Key takeaways
- Size from daily watt-hours, not panel capacity. Divide by your planned depth of discharge, then multiply by days of autonomy.
- A 240Ah 12V battery holds roughly 2880Wh but only about 1440Wh is usable at 50% DoD.
- Always check the C-rate. A 200Ah C20 battery is not a 200Ah C10 battery — solar duty draws harder than twenty hours.
- Undersized charging causes more disappointing installations than undersized storage, and it looks like a failing battery.
- MPPT controllers typically harvest meaningfully more than PWM, and matter most when panel voltage is well above battery voltage.
- Above roughly 1kW, move to 24V or 48V — it cuts current, cable size and losses.
- Design routine cycling around 50% DoD and keep the 80% capability in reserve for cloudy spells.
Across much of Africa, South Asia and the Middle East, the question is rarely whether solar generation works. Sunlight is abundant and panels have become inexpensive. The question is what happens after dark, during a cloudy week, and three years into the installation when the batteries are tired.
Generation without storage does not solve the problem it appears to solve. This guide covers how off-grid and hybrid systems are actually sized, which battery technology suits which duty, and the mistakes that most often turn a working installation into a disappointing one.
Why storage is the part that decides the outcome
Solar generates during daylight. Demand continues after dark — lighting, refrigeration, connectivity, water pumping, medical equipment. Without storage, a large share of what the array produces is simply unavailable when it is wanted, and the site falls back to a diesel generator or to nothing at all.
Storage is what converts intermittent generation into dependable supply. It is also the component that determines whether solar displaces fuel burn or merely supplements it — which is why, for an energy business, the battery is where the climate contribution actually sits.
Sizing a system, in the order that matters
1. Start with daily consumption, not panel capacity
Total the watt-hours the site needs each day. A 10W router running 24 hours is 240Wh. Six 15W LED lights for six hours is 540Wh. A small refrigerator might be 800 to 1200Wh per day. Add them up before thinking about panels or batteries.
2. Divide by the depth of discharge you plan to use
Batteries should not routinely be run flat. For lead-acid tubular, design around 50% depth of discharge for daily use. A 240Ah 12V battery holds roughly 2880Wh, of which about 1440Wh is usable at 50% DoD.
3. Multiply by days of autonomy
How many consecutive poor-weather days must the system carry? Two days of autonomy on a 1200Wh daily load points to a bank, not a single unit. This is the step most often skipped, and it is why systems that work in March disappoint in July.
4. Check the charging can keep pace
Array output and charge controller capacity must return the energy used within the available sunshine hours. If they cannot, the bank drifts steadily lower over successive days until it fails to carry the night. Undersized charging is a more common cause of poor performance than undersized storage, and it is harder to diagnose because the symptom looks like a failing battery.
A worked example
A rural household wanting lighting, a fan, phone charging, a television and a small refrigerator. Working through it in order:
| Step | Working | Result |
|---|---|---|
| Daily consumption | 6 lights, fan, TV, phone charging, small fridge | ~1,500Wh/day |
| Divide by planned DoD | 1,500Wh ÷ 0.5 | 3,000Wh of rated capacity |
| Multiply by autonomy | × 2 days of poor weather | 6,000Wh rated |
| Convert to Ah at 24V | 6,000Wh ÷ 24V | ~250Ah at 24V |
| Choose the bank | Two 240Ah 12V units in series | 240Ah at 24V |
| Check charging keeps pace | 1,500Wh + losses ÷ ~5 peak sun hours | ~400W of array, minimum |
Two observations. The battery bank is doing most of the work, and it is larger than people expect — because usable capacity is roughly half the rated figure and autonomy multiplies it again. And the array figure is a floor, not a target: it assumes every day delivers five good sun hours, which no site does. Building in headroom on the array is cheaper than discovering the bank never reaches full charge.
Read the C-rate before comparing quotes
Battery capacity depends on how fast it is discharged. C10 measures capacity over a ten-hour discharge; C20 measures it over twenty. The same battery shows a higher Ah figure at C20, because lead-acid delivers more total energy when discharged gently.
A 200Ah C20 battery is not equivalent to a 200Ah C10 battery. Solar and inverter duty draws harder than twenty hours, so C10 is the honest rating for this application. When comparing quotes, always confirm which rate is being quoted — the difference is large enough to make a cheaper battery look bigger than it is.
| Model | Capacity (C10) | Usable at 50% DoD | Typical use |
|---|---|---|---|
| Solar Max TPS1600MAX | 160Ah | ~960Wh | Solar home lighting, small rooftop |
| Solar Max TPS2200MAX | 220Ah | ~1320Wh | Household rooftop, shops, street lighting |
| Solar Max TPS2400MAX | 240Ah | ~1440Wh | Pumps, telecom hybrid, microgrids |
Charge controllers: MPPT or PWM
The charge controller sits between the array and the battery, and the choice between the two common types affects how much of your generation actually reaches storage. We do not sell controllers, so take this as straight advice rather than a pitch.
| Consideration | PWM | MPPT |
|---|---|---|
| How it works | Connects array to battery, pulling panel voltage down to battery voltage | Converts excess voltage into additional charging current |
| Harvest from the same array | Baseline | Meaningfully higher, especially in weak light |
| Panel voltage well above battery | Wastes the difference | Recovers it |
| Cost | Lower | Higher |
| Best suited to | Small systems where panel voltage closely matches battery voltage | Larger arrays, higher-voltage panels, sites where every watt-hour counts |
The practical rule: if the array is small and the panels are voltage-matched to the battery, PWM is adequate and cheaper. If the panels run at a much higher voltage than the bank — which is increasingly common, since most modern panels do — MPPT recovers energy that PWM discards. On a remote site where adding panels later means another trip, that difference compounds.
