Where This Guide Comes From
The method and every number above are written out from Gary Does Solar's home-battery sizing guide — his bill-based golden rule, the usable-capacity details, and the future-proofing reasons to go bigger:
Gary Does Solar — "How To Size A Home Battery, The Golden Rule That Will End Overpaying"
290,000+ views. A practical, bill-first guide to sizing home battery capacity, with the depth-of-discharge, minimum SOC and degradation details installers rarely explain.
Watch on YouTube →
When you ask installers for a solar-and-battery quote, you get one of two answers. Some quote you "5 kWh" or "10 kWh" or "a Powerwall 3" as if those were the standard sizes everyone should own. Others ask your budget and tell you to get the biggest battery you can afford. Neither is sizing — they're guessing.
This guide is written out from a single video, Gary Does Solar's home-battery sizing guide (290,000+ views), and it gives you the method to work out the number for your house. It's based on his own real bill, so the numbers are concrete.
Start With Your Electricity Bill, Not the Sales Pitch
The whole method starts with one number: your average daily electricity usage. It's already on your bill.
- Find your estimated annual usage on the bill. If it's not printed, log into your account and pull a chart of one full year of usage.
- Divide the annual figure by 365 to get your average daily usage in kWh.
Gary's own bill before solar: nearly 7,500 kWh a year — about twice what his home size should need (and he still doesn't know why). That's a little over 20 kWh per day.
The Golden Rule
Golden rule: your battery's usable capacity should cover your average daily electricity usage.
Following that rule literally, to cover his own 20 kWh/day, he'd look at a Tesla Powerwall 3 plus an expansion pack (27 kWh) or a SigEnergy battery with modules totaling 21 kWh. But there are three things that quietly change the real number — and this is where most people overspend or undersize.
1. Depth of Discharge (DoD)
State of charge is how full a battery is; depth of discharge is how much you've drained it since the last full charge. A battery at 70% state of charge has a 30% depth of discharge. For lithium-ion, routinely discharging beyond about an 80% DoD significantly shortens lifespan.
Here's the catch most people miss: modern home batteries are advertised with 100% DoD, but that's not because they beat the chemistry — the manufacturer builds in hidden extra capacity to deliver that "full 100% usable". So always check the data sheet for the usable capacity, which is often smaller than the headline number.
2. Minimum State of Charge
Even with 100% DoD, most batteries have a minimum state of charge the BMS enforces to protect the cells. Gary's GivEnergy battery sits at a 4% minimum; others run 10% or even 20%. That reserve is real capacity you can't touch, and it needs to be accounted for when you size.
3. Battery Degradation
Lithium cells lose capacity as they age, even with perfect care. After 10 years or so, a 10 kWh battery may only deliver 7–8 kWh. Check the warranty to see exactly what capacity is guaranteed at the end of the warranty period — that number is the honest one to plan around.
Take all three into account, and the 27 kWh Tesla solution might still work, while the 21 kWh option probably needs another module. Which is exactly why "the biggest battery you can afford" is the wrong advice — you end up either paying for unusable capacity or undersizing the usable part.
Why "Daily Usage" Isn't Quite Enough — Three Reasons to Go Bigger
Once your battery covers daily usage, the video pushes you one step further, and all three reasons are practical:
- Future electrification. Swapping a gas hob for an induction hob, adding a heat pump, or installing AC all raise your daily usage. Sizing now for where you're headed saves a rebuild later.
- Grid outages. The worst time for an outage is when your battery is running low. If it's sized only to your average daily usage, it may not have enough left to keep essentials running. Also — and this is easy to forget — not all home batteries have backup/EPS capability. Ask your installer before you buy.
- Earning money, not just saving. With surplus capacity you can export to the grid at peak hours, or even enroll part of your battery in a virtual power plant, where a third party combines your battery with thousands of others to sell grid-balancing services. Everything runs automatically and pays you.
