Every solar job passes or fails the same six families of electrical check, whatever the country and whatever the kit. A design that has not been through all six is not a design. It is a drawing with a price on it.
None of the checks is hard. Each compares a number on a datasheet with a number the site or the inverter sets. What goes wrong is who runs them, and when. This is the map: the principle at each, the mistake firms make there, and where to go deeper. The electrician signs the design off. The checks are what they are signing.
1. The string: a window with a top and a bottom, and a current the tracker will take
Four things at once. On the coldest morning the string's open-circuit voltage, which rises as the cells get colder, must stay under the inverter's DC input limit: that is the voltage that destroys inverters. On the hottest afternoon its operating voltage, which falls as the cells get hotter, must stay inside the tracker's MPPT window, or the inverter loses the maximum power point. Strings in parallel on one tracker add their current, which must stay under the tracker's input limit. And the array as a whole, DC watts against AC rating, has to be a sensible ratio: too high and the inverter clips on bright days, too low and the inverter is bigger than the roof needs.
The mistake: sizing at standard test conditions. The datasheet's Voc is measured at a cell temperature the panel never sees on a frosty morning, and its Vmp at one it never sees in July. A string checked at one temperature passes at the desk and fails on the roof. How many panels can a string take? has the four checks with the arithmetic and a worked example.
2. The DC side: cables and fuses are sized for the fault, not for the sunshine
DC cables and string fuses are not chosen for the operating current. They are chosen for the panel's short-circuit current with a margin on top, because irradiance exceeds the test standard and a fault has to clear. That design current, the system voltage and the length of the run decide the cable: its voltage rating, its current rating as installed, and the voltage it loses between roof and inverter. Voltage drop is a loss paid every day the system runs, so it is checked against a recommended limit, not only a permitted one. Then isolation and protection: a DC isolator the electrician will be able to reach, and string fuses where parallel strings need them.
The mistake: "we always use the same cable". The standard cable is right for the standard run. The inverter in the garage with the array on the far gable is not the standard run, and nobody worked out the drop because nobody measured the route.
3. The AC side and the connection: the supply decides the route, and the survey decides whether you know it
Three facts about the supply decide the AC design: the number of phases, the main fuse rating, and the consumer unit, its type, its condition and its spare ways. They are captured at the survey or guessed, and the survey checklist lists them with the reason each matters. The grid connection route then follows from the inverter's AC rating and the network operator's rules: in the UK, Engineering Recommendation G98 for the smaller systems that are fitted and then notified, G99 for the larger ones that need an application and an approval before the install. Export limiting is a choice, not a fix: it keeps a larger inverter on the simpler route at the cost of energy the customer will never sell, so it is written on the quote.
The mistake: finding out the route after the quote. A system priced on the notify-afterwards route that turns out to need an application first has gained weeks of waiting and a customer who was told a date.
4. Battery and hybrid systems: which side it couples to, how fast it moves, and where it is allowed to live
A battery brings three checks of its own. The first is coupling. A DC-coupled battery needs a hybrid inverter and shares its DC input, so the battery's voltage and the inverter's charge and discharge limits have to match. An AC-coupled battery brings its own inverter and works with any PV inverter, at the cost of a second conversion. The second is rate and capacity. The inverter limits how fast the battery charges and discharges, and if the customer wants backup, the loads kept on have to sit under the discharge rate and the hours wanted have to fit in the usable capacity. The third is location. Battery siting standards, PAS 63100 in the UK, rule some rooms out, allow others with conditions, and cap the storage in one enclosure. Decide the location at the survey, before the quote.
The mistake: the battery as a line on the quote, added on the phone after the survey, priced from a list, and never put through the checks. The inverter was not hybrid, or the loft was not permitted, and install day became redesign day.
5. Other loads on the same job: the charger and the diverter share the supply you have already used
Customers who buy solar buy EV chargers and diverters too, and all of them draw on the same main fuse and consumer unit. A three-phase charger will not go on a single-phase supply. A high-power single-phase charger, added to the house's existing load, has to sit under the main fuse with headroom. Two chargers need load balancing. A diverter, or a charger with solar diversion, needs a PV system to divert from, forgotten when the panels come off the order and the diverter does not. One supply capacity, counted once, with everything on the job added to the loads already there.
The mistake: three designs for one house. The PV was checked, the battery was checked, the charger was checked, each on its own sheet, and nobody added them up against the fuse.
6. The check as a record: every design validated before it is priced, and the result kept with the design
A check that was run and not written down was not run. Every design is validated before a price goes on it, not after. The result separates errors from warnings: an error cannot go on the roof as drawn, a warning is a decision the designer should be able to defend, such as a high ratio on an east-west roof. The result is saved with the design it describes. The checks then feed the paperwork: the electrical installation certificate under the wiring regulations (BS 7671 in the UK), the scheme certificate, the building-control notification, and the network operator's notification or application, each with a clock running from a date on the job. The design sheet can start on paper, once.
The mistake: the check in the electrician's head. The design was fine, because the electrician looked at it. Later the manufacturer asks for the string design on a warranty claim, and the only record is a drawing with a panel count on it.
Installa runs the checks for you
- Stop sizing strings by hand. Installa shows the window of panels per string and the strings each tracker will take. Every string is checked at a cold and a hot cell temperature against the inverter's DC limit and MPPT window, each tracker's strings against its input current, and the array-to-inverter ratio is reported. The same checks run in the 3D designer as the layout changes.
- Stop guessing the route. With the supply phases and any export limit on the design, the system summary says whether the job is a G98 notification or a G99 application, counting the PV inverter and the battery's own inverter together.
- Stop adding the battery as a line. A DC-coupled battery on a non-hybrid inverter is an error. Battery voltage, charge and discharge rates are checked against the inverter, and backup loads and hours against the battery. The survey assesses the proposed battery location against PAS 63100 as it is typed, and the job file carries an item to verify it.
- Stop adding up the loads by hand. An EV charger on the design is checked for its phases against the supply, its current against the main fuse, load balancing where there is more than one, and solar diversion where there is no PV to divert.
- Stop wondering whether a fault matters. Errors are listed apart from warnings, grouped by string, battery and grid, and the last result is saved with the design with the time it ran.
- Stop chasing the paperwork from memory. Once the route is set, the job file lists the DNO form, the single line diagram, the submission and the approval before install, then the commissioning notification, the EIC, the Part P notification and the MCS certificate, each with a due date counted from the contract, install or commissioning date. Overdue and due-soon items reach the home screen, and booking the install on a job whose application still needs approval brings a warning.
Cold and hot on every string, cables sized for the fault, the supply known before the route, the battery coupled and sited on purpose, the loads added once, the result kept with the design. See how electrical validation in Installa works, or try the string sizing calculator with your own kit.