Short answer: enough solar to put back what you use in a day, in the weather you actually camp in, with some extra for winter. The watts come from your daily use and what a panel makes where you'll be. Then check the second thing people skip: will the controller cope with those panels? Get that wrong and you either waste panel or damage the controller.
How do I work out how many watts I need?
Three steps.
- Know your daily use, in watt-hours or amp-hours. If you haven't worked it out yet, how to size a 12V system walks through the load list. Everything below hangs off that number.
- Find out what panels make where you go. It changes a lot with the season and where in Australia you are, so we won't give you a one-size rule of thumb. Victron's free MPPT sizing calculator shows the likely daily yield for a given array in a given place. Put in your location and a panel size, and see whether it covers your daily use.
- Size for the hard part of the year. Winter sun is weaker. If you tour in winter, size for winter, not a January afternoon.
If you also charge from the vehicle, the panels only have to cover the days you stay put. That's the DC-DC charger, solar controller or both? question.
How much solar can my controller take?
For an MPPT controller, start with the maker's nominal PV power, which depends on the battery voltage. Victron's figures for their SmartSolar range:
| SmartSolar model | Nominal PV power at 12 V | at 24 V | Max PV open-circuit voltage | Max PV short-circuit current |
|---|---|---|---|---|
| 75/10 | 145 W | 290 W | 75 V | 10 A (manual) or 13 A (datasheet) |
| 75/15 | 220 W | 440 W | 75 V | 15 A |
| 100/15 | 220 W | 440 W | 100 V | 15 A |
| 100/20 | 290 W | 580 W | 100 V | 20 A |
| 100/30 | 440 W | 880 W | 100 V | 35 A |
| 100/50 | 700 W | 1400 W | 100 V | 60 A |
(The 100/20 also takes 1160 W on a 48 V battery.)
The same controller handles twice the panel on a 24 V battery as on 12 V. And the model name is a shortcut: a 100/30 takes up to 100 V from the panels and charges at up to 30 A.
Victron's manual and datasheet disagree on the 75/10's current limit (10 A against 13 A); we'd work to the lower one.
Can I put more panel on than the controller's rating?
Yes, within limits. It's called oversizing. The controller just limits its input when there's more panel than it can use, so the extra isn't used at the sunniest moments.
Why bother? Because panels rarely hit their rating, maybe an hour in the middle of a sunny day. The rest of the day, and all winter, the extra panel makes up the shortfall.
How much extra? Victron's sizing calculator allows for 130%, which they say generally loses less than 1% of the year's energy. On the nominal figures above, that's about:
- 75/15 on 12 V: 220 W x 1.3 = 286 W of panel
- 100/30 on 12 V: 440 W x 1.3 = 572 W
- 100/50 on 12 V: 700 W x 1.3 = 910 W
Two cautions. That 130% comes from Victron's calculator, not a maximum written into the manuals or datasheets. And it never lets you break the two hard limits below. Other makers set their own figures: Enerdrive suggest at most 20% over (600 W total) on the 500 W solar input of their DC2DC40+.
What are the hard limits?
There are two, and each is checked on its own: the maximum PV open-circuit voltage and the maximum PV short-circuit current. Don't multiply them together: 100 V times 35 A isn't a 3500 W rating.
Voltage: panels in series add up, and cold makes it worse
Panel Voc times the number of panels in series must stay under the controller's maximum PV voltage. The colder it is, the higher a panel's open-circuit voltage goes, so work it out for the coldest morning you'll see. Victron's rule of thumb, in cold climates or with nights near or below 10 °C: keep an extra 10% safety margin. If you camp anywhere with cold nights, that means you.
Going over is not a small thing. Victron's manuals say the controller can be damaged, and "such damage is not covered by warranty." If it does go over, the controller stops charging with error #33 and only starts again once the PV voltage is 5 V below its maximum.
Current: panels in parallel add up
Panel Isc times the number of panels in parallel must not go over the controller's maximum PV short-circuit current. Panels in series don't change it. It matters because too much current can damage the controller if the panels are ever connected with the wrong polarity.
A worked check (made-up panel figures, for the example only)
Say your panel's label shows Voc 22 V and Isc 11 A. Yours will be different; use the figures on its label.
- Two in series on a 75/15: 2 x 22 V = 44 V, plus 10% for cold = 48.4 V. Under 75 V. Current stays at 11 A, under 15 A. Fine.
- Three in series on a 75/15: 3 x 22 = 66 V, plus 10% = 72.6 V. Technically under 75 V, but with almost no room. A 100-volt controller is the comfortable choice.
- Two in parallel on a 75/15: voltage stays at 22 V, but current is 2 x 11 = 22 A. Over the 75/15's 15 A. On a 100/30 (35 A) it's fine.
The same MPPT sizing calculator will run this check for you.
Anything else that limits the controller?
- Low light start-up. The panels have to beat the battery by 5 V before the controller starts, and by 1 V to keep going.
- Heat. Output is fully rated up to 40 °C around the controller; above that the 100/30 and 100/50 cut their output current. Where you mount it matters.
- No optimisers. Victron are blunt: panels with optimisers can cause "irreparable damage to the solar charger."
Is more panel always better?
- No. Past what the controller can use, it only helps on poor days, and it costs roof space and weight.
- If you've got more panel than your controller is comfortable with, the fix is a bigger controller or a second one, not ignoring the limits.
- Series or parallel also changes which controller suits, and whether MPPT is worth it at all. That's in MPPT vs PWM solar controllers. Where the panels go (roof, folding, blanket) is in fixed vs folding vs blanket solar.
Our solar panels list each panel's own figures, and the MPPT charge controllers are grouped together if you're matching one to the other.