Solar Battery Sizing: How to Calculate What Your Home Needs
Solar battery sizing is a four-step job: list the loads the battery must actually carry, add up their daily watt-hours, multiply by the backup days you want, then convert that need into nameplate capacity rather than guessing from a kWh figure on a spec sheet. A small Southeast Asian home runs about 3.7 kWh/day of critical loads — fridge, fans, lights, router, TV — so plan with a small buffer around 4–5 kWh/day and one to two autonomy days.
A 5 kWh/day home wanting two days of backup needs about 11 kWh of usable energy, which works out to about 13.6 kWh of nameplate capacity. In standard 51.2V module sizes that is one 16 kWh pack. Do not quote the job from nameplate: quote it from usable energy, because nameplate overstates what reaches the outlets by roughly 19%.
Most sizing tools found online won’t give you that answer. The top calculators are built around a US household that burns 28–30 kWh/day, with dishwashers, electric ovens and central air conditioning. Run a real SE-Asian critical-load list through them and the battery bank overshoots three to five times. Some divide daily energy only by depth of discharge and skip round-trip losses, which quietly leaves the bank about 10% short. The method below is built the other way: start from the loads on a worksheet, and model a real 51.2V LiFePO4 pack.
Start with a critical-load list, not the power bill
Most sizing arguments start in the wrong place: with the customer’s monthly bill or the wattage of the solar array on the roof. Those numbers describe whole-home usage and whole-home generation. A backup battery does not run the whole home. It runs the shortlist of loads the homeowner refuses to live without during an outage. The first deliverable on site is that shortlist, written down with the customer in the room.
Walk through the house room by room. Ask what actually breaks a bad night: the refrigerator, the Wi-Fi router, a few ceiling fans, LED lighting, a TV, perhaps a water pump or a medical device. In hot SE-Asian climates a fan and a fridge matter far more than a dishwasher. Write each device, its running watts, and how many hours a day it runs on battery. Ignore air-conditioning unless the customer explicitly accepts the size and bill that comes with it, because one 9,000 BTU unit changes the math more than everything else on the list combined.
One trap here is startup surge. A fridge compressor or pump can draw three to five times its running watts for a few seconds. That surge is an inverter and continuous-kW problem, not a daily-energy problem. Flag it on the worksheet, but size the energy bank from running watts and hours.
Sum the daily watt-hours
Now turn the list into one number. Watt-hours per day for each load equals running watts times hours of use. Add them up. Here is a realistic critical-load list for a small SE-Asian household:
- Refrigerator (running ~150W, cycles over 24h): about 1,000 Wh/day
- Four ceiling fans (~70W each, 6h): 1,680 Wh/day
- LED lighting (~80W total, 5h): 400 Wh/day
- Wi-Fi router + modem (~15W, 24h): 360 Wh/day
- TV + phone/laptop charging (~60W, 4h): 240 Wh/day
Total critical load: about 3,680 Wh/day, round to 3.7 kWh/day.
That 3.7 kWh/day is the heart of the whole calculation. Everything downstream multiplies off it, so if the list is wrong the bank is wrong. Add a small buffer for loads added later, then treat the result as the design daily load. A customer already running whole-home air-con lands closer to 8–10 kWh/day, and that is a different conversation about system size, not a rounding difference.
Pick your autonomy days from real outage history
Autonomy is how many consecutive days the battery must carry the critical list with no solar input and no grid. It is chosen from the site, not from a rule in a forum. Ask the homeowner how long the worst outage lasted last year and how often the grid drops out. In many SE-Asian cities the grid is up most days but load-shedding stretches into late afternoon, when the battery is already partly drained.
- Grid-tied with occasional short outages: 1 day of autonomy is usually enough.
- Frequent or multi-hour daily load-shedding: size for 2 days.
- Remote or weak-grid sites where a cloudy stretch could last days: 3 days or more.
