How many solar panels and batteries will I need for my home?

Let's start with some grounding assumptions

Right now, you're probably looking at your electricity bill, your roofline, and a growing sense that something needs to change.

Maybe it's the cost.
Maybe it's the blackouts.
Maybe it's the idea of energy independence—or just cutting ties with a grid that feels less reliable each year.

If you're asking this question, chances are you want three things:

1. Lower and more predictable energy bills—tired of watching costs climb every quarter.
2. Backup power during blackouts—keeping essentials running when the grid fails.
3. A system that's safe, reliable, and future-proof—something that works today and adapts tomorrow.

Before we size anything, here's what I'm assuming:

• You have reasonable roof space or ground area with decent sun exposure (north-facing in the southern hemisphere, south-facing up north). Although east/west configuration can work too for consitent, but lower output throughout the day.
• Your home is connected to the grid, though you might be weighing off-grid or backup scenarios.
• You're thinking beyond today's bill—this is about resilience, future-proofing, and long-term value.
• You want clarity, not hype—so we'll work through the physics, the trade-offs, and the practical realities together.

If any of those don't fit, flag it now. The sizing logic shifts fast depending on whether you're grid-tied, isolated, or somewhere in between.

The size of your solar and battery system isn't about arriving at a single number. It's about matching physics—how much sunlight falls on your roof and how much energy you actually use—with practical choices around budget, available space, and the degree of independence you're after.

Understanding your energy needs comes first

Before we calculate panel counts or battery capacity, you need to know your baseline. Pull up your last 12 months of electricity bills and look for:

• Daily energy use (kWh/day): How much power your household consumes over 24 hours.
• Peak demand (kW): Times when multiple appliances run simultaneously—air conditioning, hot water, EV charging, cooking.
• Seasonal variation: Summer cooling loads, winter heating spikes, or year-round consumption from pools or home offices.

For instance, a typical household might use 15–20 kWh per day. An efficient home with LED lighting, good insulation, and a heat pump could sit at 10–12 kWh. A poorly managed one with resistive heating and constant air-con might push 30 kWh or more.

Physics check: 1 kW of power used for 1 hour equals 1 kWh of energy. A 2 kW heater running for 3 hours consumes 6 kWh. A 100 W lightbulb running for 10 hours consumes 1 kWh. Get comfortable with this relationship—it's the foundation of every sizing decision.

Once you have this picture, you can start matching it with what solar and batteries can realistically deliver.

A framework for decision-making: three questions to guide your choices

Before diving into panel counts and kilowatt-hours, let's establish a simple decision framework. Ask yourself these three questions:

1. What am I solving for?

• Bill reduction? Solar alone, grid-connected, may be enough.
• Blackout protection? You need a battery—sized to your critical loads and desired autonomy.
• Energy independence? Off-grid or large battery systems with generous solar capacity.
• Future adaptability? Consider EV charging, potential grid export limits, and evolving technology.

2. What are my constraints?

• Budget: Solar is cheaper per kWh than batteries. Prioritise panels first, batteries second—unless resilience trumps cost.
• Roof space: Limited area? High-efficiency panels or ground-mounted arrays become necessary.
• Structural capacity: Older roofs may need reinforcement. Factor this into upfront costs.
• Local regulations: Feed-in tariffs, export limits, and rebates vary. Know your rules before you design.

3. What's my acceptable trade-off?

• Cost vs autonomy: Smaller systems cost less but rely more on the grid. Larger systems cost more but deliver independence.
• Complexity vs simplicity: Grid-tied solar is straightforward. Off-grid adds generators, load management, and maintenance.
• Present savings vs future resilience: Maximising ROI today might mean under-sizing for tomorrow's needs—or vice versa.

This framework cuts through the noise.
Every sizing decision flows from these three questions.
Keep them in mind as we work through the technical details.

For additional context, see the video below (sourced from Bluetti).
It compares solar only option vs solar + battery (assuming you have high demand in the evening hours):

Rooftop solar: how many panels is enough?

