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At dusk on a campsite, with the phone dead and the panel on the roof done for the day, solar charge controller size is the question that decides whether the battery fills again tomorrow. Size comes down to matching the controller amp figure against the panels, the max PV voltage against the array, and the battery voltage against the bank. The listings below show the figures makers state, so you can match a controller to the panels and battery you already have.
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The quick answer
Start with the panels: divide the panel watts by the battery volts to get the charge current, then pick a controller with an amp figure above that current. Then check the array voltage against the controller max PV voltage, and check the battery voltage against the controller battery range. A controller that fits on every match is the right size, and one that falls short on any of them is not.
Step 1: Work out the charge current
The charge current is what the controller has to pass from the panels to the battery, so it sets the amp figure you need. Divide the panel watts by the battery volts to get that current, then look for a controller with an amp figure above it. The controllers in these listings are rated 15A, 30A, 45A and 60A, so the range covers small arrays up to the PowMr, which lists a 2800W max input. A controller with too low an amp figure cannot pass all the panel current, so the extra power is not used.
Step 2: Check the array voltage against the max PV voltage
Every controller lists a max PV voltage, and the array voltage must stay under it. The listings here state 75V, 100V, 150V and 160V max PV voltage. Panels wired in series add their voltage together, so check the array open-circuit voltage against the controller figure before you wire it. The VictronConnect app on the Victron controllers shows real-time data and a 30-day performance history, so the array voltage is visible once the system runs.
Step 3: Match the battery voltage
The controller has to list your battery voltage, or it cannot charge the bank. The Victron 100V 30 amp and the Victron 75V, 15A list 12/24V, while the Victron 150V, 45A and the PowMr list 12/24/36/48V. The PowMr detects the battery voltage on its own and lists auto detection for 12V, 24V, 36V and 48V, with a backlight LCD that shows PV voltage, output power, battery voltage, charging current, working mode and temperature. Pick the controller that lists your battery voltage, and check the battery type too: the PowMr lists lead-acid and lithium, with user settings for lithium. See the MPPT solar charge controllers for lithium batteries for lithium setups. The Victron controllers also list an intelligent load output that powers devices directly and disconnects the load if the battery voltage drops too low.
Solar charge controller size: what the listings state
The table below lines up the stated figures for the controllers in these listings, so you can see the amp figure, max PV voltage and battery voltage side by side.
| Controller | Max PV voltage | Charge current | Battery voltage | Max input |
|---|---|---|---|---|
| Victron SmartSolar 100V 30 amp | 100V | 30A | 12/24V | Not stated |
| Victron SmartSolar 75V, 15A | 75V | 15A | 12/24V | Not stated |
| Victron SmartSolar 150V, 45A | 150V | 45A | 12/24/36/48V | Not stated |
| PowMr 60A | 160V | 60A | 12/24/36/48V | 2800W |
The Victron 100V 30 amp states a 2-year manufacturer warranty. The other listings state no warranty term.
Ways to check a controller before you buy
- Match the amp figure first. Work out the charge current from the panel watts and battery volts, then pick a controller with an amp figure above it.
- Check the max PV voltage. The array voltage must stay under the controller figure, so check the array open-circuit voltage before you buy.
- Check the battery voltage and type. The controller has to list your battery voltage, and the PowMr lists lead-acid and lithium types with user settings for lithium.
- Look for the display and app. The PowMr lists a backlight LCD, and the Victron controllers work with the VictronConnect app for real-time data and a 30-day performance history.
- Check the warranty term. The Victron 100V 30 amp states a 2-year manufacturer warranty, while the other listings state no warranty term.
FAQ: power away from the grid
What size charge controller for a 400W solar panel?
Divide the panel watts by the battery volts to get the charge current, then pick a controller with an amp figure above that current and a max PV voltage above the array voltage. The method is the same for any panel size: the watts and the battery volts set the current, and the controller has to cover it.
What happens if the controller amp figure is too low?
The controller cannot pass all the panel current, so the extra power is not used and the battery charges slower than the panels allow. Pick a controller with an amp figure above the charge current, so the full array output reaches the battery.
Can a 12V controller charge a 24V battery?
Only if the controller lists 24V among its battery voltages. The Victron 100V 30 amp and the Victron 75V, 15A list 12/24V, while the Victron 150V, 45A and the PowMr list 12/24/36/48V, so pick the controller that lists your battery voltage.
What is the max PV voltage on these controllers?
The listings state 75V, 100V, 150V and 160V max PV voltage: the Victron 75V, 15A at 75V, the Victron 100V 30 amp at 100V, the Victron 150V, 45A at 150V and the PowMr at 160V. Check the array open-circuit voltage against the controller figure.
Do these controllers state a warranty term?
The Victron 100V 30 amp states a 2-year manufacturer warranty. The other listings state no warranty term, so check the listing before you buy if a warranty term matters to you.
Related solar guides
See every charge controller guide and all controllers and inverters guides. For picking a controller, see our MPPT charge controllers for 24V systems and MPPT solar charge controllers for RVs, and for the battery side see 12V LiFePO4 batteries.