If nobody knows the exact history, 2 days is a defensible default for a residential backup quote. It is not the cheapest option, but it is the one that does not get you a call during the second rainy week. Autonomy compounds fast: going from 1 day to 2 days roughly doubles the energy bank, so make sure the customer actually needs that second day.
Convert nameplate kWh into the energy you can actually use
This is the step most online calculators gloss over and the one that loses installers the job. A battery advertised as 16 kWh does not hand you 16 kWh. Two frictions sit between the nameplate and the outlets: depth of discharge and round-trip efficiency.
Depth of discharge (DoD) is how much of the bank you may cycle down to before recharging. LiFePO4 packs are typically rated for a 90% DoD operating window, so use 0.90 as a conservative design factor. Round-trip efficiency covers losses charging through the charger and discharging through the inverter; a well-matched LFP system lands around 90%, so apply another 0.90. The usable energy the customer can actually count on is given by the formula:
Usable kWh = nameplate kWh × 0.90 DoD × ~0.90 round-trip efficiency
So a 16.08 kWh nameplate pack delivers about 16.08 × 0.90 × 0.90 = 13.0 kWh of usable AC energy. Quote the job from that 13.0 kWh, not from 16.08. If a datasheet states a different DoD or efficiency, use the manufacturer’s numbers and confirm them, because assuming a round number is how you end up 15% short on the first long outage.
Worked example: 5 kWh/day, 2 days backup on a 51.2V pack
Walk through a full quote. The customer’s critical list sums to 5 kWh/day. They want 2 days of backup. First find the usable energy required: 5 × 2 = 10 kWh usable. Add a small margin for load growth and hot-day derating, and plan for about 11 kWh usable.
Now convert that usable target back to nameplate. With 0.90 DoD and 0.90 efficiency, divide the other way: 11 / (0.90 × 0.90) = 11 / 0.81 = about 13.6 kWh of nameplate needed. Real 51.2V residential packs come in standard sizes, so round up to the nearest available module. A 51.2V × 314Ah pack is 51.2 × 314 / 1000 = 16.08 kWh nameplate, which yields roughly 13.0 kWh usable. That comfortably covers the 11 kWh target, and leaves headroom for another day or a new load later.
Check the math the other way to be sure. Runtime equals usable Wh divided by load watts. The 13.0 kWh pack on a 3.7 kWh/day list runs about 13.0 / 3.7 = 3.5 days. On a heavier 5 kWh/day list it runs about 2.6 days. Either way it clears the 2-day requirement. That cross-check takes two minutes and catches a factor-of-two error before the customer signs.
Standard 51.2V module sizes and what they cover
Residential LFP modules are sold in a small set of standard configurations. The usable column applies the same 0.90 × 0.90 factors:
| Pack | Nameplate | Usable (~0.81) | What it covers |
|---|---|---|---|
| 51.2V 100Ah | 5.12 kWh | ~4.1 kWh | 1 day for a small 3.7 kWh/day home |
| 51.2V 200Ah | 10.24 kWh | ~8.3 kWh | 1 day for a heavier home / TOU shifting |
| 51.2V 300Ah | 15.36 kWh | ~12.4 kWh | 2 days of critical loads |
| 51.2V 314Ah | 16.08 kWh | ~13.0 kWh | 2 days with headroom |
| Two packs stacked | 30+ kWh | ~24+ kWh | whole-home backup / long outages |
Leave expansion headroom before you quote
Size the bank the customer needs today, but choose a platform that can grow. Stackable 51.2V packs let you start with one module and add another later, which keeps the first quote affordable without designing a dead-end system. Confirm the inverter’s parallel and communication limits before promising a second pack in year two; an inverter capped at four modules cannot honor a promise of twelve. Pick a footprint and wall space that already has room for the next one.
