Solar String Calculator
Enter your panel specs and string layout. We calculate temperature-adjusted voltage, current, and power, then check everything against your inverter or MPPT limits.
Getting the string config wrong can fry your inverter on a cold morning or leave half your panels underperforming. This calculator does the voltage maths (including cold-weather adjustment) and flags any limits you're about to break. Use it before you buy anything.
Panel Specs
Enter the specifications from your panel's datasheet. All values are at STC (Standard Test Conditions).
String Configuration
How are your panels wired together?
Inverter / MPPT Limits
Get these from your inverter datasheet. Never guess.
Quick reference: common UK inverter input limits
| Inverter / MPPT | Max Voc (V) | Max Isc (A) | Max DC Power (W) |
|---|---|---|---|
| Victron SmartSolar 150/35 | 150 | 35 | 2000 @ 48V |
| Victron SmartSolar 250/60 | 250 | 60 | 3440 @ 48V |
| Victron MultiPlus-II 48/3000 | 450 | 18 | 2400 |
| Victron MultiPlus-II 48/5000 | 450 | 18 | 4000 |
| Solis S5 3.6kW | 600 | 16 | 4680 |
| Solis S5 5kW | 600 | 16 | 6500 |
| GivEnergy Hybrid 3.6kW | 550 | 13 | 4680 |
| GivEnergy Hybrid 5kW | 550 | 13 | 6500 |
| SunSynk 3.6kW | 500 | 18 | 4680 |
| SunSynk 5.5kW | 500 | 22 | 7150 |
| Fronius Primo 3.0-1 | 600 | 18 | 4500 |
| Growatt SPH 3600 | 550 | 13 | 5000 |
Always check your specific model datasheet. Limits vary between firmware versions and regions. This is a reference, not a guarantee.
How string sizing works
Your inverter or MPPT has three hard limits you must never exceed:
- Max Voc (open-circuit voltage) - This is the most critical one. Panel voltage rises in cold weather. A typical panel's Voc increases by roughly 0.3% for every degree below 25°C. At -5°C, that's a 9% voltage boost. If your temperature-adjusted array Voc exceeds the inverter's max, you'll damage the electronics. Always leave at least 5% headroom.
- Max Isc (short-circuit current) - Your parallel strings add current. If you have two strings in parallel and each has 10.5A Isc, the input sees 21A. This must stay under the MPPT's rating.
- Max DC power - You can over-panel (connect more watts than the inverter can use) as long as voltage and current limits are respected. The inverter simply clips excess power. Over-panelling by 30-50% is standard UK practice to get decent winter output.
Critical: Cold weather voltage spike
The most common way people destroy inverters is forgetting that panels produce more voltage when it's cold. A 400W panel rated at 41V Voc at STC (25°C) will hit ~44.7V at -5°C. On an 8-panel string, that's 358V instead of 328V. Always size for your coldest expected temperature, not the datasheet number.Common questions
How many panels can I put on one string?
Divide your inverter's max Voc by your panel's temperature-adjusted Voc, then round down. For example: 600V max / 45V adjusted Voc = 13.3, so 13 panels maximum. Always use the cold-adjusted Voc, not the STC value. At -5°C, a typical 41V panel becomes about 44.7V.
What happens if I exceed the max Voc?
You can permanently damage the inverter or MPPT. The input capacitors and MOSFETs have absolute voltage ratings. Exceeding them causes immediate failure, and it's not covered by warranty. This is the number one way people blow up their charge controllers.
Can I mix different panel models on the same string?
It's not recommended. In a series string, current is limited by the lowest-current panel. In parallel, voltage is dragged down by the lowest-voltage string. Mismatched panels waste power. If you must mix, use identical panels per string and separate MPPT inputs for different string types.
Do I need to worry about the MPPT voltage range?
Yes. Inverters have both a minimum start voltage and an MPPT operating range (e.g. 120V-500V). Your string Vmp (at operating temperature) needs to sit inside that range. If your voltage drops too low on hot days, the MPPT stops tracking. If it goes too high, you exceed limits. The calculator checks both extremes.
Switch to Octopus Energy for top export rates
Once your string configuration is sorted, make sure you're on a tariff that pays you properly for your solar exports. Octopus Energy offers competitive SEG rates and smart time-of-use tariffs that pair perfectly with solar and battery storage. Both you and DIY Solar UK get £50 credit when you switch.
Switch to Octopus →