Budget-Conscious? Model It Before You Buy
If financial return is your main goal, the more you spend, the longer the payback. So before you commit, Gary strongly recommends modeling the installation. If your solar array is big enough, you might still generate enough on a cloudy winter day to avoid buying extra battery at all.
His own model of a cloudy December day still generated around 10 kWh — potentially 10 kWh of battery capacity he doesn't need to buy. He built a modeling tool (Solar Rasma) for his supporters, but the principle applies with any decent simulator: size against your worst-case winter day, not your sunny summer one.
One more lever: many of the newer home batteries (SigEnergy's Sig in store is his example) are modular. You buy a certain capacity today and add modules later — you don't need to get the sizing perfect the first time, and you can grow as your needs (or budget) do.
What Viewers Actually Asked Under This Video
The comment section under Gary's video is where the real questions live — over 500 replies, most of them from people who already had a quote in hand. Below are the ten most-liked comments, ranked by how many viewers found them useful, with straight answers after each one.
@oakfield.farm · 1 year ago · 60 likes
"Glad you included future requirements. This is so often forgotten about! The only other thing I'd add is that it's also important to consider the battery's ability to deliver sufficient power (kW). It's no use having a large energy store (kWh) if it can't supply enough power to meet your peak loads (kW)."
Creator's reply: "Great point and I totally agree!"
@YewDuct · 1 year ago · 46 likes
"A great big gotcha on stackable batteries is that they fall under home contents insurance. If solar kit is wall mounted or bolted to the floor, it comes under buildings insurance. The policy limits are typically the rebuilding cost of your home, so no worries. However, stackable batteries are insecure and can just be carried away by strong enough people. They come under home contents insurance. Unfortunately, doing the right thing from the fire risk perspective by installing them in the garage..."
Creator's reply: "My understanding is that if you turn your house upside down, what doesn't fall out is buildings insurance."
@cybetica · 1 year ago · 40 likes
"Thanks for showing a battery only option with cheaper overnight charging. This may be a great solution for some people, who can't add solar panels"
Creator's reply: "Cheers!" (to @DavidBkiwee in this thread)
@timl9730 · 1 year ago · 22 likes
"Great introduction to the subject! Thanks to Gary for the great, informative nature of this channel. However, I think with this subject it's virtually impossible to give easy principles to apply. I would strongly contest the 'make sure your usable battery capacity at least covers your average daily electricity use' Golden Rule. If you have solar generation, I would suggest starting with answering the question 'what do you typically consume between dusk and midnight on a winter's day'..."
Our take: This is the strongest pushback in the thread, and it's worth taking seriously. The golden rule is written for the battery-only case — it assumes the battery has to carry your whole day. Add solar and the math changes: during daylight your panels feed the house directly, and the battery's real job becomes covering the gap from dusk to midnight (and a little into the morning). On a winter day in the UK that gap might be 4–6 kWh, not your 20 kWh daily average. So the honest method is: run the golden rule to get your ceiling, then re-run it on your winter dusk-to-midnight draw to see how much smaller the battery can realistically be. The rule is a starting point, not a verdict.
@timboz88 · 7 months ago · 12 likes
"I live in New Zealand and we have much higher incidence of sunny days etc. Thanks Gary, the logic and in depth explanations are very valuable, cheers!"
Our take: The golden rule travels. Whether you're in New Zealand with year-round sun or Scotland with a grim winter, the method is the same — read your own bill, size the usable capacity to a typical day, then add margin for the future. What changes is only the solar fraction, not the logic.
@DOCTORAPPS-y5i · 3 weeks ago · 1 like · 1 reply
"Thanks for ur amazing clarification."