Match voltage to the inverter you are already buying. Most residential SE-Asian systems run at 48V nominal, and 51.2V LFP packs are the standard 16-cell configuration for that bus. Mixing a 51.2V pack with a 48V-only inverter profile is a common site mistake, so check battery voltage against the inverter manual early in our 51.2V vs 48V home storage guide.
Mistakes that cost installers money
The undersized bank is the classic: quote from the bill instead of the load list, add no DoD or efficiency factor, and the customer hits empty on the first cloudy day. The oversized bank is the quiet one: a homeowner convinced they need whole-home backup gets a 20 kWh system that only cycles through about a fifth of its capacity on a normal night. The cells are fine; it is the payback that suffers, because most of that purchase sits idle. Both mistakes are fixed by doing the load list honestly.
Heat is the other one. In tropical installs, a pack mounted in a tin shed or unvented cupboard loses usable capacity and ages faster than the datasheet assumes. Ventilate the battery room, keep it out of direct sun, and derate expectations for summer.
Related guides
For how long a given usable bank actually lasts on a household load, see our runtime walkthrough and the outage-sizing angle. The pillar overview of LFP for solar ties the chemistry back to these numbers, and our 15 kWh capacity guide works the same load-matching method. A practical starting module for the worked example is the 51.2V wall-mounted pack, with the full wall-mounted range available for larger quotes.
About this guide
This guide is written by the technical team at JM Batteries, a LiFePO4 battery manufacturer for residential and commercial storage. The four-step method above is the same worksheet our team walks through with installers when sizing systems. We quote usable energy rather than nameplate because that is the number that holds up on the second night of an outage. Where a product datasheet states different DoD, efficiency or cycle figures, use the datasheet and confirm them with the supplier（JM Batteries）.
Frequently Asked Questions
How do I calculate what size solar battery I need?
List every load the battery must carry and write running watts and daily hours. Sum watts × hours for daily watt-hours, multiply by the number of backup days, then divide by DoD times efficiency (about 0.81 for LFP) to get nameplate kWh. Round up to the nearest standard module. For a 5 kWh/day home with 2 backup days: 5 × 2 / 0.81 ≈ 12.3 kWh, so a 16 kWh pack with margin.
What size solar battery do I need for a 5kW solar system?
Panel wattage does not set battery size — the critical-load list and autonomy days do. A 5 kW array can charge a wide range of banks, but a home using 5 kWh/day of critical loads with 2 backup days still needs roughly a 13–16 kWh nameplate pack regardless of the 5 kW panels.
How many solar batteries do I need to power a house?
Divide required usable kWh by the usable capacity of one module. If you need 11 kWh usable and each 51.2V pack delivers about 13 kWh usable, one module clears it. For a heavier whole-home list, stack two. Always round up, never down.
Should I oversize my solar battery?
Not as a starting point. Oversizing ties up capital in capacity that rarely cycles and stretches out payback. In a hot climate, a bank left sitting at a high state of charge day after day also loses some calendar life compared with one that cycles normally. The exception is when you already know new loads are coming — in that case pick a stackable platform and add modules later rather than buying one oversized box up front.
How many days of backup should I plan for?
One day for rare short outages, two days for regular load-shedding, and three or more for remote weak-grid sites. Base it on the site’s actual outage history, not a generic rule. Going from one day to two roughly doubles the bank, so confirm the customer needs it.
Should I use 48V or 51.2V for a home battery?
They are effectively the same nominal bus. A 51.2V LFP pack is the 16-cell configuration, charging to roughly 54–58V, and appears as 48V nominal in most inverter menus. Confirm the inverter supports the LFP charge profile before ordering.
What depth of discharge should I design around?
LiFePO4 packs are rated for about a 90% DoD window; lead-acid is closer to 50%, which is why the same usable kWh needs about twice the lead-acid nameplate. The common 80/20 rule — use about 80% and keep 20% in reserve — is the conservative version of the same idea and buys extra cycle life. Always confirm the exact figure on the manufacturer’s datasheet.