Whichever type, check the controller can be set to the correct charge profile for your battery — and specifically that it has a lithium setting if you are installing LiFePO4. Older controllers often do not, and a lead-acid profile will not charge a lithium bank correctly.
Charge current matters as well as array size
Tubular lead-acid batteries want a reasonable charge current to reach a full charge and avoid sulphation — commonly around a tenth of the Ah rating as a starting point, though you should confirm against the specification for the model you are fitting. An array and controller that trickle in less than that will keep the bank in a chronic partial state of charge no matter how many hours of sun the site gets.
System voltage and wiring the bank
A battery bank is built from individual units wired to reach the system voltage and capacity you need. Two rules govern it:
Series raises voltage. Two 12V batteries in series give 24V at the same Ah. Parallel raises capacity. Two 12V batteries in parallel give 12V at double the Ah. Larger banks combine both.
Which system voltage?
| System voltage | Typical scale | Why |
|---|---|---|
| 12V | Small — lighting, a few appliances | Simplest, widest component availability |
| 24V | Household rooftop, small shop | Halves current for the same power |
| 48V | Larger installs, pumps, telecom, microgrids | Lowest current, thinnest cable, least loss |
The reason to go higher is current. The same power at 48V draws a quarter of the current it does at 12V, which means thinner cable, smaller fuses, less heat and lower resistive loss in the wiring. On a system pushing beyond about a kilowatt, staying at 12V means heavy expensive cable and losses that quietly eat the generation you paid to harvest.
The LiON Max range is offered in 12.8V, 25.6V, 48V and 51.2V configurations for this reason, and lead-acid banks are built up from 12V units in series.
Every battery in a bank should be the same age, capacity, type and ideally the same batch. Mixing a new unit into an ageing bank drags the new one down to the level of the weakest — which is why banks are replaced together rather than one at a time.
Choosing the battery technology
Solar duty is harder on a battery than grid-charged backup, in a specific way: the battery frequently sits at partial state of charge, is charged at a varying rate as cloud passes, and may not reach a full charge for days in poor weather. That combination is what wears out an ordinary battery pressed into solar service.
| Consideration | Solar Max tubular | LiON Max LiFePO4 |
|---|---|---|
| Cycle life | 3000+ at 50% DoD | 5000+ at 80% DoD |
| Usable capacity | ~50–80% | 100% |
| Weight for same usable energy | Higher | 60–70% lower |
| Purchase price | Lower | Higher |
| Maintenance | Periodic water topping | None |
| Best where | Budget-led, ventilated space | Weight or space constrained, heavy daily cycling |
Both are legitimate choices. Tubular lead-acid remains the pragmatic answer for most residential and small commercial rooftop installations, particularly where capital cost governs. Lithium earns its price where the bank cycles hard every day, where weight or space is constrained, or where a remote site makes maintenance visits expensive.
Depth of discharge is a design decision, not a limit
Solar Max supports up to 80% depth of discharge and is rated for 3000 or more cycles at 50%. Those two numbers are meant to be used together.
Design so routine daily use sits near 50%, and keep the 80% capability in reserve for extended cloudy periods. That gives long backup when it is needed and long service life the rest of the time. Designing to 80% as the everyday case spends the reserve you will want in the third week of a bad monsoon.
What actually goes wrong on site
Chronic partial state of charge
The most damaging condition for lead-acid. If the array cannot return a full charge, sulphation accumulates and capacity is lost permanently. It presents as gradually shortening backup, and is usually a charging problem rather than a battery fault.
Heat
Heat accelerates grid corrosion and water loss. Life figures are quoted at 25°C, and sustained operation well above that shortens service life substantially. A ventilated battery enclosure is not a luxury in a hot climate — Solar Max operates from -10°C to +55°C, but operating at the top of that range costs life.
Mixed or mismatched banks
Batteries in a bank should be the same age, capacity and type. Adding one new battery to an ageing bank means the new unit is dragged down to the level of the weakest. Replace banks together.
Skipping maintenance that was assumed
Enhanced water retention and ceramic vent plugs extend topping-up intervals to six months or more, but not indefinitely. Use distilled or demineralised water only, keep terminals clean and tight, and check more often at the hot end of the operating range.
Where these systems earn their keep
Solar plus storage is doing serious work across the markets we supply: rural household electrification and solar home lighting; agricultural irrigation pumps, where the sun and the demand conveniently coincide; telecom towers running solar hybrid to cut diesel deliveries to remote sites; solar street lighting; cold storage for produce and vaccines; and community microgrids serving several households from one installation.
What these share is that the alternative is usually a diesel generator, or no power at all. That is the comparison worth making — not solar against grid supply, but stored solar against fuel that has to be bought, transported and burned.
A short checklist before you commit
Total the daily watt-hours honestly, including standby loads. Decide the days of autonomy the site genuinely needs. Size the bank from usable capacity at your planned depth of discharge, using C10 figures. Confirm the array and controller can refill it within available sunshine. Plan ventilation for the climate, not the datasheet. And keep the bank matched in age and type.
Systems that disappoint rarely do so because the panels underperformed. They disappoint because the storage was sized from the wrong number, or the charging could never keep up with the load in the first place.
Still deciding between tubular lead-acid and lithium for the bank itself? The trade-offs are set out in full in LiFePO4 vs Lead-Acid: Which Battery Technology Is Right for Your Application?