Panels and controllers for this array
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Assumptions used
Array watts = daily Wh ÷ (peak sun hours × loss factor).
Loss factor 0.80 for MPPT, 0.68 for PWM, covering soiling, wiring, heat derate and controller efficiency.
How this calculator sizes a solar array
Array watts = daily consumption ÷ (peak sun hours × system loss factor). Two of those three inputs are routinely misunderstood, which is why so many arrays disappoint.
Peak sun hours are not daylight hours
A peak sun hour is one hour of 1,000 W/m² irradiance. A bright fourteen-hour summer day in northern Europe might deliver five peak sun hours; an overcast December day in the same place might deliver one. Daylight length tells you almost nothing.
The winter figure is the one that matters
This is the single most expensive mistake in off-grid solar. Sizing to the annual average leaves you short for months.
| Region | Annual avg PSH | Worst month PSH | Array for 1,450 Wh/day |
|---|---|---|---|
| Northern UK / Pacific Northwest | 3.4 | 1.2 | 533 W → 1,510 W in winter |
| Northern US / northern France | 4.0 | 1.8 | 453 W → 1,007 W |
| Central US / central Europe | 4.6 | 2.4 | 394 W → 755 W |
| Southern US / Mediterranean | 5.4 | 3.4 | 336 W → 533 W |
| Desert Southwest / Australia | 6.2 | 4.4 | 292 W → 412 W |
In northern latitudes covering winter needs roughly three times the panel of the annual average. At that point most people add a second charge source — alternator charging or a generator — rather than buy an array that sits idle for eight months.
System losses
Panels never deliver their rated output in service. The loss factor covers soiling, cable resistance, heat derate, panel mismatch and controller efficiency. This calculator uses 0.80 for MPPT and 0.68 for PWM.
That gap is the real argument for MPPT. A PWM controller clamps the panel to battery voltage and discards the difference, which on a 36-cell panel charging a 12 V bank is a meaningful share of what you paid for. Above roughly 600 W of array, the extra panels PWM demands cost more than the MPPT controller you avoided buying.
Common questions
How many solar panels do I need to run a house off-grid?
For a typical off-grid home consuming 8 kWh a day in a region averaging 4.6 peak sun hours, you need roughly 2,175 W of panel — about six 400 W modules, occupying around 12 m². Covering the same load in the worst winter month at 2.4 peak sun hours requires closer to 4,170 W. Most off-grid homes at that latitude pair a moderate array with a generator rather than sizing for December.
What is a peak sun hour?
One hour of solar irradiance at 1,000 watts per square metre — the condition panels are rated under. It is a measure of energy delivered, not of time. Five peak sun hours might be spread across a fourteen-hour summer day.
MPPT or PWM — which controller should I buy?
MPPT for anything above a few hundred watts. It converts excess panel voltage into additional charging current instead of discarding it, typically yielding 15–25% more energy from the same panels, with the advantage largest in cold and low light. PWM is only sensible on very small arrays where the controller price is a large share of total cost.
Can I add more solar panels later?
Usually yes, provided your charge controller has headroom on both voltage and current, and you match the new panels to the existing string. Mixing panels of different voltage or current in one string drags the whole string down to the weakest module. Buying a controller with spare capacity from the start is cheap insurance.
Do solar panels work in cloudy weather?
They produce, but far less — commonly 10–25% of rated output under heavy cloud. This is why days of autonomy in the battery calculation matter: the array is not the thing carrying you through a bad week, the bank is.
These figures are planning estimates. Electrical installation carries real risk of fire and injury — have your design checked against the regulations that apply where you are, by someone qualified.