Expansion is not a second system. The calculator keeps what you already own and sizes only the extra panels and inverter capacity needed for the new load.
Start from the units you still buy. Your remaining grid kWh are the gap the new solar has to close; the appliance list explains what those units are.
The inverter is often the real constraint. An existing inverter can usually host up to about 1.35× its rating in DC panels. Beyond that, the upgrade is forced.
What the expansion calculator answers
You already have solar. Now you are adding an air conditioner, an EV charger, a water pump, induction cooking or a workshop. The array that covered last year’s bill may no longer cover next year’s. The calculator answers:
- How much additional solar capacity the new load needs.
- How many extra panels that means at your panel wattage.
- Whether your existing inverter can host the extra DC power or must be replaced.
- What the expansion costs now versus later, including a retrofit premium.
- When to do it, based on your appliance timeline.
It sizes solar and inverter capacity only. Batteries are handled separately because backup requirements depend on outage length and the loads you must keep running.
Two ways to describe the same gap
Method 1: the units you still buy
This is the fastest and usually the most accurate input. Look at a recent bill after your existing solar is working and enter the kWh you still import. Those are the units the new solar has to cover. It already includes your real behaviour, appliance duty cycles and seasonal variation, so there is less to guess.
Method 2: the appliances you plan to add
Build a list of future loads with a power rating and daily running hours:
Monthly kWh = daily kWh × 30.42
You can enter amps instead of kW if that is what the label gives you. The conversion is:
Three-phase: kW = A × 415 V × 1.732 × power factor ÷ 1000
If both methods are used, the sizing follows the grid units and the appliance list explains where those units come from. If the two disagree by more than 15%, the calculator flags it rather than silently picking one. A big disagreement usually means an appliance is rated at a duty cycle you did not account for, or the bill includes loads you have not listed.
How the extra solar is sized
The model first works out how much energy the new solar must produce, then converts that into kWp using a specific yield:
Specific yield = your existing monthly generation ÷ existing kWp
Default when generation is blank: 130 kWh/kWp/month
Using your own measured generation is better than the default because it captures your location, tilt, shading and soiling. The calculator flags values outside a 60–220 kWh/kWp/month sanity band instead of silently clamping them. That flag is worth investigating: a very low yield may mean a shading problem, a faulty inverter or a meter reading in the wrong units.
The extra kWp is rounded up to the next 0.5 kWp, then converted to whole panels at your chosen panel rating. The panel count also assumes about 2.58 m² per panel with a 30% allowance for fire setback and access, so it is a planning area, not a precise layout.
The inverter check
An inverter’s DC input can usually accept more panel capacity than its AC rating. The calculator assumes the existing inverter can host up to 1.35× its rated kW in DC panels. That is a planning convention, not a universal datasheet limit.
Future total DC = existing kWp + extra kWp
Upgrade needed if future total DC > existing inverter kW × 1.35
When an upgrade is needed, the tool selects the smallest standard size from 3, 5, 6, 8, 10, 12, 15, 20, 25, 30, 40 or 50 kW that keeps the DC/AC ratio inside 1.15–1.35 and carries the worst-case property peak. The peak check adds the new appliances together and, if you provided it, your existing peak load.
This is why the answer is sometimes “add panels” and sometimes “add panels and replace the inverter”. Replacing the inverter is the expensive part, and doing it twice is worse than doing it once at the right size.
A worked example
An existing 5 kWp system with a 5 kW inverter still imports 300 kWh/month. You plan to add a 1.5-ton inverter AC and an EV charger. Use 550 W panels and the default 130 kWh/kWp/month yield.
Extra panels = 2,500 ÷ 550 = 4.5 → 5 panels
Future total DC = 5.0 + 2.5 = 7.5 kWp
Existing inverter headroom = 5 × 1.35 = 6.75 kWp
7.5 > 6.75 → inverter upgrade needed
Smallest standard inverter for 7.5 kWp at 1.15–1.35 DC/AC: 6 kW (ratio 1.25)
The plan is five extra panels plus a 6 kW inverter. If the existing inverter were 6 kW, its headroom would be 8.1 kWp and the 7.5 kWp total would fit without replacement — a useful reason to size the original inverter generously.
The cost estimate uses your own installed cost per watt, not a market average:
Later cost = extra capacity cost × (1 + inflation)years
Inverter work = inverter kW × $/kW, plus a retrofit premium if done later
These are illustrative. A real quote depends on scaffolding, cable runs, switchgear, permits and whether the existing rails and roof fixings can take another row of panels.
Timing the expansion
The appliance timeline (Now, 1–2 years, 3–5 years, Later) does not change the physics, but it changes the economics. Waiting can mean higher equipment prices and a retrofit premium, but it also avoids buying capacity you do not need yet. The calculator shows both “today” and “later” costs so you can see the trade.
Where the model stops
Use the result to decide whether an expansion is a panel-only job or an inverter project, and to get a realistic budget before requesting quotes. Then check the roof with the roof space calculator and the production with the yield calculator.
Frequently asked questions
How much extra solar do I need for an EV charger?
Start from the kWh the car will use. A car driven 40 km/day at 6 km/kWh needs about 6.7 kWh/day, or roughly 200 kWh/month before charging losses. At a planning yield of 130 kWh/kWp/month, that is about 1.5–1.8 kWp of extra solar, or 3–4 panels of 550 W. The calculator uses your actual remaining grid units and yield if you provide them.
Can I add panels without replacing my inverter?
Often yes, up to about 1.35× the inverter’s rated kW in total DC capacity. A 5 kW inverter can usually host around 6.75 kWp, so a 5 kWp array has roughly 1.75 kWp of headroom. The exact limit depends on the manufacturer’s maximum DC input power and voltage window, so check the datasheet rather than relying on the rule alone.
Why does the calculator add a retrofit premium?
Because doing inverter work later usually costs more than doing it during the original installation: scaffolding may be needed again, the existing system must be isolated, and the installer may charge a call-out or re-commissioning fee. The default 40% premium is a planning assumption and can be set to zero if you have a fixed quote.
Should I use my remaining grid units or list every appliance?
Use the grid units when you have a reliable bill: they are measured and already include real behaviour. Use the appliance list when you do not know the units, or when you want to understand which load is driving the need. Using both is best: the units size the system and the list explains the gap.
Does the calculator include a battery?
No. It sizes solar and inverter capacity only. Battery backup depends on which loads must stay on during an outage and for how long, so it is a separate calculation. Adding an EV or AC may also change the backup requirement, so revisit the battery backup calculator after the expansion.
Will the extra panels reduce my bill immediately?
Only to the extent they produce during the hours you import power. If the new load runs at night and there is no battery or net metering, extra daytime solar may be exported at a low credit rather than offsetting the new consumption. The expansion calculator sizes generation; the grid-versus-battery and ROI tools show how much of it you actually use.