How to size a home solar system properly

Most disappointment with solar is not about panel quality; it is about sizing. An oversized system simply cost more than it needed to, and an undersized one never delivers the expectation that sold it. Getting it right is not complicated, and it starts with your own bills.

# Step one: know your real consumption

Lay out the last twelve months of bills and note the kilowatt hours for each period. A full year matters, because summer with air conditioning and winter with electric heating look nothing alike. Divide the annual total by 365 for an average daily figure.

# Step two: local irradiance, not national

This is where most of the error enters. The figures in advertising are usually based on central and desert Iran. Gilan has more cloud, more rain and fewer useful sun hours. Size an array on a Yazd number and it will underperform here.

A proper calculation uses peak sun hours for your actual coordinates, which in Gilan are appreciably lower than on the central plateau.

# Step three: do not ignore the losses

There is a gap between what a panel produces on paper and what reaches your socket. The real losses come from:

• Inverter efficiency, typically 95 to 98 per cent
• Voltage drop in the DC and AC cabling
• Panel efficiency falling as cell temperature rises in summer
• Dust and soiling between washes
• Partial shade from a tree, chimney or neighbouring building for part of the day
• Gradual power degradation over the panel’s life, roughly half a per cent a year

A conservative calculation puts these together at around 20 to 25 per cent, which means the nameplate capacity has to exceed the net requirement.

# A worked example

Say a household averages 12 kWh a day and wants all of it from solar. With 3.5 peak sun hours and a 20 per cent loss allowance, the capacity works out roughly like this:

# Do you need a battery?

This decision moves the cost more than any other. If the aim is to cut the bill and the grid is stable, a grid-tied system with no battery is the sensible choice; the grid does the storing. If your supply fails often, or there is no supply at all, a battery is necessary and must be designed for several overcast days in a row.

# Notes for Gilan

• Winter snow load has to be designed into the mounting structure
• High humidity means galvanised structure and sealed connections
• Cable routes must not let water pool
• Frequent lightning makes surge arrestors on both DC and AC sides essential
• The rain has an upside: it washes the array, so soiling losses are lower than in dry regions
• Keep the panels standing off the roof so it can drain and breathe

# Questions worth asking any installer

1. What consumption figure and what peak sun hours did you size this from?
2. What loss factor is in the calculation?
3. Was a shading survey done, or just the roof area measured?
4. What does the DC-side protection consist of?
5. What documentation comes at handover, and does it include a single-line diagram and a commissioning report?
6. Whose warranty covers the panels and inverter, and how would a claim actually be made?

Vague answers usually mean no calculation was done. For a consumption review and feasibility check in Gurab Zarmikh, Soumeh Sara and the surrounding area, get in touch.