Because it decouples array voltage from bank voltage, panels can be wired in longer strings at higher voltage, which means lower current in the wire between array and controller. That single decision reduces conductor size and cost across what is often the longest run on the property.
Sizing the controller
Carrying the property forward: a 6,000 watt array from page one, on the 48 volt bus established in battery banks.
6,000 ÷ 48 = 125 amps flowing into the bank at full production. The controller has to be rated for at least that on its output side.
A 150 amp controller covers it. Two smaller controllers on split array strings is often the better answer, because it keeps the system running when one fails and allows different roof or ground orientations.
Separate limit, separate calculation. The controller has a maximum photovoltaic input voltage, and exceeding it destroys the unit. String length is set by that ceiling.
Photovoltaic open-circuit voltage rises as temperature falls. A string comfortably inside its controller's limit on a warm afternoon can exceed that limit at dawn on the coldest day of the year, before any load has been drawn.
String length is calculated against the record low temperature for the site, using the panel's temperature coefficient of voltage from its datasheet. Not the average low. The record.
Inverter capacity
Motors draw far more current starting than running. A well pump, a compressor, a table saw. The inverter has to survive that spike or the load simply never starts, and the inverter shuts down trying.
Built around a heavy transformer. Typically deliver around 300 percent of rated output for several seconds, which is what starting a submersible pump asks for. Heavier, more expensive, and they idle at a higher standing draw.
Solid-state switching. Lighter, cheaper, and lower idle draw, which matters because that draw is in the daily load figure. Surge is usually 150 to 200 percent and lasts a fraction of a second.
Put numbers on it. A 3,000 watt low-frequency inverter offers roughly 9,000 watts of surge. The same 3,000 watt rating in a high-frequency unit offers about 6,000, or 4,500 at the lower end of the range. Identical continuous ratings, very different behaviour at the moment a motor starts.
The power needs calculator works out startup surge across a device list and reports whether a given unit clears it, sits marginal, or falls short. Worth running before choosing between these two families.
Choosing the inverter
Pure sine wave is the practical requirement for a property running motors, electronics and anything with a transformer. Modified sine wave makes motors and transformers run hotter and less efficiently, and some equipment refuses it outright.
Continuous rating comes from the largest realistic combination of loads running at the same time. Not the sum of everything owned, and not a single appliance either. Think about a winter evening with the pump cycling.
Surge is checked against the hardest-starting motor on the property, with whatever else is likely running at that moment already accounted for.
An inverter left on consumes power whether the house is using any or not, and low-frequency units consume more of it. That standing load belongs in the daily watt-hour figure that sized the array, so choosing a heavier inverter slightly increases the panel count. It is a small effect, and it is real.
Limits
Many systems now combine controller, inverter and transfer gear in one enclosure. That changes the shopping, not the arithmetic. The same two calculations still have to be satisfied.
The cold voltage calculation needs your specific panel's temperature coefficient and your site's record low. Both are lookups, and both belong to the equipment you actually buy.
The 125 amps calculated above has consequences for wire gauge and overcurrent protection that belong on the wiring and code page.
Inverter-chargers that start and manage a backup generator are a real part of many off-grid systems, and they are covered in the continuous-duty generation section.
Common mistakes
A 6,000 watt array does not call for a 6,000 watt inverter. The inverter serves the house, the array serves the bank. Different questions.
The amp rating is the number people compare. The maximum photovoltaic input voltage is the one that destroys the controller on a cold morning.
Two inverters with the same rating can differ by 4,500 watts at the moment a pump starts. The spec that matters is the one most listings mention last.
The saving is real and small. The energy given up recurs every day for the life of the system, and it is worst in the month the array can least afford it.
Next
125 amps on the bank side is the number the next pages work from. These are in progress and will link here as they publish.
Where the array physically goes, and how orientation and string layout interact with controller choice.
Conductor sizing and overcurrent protection for the currents established on this page.
Where cold open-circuit voltage, short days and a working bank all meet.
Back to Solar at Property Scale, or to battery banks for the bus voltage this page uses.
Sources
Surge percentages describe typical behaviour by inverter family and vary between products. The rating that governs your system is the one on the datasheet of the unit you buy.