Solar panels are measured in watts (W). A typical modern panel produces around 400–450 W under ideal conditions. Ten panels give you roughly 4–4.5 kW of capacity. Multiply that by your average sunlight hours per day—say 4–5 hours in Melbourne, 5–6 in Brisbane—and you get your daily generation potential.

Panels themselves are modular, so scaling up is mostly a matter of available space and inverter sizing.

Basic formula:

Daily solar generation (kWh) = System size (kW) × Peak sun hours × Efficiency factor

The efficiency factor accounts for inverter losses, shading, temperature effects, and panel degradation. Use 0.75–0.85 as a conservative estimate.

Sizing by household type

• Small home (low demand, efficient appliances): 3–5 kW system, roughly 8–12 panels.
• Average family home: 6–10 kW system, approximately 15–25 panels.
• Large home with pool or EV charging: 10–15 kW system, around 25–40 panels (if your roof allows).

Example:

You use 15 kWh per day. You're in Melbourne with an average of 5 peak sun hours daily. You want solar to cover 100% of your demand.

15 kWh ÷ (5 hours × 0.8) = 3.75 kW system

That's about 9–10 panels at 400 W each.

But here's the nuance: If you're grid-connected, oversizing intentionally—say, 5–6 kW—lets you maximise feed-in credits, charge batteries faster, and buffer against inefficiencies or cloudy stretches. If you're off-grid, size for your worst-case winter day, not the annual average. That might mean doubling or even tripling your array to survive consecutive grey days when generation drops and heating demand climbs.

Home batteries: backup vs independence

Batteries store solar energy for when the sun isn't shining. Their capacity is measured in kilowatt-hours (kWh). The right size depends on what matters to you: riding through a brief blackout, covering your evening loads, or achieving multi-day autonomy.

Sizing by use case

• Backup-only (fridge, lights, Wi-Fi during blackouts): 5–7 kWh.
• Bill savings and evening use: 10–15 kWh.
• Partial independence (reduced grid reliance): 15–25 kWh.
• Full off-grid or large household: 30+ kWh, often paired with a backup generator for extended cloudy periods.

Basic formula:

Battery capacity (kWh) = Daily evening/night demand (kWh) × Days of autonomy ÷ Usable depth of discharge (DoD)

Most lithium batteries safely discharge to 80–90%. Lead-acid? Only 50%.

Example:

You use 8 kWh between sunset and sunrise. You want 1 day of autonomy for grid-connected backup. Your battery has 90% usable capacity.

8 kWh × 1 ÷ 0.9 = ~9 kWh battery

A 10 kWh system (like a Tesla Powerwall 2) handles this comfortably.

Off-grid twist:

Same 8 kWh nightly demand, but you want 3 days of autonomy to survive cloudy stretches.

8 kWh × 3 ÷ 0.9 = ~27 kWh

Now you're looking at multiple batteries or a single large unit—and costs escalate quickly.

The right size isn't about maxing out capacity. It's about matching your actual evening consumption with a realistic buffer for autonomy, then deciding whether the grid (or a generator) fills the gaps.

Off-grid, grid-connected, and everything in between

Your relationship with the grid fundamentally shapes your system design. Let's break down the spectrum.

Grid-connected solar (most common)

You generate power during the day, use what you need, and export the excess. At night or on cloudy days, you import from the grid. The grid acts as an infinite battery—storing your surplus and supplying shortfalls.

Physics check: Solar panels generate in real time but don't store energy. Without a battery, you're tied to when the sun shines versus when you need power.

What this solves: Lowers bills, reduces grid reliance during daylight hours, cuts carbon footprint.

What it doesn't solve: Blackouts, evening peak demand, or independence.

Grid-connected with battery

Now you're storing excess solar in a home battery. Panels charge the battery and power your home during the day. At night, the battery takes over. If the battery runs low, the grid fills the gap.

What this solves: Bill reduction, time-shifting (using cheap solar at night), partial blackout protection, and peak shaving (avoiding expensive grid power during high-demand windows).

What it doesn't solve: Multi-day outages, unless you've sized generously and managed demand tightly.

