Sizing Backup Water for a 3-Day Outage

If Johannesburg loses pressure for three days — whether from a burst main, load-shedding knocking out pump stations, or the city’s roughly 27-hour reservoir buffer running dry during a prolonged incident — you need stored water before the taps go quiet, not after. This guide works through the numbers so you can size your storage with confidence rather than guesswork.

Why three days is the right planning horizon

Three days is a practical middle ground. A single overnight or same-day restoration is inconvenient but manageable with a few jerrycans. Beyond 72 hours, however, basic sanitation starts to break down and the health risks become real. Planning for three days means you’re covered for the vast majority of supply interruptions without over-engineering an expensive system for a once-in-a-decade disaster.

Johannesburg Water’s own figures show non-revenue water sitting at 44.8% for 2024/25 — a sign that the distribution network is under significant stress. Ageing pipes and pump failures are not hypothetical; they happen regularly across the city.

The baseline: how much water does your household actually use?

South Africans use roughly 237 litres per person per day on average, well above the global average of around 173 litres. That figure includes garden irrigation, car washing, long showers — all the things you’ll stop doing the moment supply is cut. During an outage, consumption drops sharply.

A useful planning split for outage conditions:

Use category	Litres per person per day
Drinking and cooking	5–10
Basic hygiene (hand-washing, face)	5–10
Toilet flushing (reduced)	15–30
Minimum realistic total	25–50

The lower end of that range (25 L/person/day) assumes bucket-flushing toilets once or twice daily and strict discipline with hygiene water. The upper end (50 L/person/day) is more comfortable and realistic for families with young children or elderly members.

For a deeper look at normal household consumption patterns, see how much water a family uses — it’s worth knowing your baseline before you calculate your outage storage.

Worked examples by household size

Single person: 25–50 L/day × 3 days = 75–150 litres. A 200 L tank is sufficient with room to spare.

Couple (2 people): 50–100 L/day × 3 days = 150–300 litres. A 500 L tank gives comfortable headroom and is worth buying over a 300 L unit given the small price difference.

Family of 4: 100–200 L/day × 3 days = 300–600 litres. A 500 L tank covers the lean scenario; a 750 L or 1,000 L unit covers realistic use without rationing anxiety.

Family of 6: 150–300 L/day × 3 days = 450–900 litres. Plan for at least 1,000 L, or two 500 L tanks in series.

These are planning ranges, not guarantees. Your actual number depends on toilet type, whether you have a gas hob (reducing washing-up water), and how strictly your household can follow outage protocols.

Choosing the right tank size

Once you have your litre target, round up to the next standard tank size. Common sizes available in Johannesburg are 500 L, 750 L, 1,000 L, 2,500 L, and 5,000 L. The jump from 1,000 L to 2,500 L is significant in both cost and footprint, so only go there if your household genuinely needs it or you want a buffer for the garden.

For guidance on selecting the right capacity for your property, how big a water tank do I need walks through the decision in detail.

Two practical points:

• Elevation matters. A gravity-fed tank must sit high enough to deliver adequate pressure. If it’s at ground level, you’ll need a booster pump to get water to your taps and showerhead.
• Top-up strategy. A tank that fills automatically from mains supply when pressure is available is far more useful than one you have to fill manually. A ball-valve inlet and overflow pipe are standard on proper backup installations.

Sizing your tank is step one — getting it installed correctly is step two. Compare 3 free quotes — vetted installers, no obligation.

What a complete backup system looks like

A standalone tank is only part of the picture. A functional backup water system for outage conditions typically includes:

• Storage tank — sized per the examples above
• Inlet valve — allows mains to refill the tank when supply is available
• Booster pump — pressurises water from the tank to your household plumbing; typically R15,000–R35,000 installed
• Non-return valve — prevents stored water from back-flowing into the municipal supply
• First-flush or basic filtration — particularly if water sits in the tank for extended periods

Complete tank-and-pump backup systems typically run R20,000–R40,000 for simpler setups, with whole-house installations coming in around R55,000. Get itemised quotes so you can compare like for like. Our backup water systems overview explains the main system types and what each one covers.

Water quality in storage

Stored municipal water is treated to SANS 241 (2024 edition) standards when it leaves the network, but quality can degrade over time in a tank, particularly in warm conditions. Keep tanks out of direct sunlight where possible, ensure lids are sealed against insects and debris, and if water has been sitting for more than a week, run taps briefly before drinking. For drinking water specifically, a countertop filter or stored bottled water for the first 10 litres per day is a sensible belt-and-braces measure during a long outage.

Ready to find out what a properly sized backup system costs for your home? Get your free quotes here — no obligation, no spam.

Before you buy anything

Measure your available space before you settle on a tank size. A 2,500 L vertical tank is roughly 1.4 m in diameter and 2.3 m tall — it won’t fit in every yard. Check with your local municipality whether any permits are required for structural supports or plumbing connections. And confirm with your installer exactly how the system integrates with your existing plumbing so there are no surprises about pipe rerouting costs. Asking for a written scope of work before signing anything is always worth the five minutes it takes.