Creator's reply: "You're very welcome, glad I could help!😊"
@GazandHisCamera · 1 year ago · 10 likes
"I have a Powerwall3 plus the expansion pack (and Tesla Gateway) which gives me 27KW. The Powerwall can push 11.5kw into my house at any time which covers usage when we have the oven, hob on etc. I have the Powerwall3 set to provide 45% backup, the rest used for daily usage. Covers my usage easily even on a zero generation day. Come winter I will charge the batteries overnight with Octopus Go at 8p per KW using the Tesla app set to 'Time-Based Control'..."
Our take: A nice real-world config that answers both questions at once: 27 kWh of capacity and an 11.5 kW discharge ceiling, so it never runs out of "how long" or "how hard." The 45%-backup / rest-for-daily-use split is a sensible layer cake, and the winter play — charging overnight on Octopus Go at 8p/kWh with the app on Time-Based Control — is exactly how UK households make the tariff work for them. The key habit worth copying: let the software schedule charging, rather than manually chasing cheap hours.
@DJ67mc · 1 year ago · 9 likes
"Home battery with no solar, is the option I've been considering in recent months. In my research I had considered everything you covered here (a lot of time on chat GPT), but it's great to have all the important info condensed into 10 minutes. Also to get objective confirmation that the conclusions I reached were valid. Great job Gary, thanks."
Our take: Battery-only households are a fast-growing segment — renters, shaded roofs and people who simply can't add panels still want the night-rate arbitrage. His workflow is worth copying too: research it yourself, then use an independent video to confirm or challenge your conclusion before you spend. That beats trusting a single installer's quote every time.
@aneeseldeen3467 · 1 year ago · 7 likes
"Very informative. Perhaps you can expand on giving some explanations on how to work the various apps to control the inverter"
Our take: The inverter app is the second learning curve nobody warns you about. Most mainstream apps (including the ones paired with our BMS) have three screens worth learning first: live power (what the house is drawing right now), charge/discharge scheduling (when the battery fills and when it discharges), and the alarm/event list. Ask your installer to walk through those three with you on handover day and screenshot each one — after that you'll rarely need the manual.
@iRemaker · 8 months ago · 6 likes
"Your ability to simplify concepts is impressive. I wonder if you taught at university! Very impressive video, I learnt so much in that, particularly about tariffs"
Our take: "Tariffs" is where the UK battery maths actually lives: the wider the gap between cheap night rates and peak daytime rates, the faster a battery pays for itself. Smart tariffs like Octopus Go and Agile push overnight charging down to 7–8p/kWh while peak rates can sit above 30p — that spread is the battery's margin. If you're sizing a battery purely for savings, model your own tariff's spread first, not a national average.
@GuyH77 · 1 year ago · 5 likes
"Good information as usual. I used your Solarazma suite to model my expected use during winter... Sigestor... add an extra module when needed"
Our take: Modelling the worst winter day before buying is exactly the habit this guide recommends, and it's what modular batteries are designed for. Start with a base module sized to what you can prove from your own data, then add capacity later as real winter numbers come in — that's cheaper than over-buying on day one, and it's why stackable systems (Sigenergy's and our own stackable series) exist in the first place.
And if you're not hunting for a consumer brand but need the home battery storage itself — a custom LiFePO4 pack at a specific voltage and capacity, an OEM or ODM project, or wholesale supply for your own product line — that's what we do. Dajiu Energy is a Chinese manufacturer building LiFePO4 batteries for homes, RVs and commercial storage since 2017. See our wall-mounted home energy storage systems or more home storage guides — tell us your voltage, capacity and size, and we'll engineer the pack to hit it.
Reviewed by Dajiu Energy Engineering Team — a Chinese LiFePO4 battery manufacturer building LiFePO4 batteries for homes, RVs and commercial storage since 2017, serving OEM/ODM projects with CE, UN38.3, MSDS and ISO9001 certified builds.
Bottom Line
Don't let an installer quote you "a Powerwall" or "the biggest you can afford." Read your own bill, divide by 365, size the usable capacity to cover a typical day, then add a margin for the future and the occasional outage. That's the golden rule — and it's how you stop paying for battery you can't use.