Off-grid (no grid connection at all)

You're completely disconnected. Solar panels charge batteries during the day. Batteries run your home at night and through cloudy periods. There's no grid safety net.

Physics check: You must account for worst-case scenarios—consecutive cloudy days in winter, when solar generation is lowest and heating demand peaks. Oversizing is non-negotiable. Most off-grid systems include a backup generator for those rare but inevitable gaps when batteries run dry and solar can't keep up.

What this solves: Total independence, no bills, no blackouts (unless your system fails).

What it doesn't solve: Cost and complexity. Off-grid systems are expensive, demand meticulous energy management, and require regular maintenance. You're also your own utility—troubleshooting, load balancing, and fuel management all fall to you.

Off-grid can deliver full self-sufficiency, but it's costlier and higher maintenance than grid-tied alternatives. For most people, unless grid connection is unavailable or prohibitively expensive (think remote rural or island sites), staying connected offers better value and flexibility.

Return on investment and demand reduction

Let's talk money. Solar is often cheaper than buying electricity from the grid, especially as retail prices climb. Payback periods for panels alone typically run 3–6 years depending on location, electricity rates, and feed-in tariffs. Batteries extend this timeline—sometimes to 8–15 years—but they add resilience and bill smoothing in return.

Grid-connected solar (no battery)

• Payback: 4–8 years on average.
• Lifetime value: Panels last 25+ years. After payback, it's mostly profit.
• Key variables: Retail electricity price, feed-in rate, system cost ($/W), and how much solar you self-consume versus export.

Grid-connected solar + battery

• Payback: 8–15 years, sometimes longer.
• Lifetime value: Batteries last 10–15 years (lithium). You'll likely replace them once over the panel lifespan.
• Key variables: Battery cost ($/kWh), depth of discharge, cycle life, and whether you're avoiding peak tariffs or prioritising backup over pure ROI.

Off-grid

• Payback: Often doesn't "pay back" in strict financial terms—especially if grid connection is viable. But if the grid is unavailable or costs tens of thousands to connect, off-grid wins by necessity.
• Lifetime value: You're buying autonomy, resilience, and freedom from rising electricity prices. Putting a dollar figure on that depends entirely on what independence means to you.

Reality check: If ROI is your primary driver and you're on the grid, solar alone beats solar + battery almost every time. Batteries shine when you value backup, peak shaving, or independence over pure financial return.

The cheapest kilowatt-hour is the one you don't use

Here's something installers rarely emphasise enough:

‍reducing demand often outperforms adding capacity.

Before you buy a single panel, ask:

• Can I insulate better? (Reduces heating/cooling load.)
• Can I shift loads to daylight hours? (Cuts battery demand.)
• Can I replace resistive heating with a heat pump? (Slashes consumption by 60–70%.)
• Can I install LED lighting, efficient appliances, and smart controls? (Shaves kWh across the board.)

Physics check: A heat pump moves heat rather than generating it. It can deliver 3–4 kW of heating for every 1 kW of electricity consumed. A resistive heater delivers 1 kW of heat for every 1 kW consumed. The difference compounds fast.

I've seen homes drop from 25 kWh/day to 12 kWh/day through insulation, heat pumps, and behavioural changes alone. That's a 6 kW solar system instead of 12 kW—half the cost, half the roof space, faster payback. Investing in efficient appliances, insulation, and smart demand management (like running the dishwasher at midday when solar is abundant) often outperforms simply adding extra panels or batteries.

What about EVs and grid interactivity?

Electric vehicles are blurring the line between car and battery. Many EVs now support vehicle-to-home (V2H) or vehicle-to-grid (V2G) functionality, letting you tap into the car's substantial battery—often 40–100 kWh—as backup storage or grid-interactive capacity.

If you're planning an EV, your solar system should be sized to cover both household consumption and vehicle charging. Picture this: a 60 kWh EV battery sitting in your driveway. That's six times the capacity of a typical home battery—and it's mobile.

Physics check: Bidirectional charging requires compatible hardware (a bidirectional inverter and a V2H/V2G-capable vehicle). Not all EVs support this yet, but the technology is maturing rapidly.

What this means:

• Your EV becomes a rolling battery. Charge it with cheap daytime solar, discharge it at night to power your home.
• Potentially eliminates the need for a dedicated home battery.
• Opens up grid interactivity—you could sell stored energy back to the grid during peak demand and earn premium rates.

What to watch:

• Warranty implications (some manufacturers limit V2H cycling).
• Cycle degradation (every charge/discharge cycle wears the battery slightly).
• Software and hardware compatibility (this space is evolving fast).

If you're buying an EV within the next 2–3 years, size your solar generously but hold off on a home battery until you see how the EV performs in practice. The vehicle might cover your backup and evening needs entirely.

Things to consider before you commit

Roof and space availability

Solar panels need sun-exposed area. A 5 kW system typically requires 25–30 m² of roof space (or ground-mounted equivalent). Check for:

• Shading: Even partial shade can cripple output. Trees, chimneys, neighbouring buildings—all matter. Tools like Google Sunroof or a site visit from an installer will reveal problem areas.
• Structural integrity: Older roofs may need reinforcement. Budget for this if your roof is 15+ years old.
• Orientation: North-facing is optimal (southern hemisphere). East and west work but sacrifice 10–20% efficiency. South-facing (southern hemisphere) is rarely worth it unless you have no alternatives.

What if my roof is shaded? Partial shading used to be a dealbreaker, but smart inverters and micro-optimisers have changed the game. These technologies isolate shaded panels so they don't drag down the entire array. You'll still lose output on those panels, but the rest of your system keeps performing. In heavily shaded scenarios, ground-mounted arrays or alternative locations (carports, sheds) might be worth exploring.

Battery safety and insurance implications

Lithium batteries are generally safe, but thermal runaway—overheating leading to fire—is a real, if rare, risk. Modern lithium-ion systems meet strict standards (UL 1973, IEC 62619, or local equivalents), and incidents are uncommon when systems are properly installed and maintained.

Look for:

• Certified systems: Compliance with UL 1973, IEC 62619, or equivalent local standards.
• Thermal management: Quality systems include active cooling or thermal monitoring to prevent overheating.
• Installation location: Avoid enclosed, poorly ventilated spaces. Garages, basements with airflow, or external mounting are safest.

Insurance check: Some insurers may adjust premiums for homes with battery storage. Others don't. Ask explicitly before installation, and factor any increase into your ROI calculations. Certified installers and compliant systems generally minimise insurance concerns, but it's worth confirming in writing.

Local regulations and incentives

Feed-in tariffs, rebates, net metering rules, and export limits vary widely by region. In some places, you're paid handsomely for every kWh you export. In others, the grid operator caps how much you can send back—or pays almost nothing.

Check:

• Feed-in rates: What you're paid per kWh exported to the grid.
• Net metering: Whether excess solar offsets your consumption 1:1, or if credits expire seasonally.
• Export limits: Some grids cap exports to 5 kW or less, especially in areas with high solar penetration.
• Rebates and incentives: Federal, state, or local programs can cut upfront costs by 20–50%. These change frequently, so verify current offers.

Understanding your local rules isn't just about maximising savings—it's about avoiding surprises when your system doesn't perform as expected because of regulatory caps you didn't know existed.

Yes, but…

Let's take a small but important detour to address common worries and practical counterpoints.

"What if I move?"

Solar panels can increase property value—studies consistently show homes with solar sell faster and often command a premium, especially in markets where energy costs are high or rising. Batteries add appeal in areas prone to blackouts, though the ROI for batteries is harder to recoup if you move within a few years.

Practical answer: If you're planning to sell within 3–5 years, solar alone is a safer bet. Batteries may not pay back in that window, but they won't hurt resale—they just won't guarantee higher returns. Off-grid systems are trickier; not every buyer wants the responsibility of managing their own power generation.

"What if I oversize and waste money?"

Oversizing solar is rarely a mistake if you're grid-connected. Excess generation earns feed-in credits, and solar costs have dropped so much that the marginal cost of extra panels is minimal. Oversizing batteries, though? That's riskier. You're paying for capacity you might never cycle through, and batteries degrade over time whether you use them heavily or not.

Practical answer: Size solar generously. Size batteries conservatively unless you're off-grid or blackouts are frequent. Better to add a second battery later than to overbuy capacity upfront that sits idle.

"Will I still get a bill?"

Even with large solar and battery systems, you'll likely draw from the grid occasionally—especially in winter when generation drops and heating demand rises. Going truly bill-free usually means oversizing your system significantly, which drives up costs and may not pencil out financially.

Practical answer: Aim for 70–90% self-sufficiency if you're grid-connected. That balances cost, complexity, and resilience without chasing the last 10% at diminishing returns. True zero bills are possible, but they're expensive and often require behavioural compromises (like limiting heating in winter or staggering appliance use).

"What if the technology improves next year?"

It will. Solar efficiency edges up a few percent annually. Battery costs drop 5–10% per year. But waiting means you're still paying retail electricity rates today, and those costs compound. I've watched people defer for a decade, chasing the "perfect" moment, only to realise they've spent more on power bills than they would've saved by installing years earlier.

Practical answer: If the payback is reasonable now, install now. Technology improvements are incremental, not revolutionary. You'll benefit more from 10 years of generation than from waiting for 5% better panels.

"What about maintenance?"

Solar panels are remarkably low-maintenance. Rain handles most cleaning, though annual inspections and occasional manual cleaning (especially in dusty or coastal areas) help maintain output. Inverters last 10–15 years and may need replacement once. Batteries require monitoring—most systems do this automatically via app—and eventual replacement.

Practical answer: Budget for inverter replacement around year 10–12, and battery replacement around year 12–15. Even with these costs, total lifecycle expenses remain far below grid electricity over 25 years.

Quick checklist: before you sign anything

• Know your current demand (kWh/day, peak kW, seasonal swings)
• Reduce demand first (insulation, heat pumps, LED lighting, efficient appliances)
• Assess your roof (area, orientation, shading, structural integrity)
• Clarify your goal using the three-question framework (What am I solving for? What are my constraints? What's my acceptable trade-off?)
• Size solar for realistic generation (conservative for grid-tied, worst-case for off-grid)
• Size batteries for actual evening demand (not aspirational autonomy)
• Check local feed-in rates, rebates, and export limits
• Confirm battery certifications and insurance implications
• Plan for EV interactivity if you're buying an EV within 2–3 years
• Get at least 3 quotes from accredited installers (compare warranty terms, not just price)
• Model payback with realistic assumptions (don't trust the installer's optimistic projections—run your own numbers)

Go deeper: credible resources for further research

• Solar and battery sizing calculators: PVWatts (NREL), Solar Choice, local utility tools
• Energy efficiency guides: Your Home (Australian Government), Energy Star (US)
• Off-grid design: Homepower Magazine archives, Solar Electric Handbook
• Battery safety standards: IEC 62619, UL 1973, local electrical codes
• EV bidirectional charging: Vehicle-to-Grid Hub, manufacturer specs for your EV model
• Local incentives: DSIRE (US), Solar Victoria, your state/regional energy authority

Final thought: beyond the kilowatt-hours

Right now, you're probably still thinking in terms of panels and batteries, payback periods and blackout protection. That's where we started, and it matters. But let me leave you with something broader.

A well-designed solar and battery system isn't just about cutting your bill or surviving an outage. It's about resilience—the capacity to adapt when the grid falters, when prices spike unpredictably, when the climate shifts in ways we can't fully anticipate. It's about self-sufficiency, not as an off-grid fantasy, but as a practical hedge against uncertainty. And, if you size thoughtfully and manage demand well, it's about moving toward a positive-energy home—one that generates more than it consumes, that feeds stability back into the grid rather than draining it, that becomes a node of strength rather than a point of vulnerability.

That's the takeaway messeage here. The technical details matter deeply, but the real question is: what kind of energy future do you want to live in? Because the system you install today is a vote for the grid you'll rely on tomorrow—or the grid you won't need at all.

Need a hand navigating all this?

We're here to help you get that right. Let's build something that lasts.

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